Teleost Metamorphosis: The Role of Thyroid Hormone

The multi-level effect of chlorpyrifos during clownfish metamorphosis

Chemical pollution in coastal waters, particularly from agricultural runoff organophosphates, poses a significant threat to marine ecosystems, including coral reefs. Pollutants such as chlorpyrifos (CPF) are widely used in agriculture and have adverse effects on marine life and humans. In this paper, we investigate the impact of CPF on the metamorphosis of a coral reef fish model, the clownfish Amphiprion ocellaris, focusing on the disruption of thyroid hormone (TH) signalling pathways. Our findings reveal that by reducing TH levels, CPF exposure impairs the formation of characteristic white bands in clownfish larvae, indicative of metamorphosis progression. Interestingly, TH treatment can rescue these effects, establishing a direct causal link between CPF effect and TH disruption. The body shape changes occurring during metamorphosis are also impacted by CPF exposure, shape changes are less advanced in CPF-treated larvae than in control conditions. Moreover, transcriptomic analysis elucidates CPF's effects on all components of the TH signalling pathway. Additionally, CPF induces systemic effects on cholesterol and vitamin D metabolism, DNA repair, and immunity, highlighting its broader TH-independent impacts. Pollutants are often overlooked in marine ecosystems, particularly in coral reefs. Developing and enhancing coral reef fish models, such as Amphiprion ocellaris (Cuvier, 1830), offers a more comprehensive understanding of how chemical pollution affects these ecosystems. This approach provides new insights into the complex mechanisms underlying CPF toxicity during fish metamorphosis, shedding light on the broader impact of environmental pollutants on marine organisms.

Functions of thyroid hormone signaling in regulating melanophore, iridophore, erythrophore, and pigment pattern formation in spotted scat (Scatophagus argus)

BACKGROUND

Spotted scat, a marine aquaculture fish, has variable body color development stages during their ontogenesis. However, the regulatory mechanism of body color patterns formation was poorly understood. Thyroid hormones (TH) function as an important endocrine factor in regulating metamorphosis. In this study, exogenous thyroid hormones 3,5,3'-L-triiodothyronine (T3) and its inhibitor thiourea (TU) were used to treat spotted scat juveniles during the metamorphosis stage (from 60 to 90 dpf). The function and molecular mechanism of thyroid hormone signaling in regulating body color patterns formation was revealed, using the micro-observation of pigments cells distribution, colorimetric evaluation and carotenoids concentration measurement by spectrophotometry, and comparative transcriptome analysis.

RESULTS

Spotted scat body color patterns consisted of whole body black color, black bar, black and red spots, and its final pattern was formed through the metamorphosis. When spotted scat were treated with the inhibitor TU to disrupt thyroid hormone signaling, the levels of T3 and T4 were significantly decreased, the melanophores numbers were significantly increased, as well as the expression of genes involved in melanin synthesis and melanophore differentiation (tyr, tyrp1, dct, mitf, pmel, oca2, slc24a5, and erbb3) was significantly increased. Besides, the expression of genes associated with carotenoids and pteridine metabolism (apod, pnpla2, rdh12, stard10, xdh, abca1, retsat, scarb1, rgs2, and gch1) and carotenoids accumulation were stimulated, when thyroid hormone signaling was disrupted by TU. On the contrary, the levels of T3 and T4 were significantly elevated in spotted scat treated with T3, which could weaken the skin redness and reduce the number of black spots and melanophores, as well as the number and diameter of larval erythrophores. Notably, unlike melanophores and erythrophores, the differentiation of iridophore was promoted by thyroid hormones, gene related to iridophore differentiation (fhl2-l, fhl2, ltk, id2a, alx4) and guanine metabolism (gmps, hprt1, ppat, impdh1b) were up-regulated after T3 treatment, but they were down-regulated after TU treatment.

CONCLUSIONS

Above results showed that thyroid hormone signaling might play critical roles in regulation pigments synthesis and deposition, thereby affecting pigment cells (melanophores, iridophores and erythrophores) formation and body color patterns. The mechanisms of hyperthyroid and hypothyroid on different pigment cells development were different. Excess thyroid hormone might impact the rearrangement of melanophore by regulating cell cycle, resulting in the abnormalities of black spots in spotted scat. Meanwhile, the excessed thyroid hormone could reduce the number and diameter of larval erythrophores, as well as weaken the skin redness of juvenile erythrophores, but they were enhanced by the disruption of thyroid hormone. However, the formation of iridophore differentiation and guanine synthesis genes expression were stimulated by thyroid hormones. These findings provide new insights for exploring the formation of body color patterns in fish, and help to elucidate the molecular mechanism of thyroid hormone in regulating pigment cell development and body coloration, and may also contribute to selective breeding of ornamental fish.

A transient neurohormonal circuit controls hatching in fish

Hatching is a critical event in the life history of oviparous species. The decision to hatch is often carefully timed to coincide with favorable conditions that will improve survival through early life stages. However, how the relevant cues are relayed to trigger hatching remains unknown. In this work, we show that thyrotropin-releasing hormone (Trh) is the neuroendocrine activator of hatching in zebrafish. To elicit hatching, Trh neurons form a transient circuit that deposits the peptide into the embryo's circulation. Trh also activates hatching in a distantly related fish species that separated more than 200 million years ago. Our results reveal an evolutionarily conserved neuroendocrine circuit that controls a major life event in oviparous fish species.

Gene expression in the hypothalamic-pituitary-thyroid axis in Seriola rivoliana early larvae development at different temperatures

We analyzed the expression of genes involved in the hypothalamic-pituitary-thyroid axis (HPT-axis) in the longfin yellowtail Seriola rivoliana early larva, including temperature effects (22, 26 and 28 °C) and days of development (day one, day two, and day six after hatching). We aimed to determine if egg and larval incubation at different temperatures could disrupt this critical endocrine axis, which, in an aquaculture context, it could provoke mortality during early metamorphosis. There was a significant interaction between temperature and developmental timing on the relative expression of thyrotropin releasing hormone (trh). Larvae at 22 °C was the longest and increased more trh expression than larvae at higher temperatures. Interestingly, thyrotropin stimulating hormone (tsh) was highly expressed after hatching. Subsequently, it was downregulated at any temperature at least until day four, suggesting a temporal inhibition of the HPT axis. Therefore, we suggest that tsh-binding (tshr) to follicles should have occurred from hatching, creating a further "cascade effect" of upregulation of larval thyroglobulin (tg) from day two in a temperature-dependent manner. Consequently, new thyroid hormones should have been produced after yolk sac absorption. The above may indicate a narrow window of larval survival, where the larval transition from endogenous to exogenous feeding would depend on the correct timing to synthesize tg. Temperature significantly affected the expressions of deiodinase 1 (dio1-downregulated) and deiodinase 2 (dio2-upregulated) after hatching. The expressions of thyroid receptors alpha (trα) and beta (trβ) remained constant after hatching without significant effects of temperature and days of development. Then, the differential expression on day six showed that all HPT-axis transcripts increased their expressions as larvae developed, which suggested a functional HPT. Finally, there was no evidence that any temperature would disrupt the endocrine's larval axis, which indicated that the longfin yellowtail has a wide temperature adaption. Nevertheless, based on tg upregulation, we suggest that larvae should be maintained around 25-26 °C after hatching for a better chance of survival and development.

Thyroid Endocrine Disruption and Mechanism of the Marine Antifouling Pollutant 4,5-Dichloro-2-n-octyl-4-isothiazolin-3-one

The antifoulant 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) is an emerging pollutant in the marine environment, which may disrupt the thyroid endocrine system. However, DCOIT toxicity in relation to thyroid endocrine disruption and the underlying mechanisms remains largely unclear. In this study, in vivo, in silico, in vitro, and ex vivo assays were performed to clarify DCOIT's thyroid toxicity. First, marine medaka (Oryzias melastigma) were exposed to environmentally realistic concentrations of DCOIT for an entire life cycle. The results demonstrated that DCOIT exposure potently stimulated the hypothalamic-pituitary-thyroid axis, characterized by hyperthyroidism symptom induction and prevalent key gene and protein upregulation in the brain. Moreover, the in silico and in vitro results evidenced that DCOIT could bind to thyroid hormone receptor β (TRβ) and interact synergistically with triiodothyronine, thus promoting GH3 cell proliferation. The CUT&Tag experiment found that DCOIT interfered with the affinity fingerprint of TRβ to target genes implicated in thyroid hormone signaling cascade regulation. Furthermore, ex vivo, Chem-seq revealed that DCOIT directly bound to the genomic sequences of thyrotropin-releasing hormone receptor b and thyroid-stimulating hormone receptor in marine medaka brain tissues. In conclusion, the current multifaceted evidence confirmed that DCOIT has a strong potency for thyroid endocrine system disruption and provided comprehensive insights into its toxicity mechanisms.

Effects and phenotypic consequences of transient thyrotoxicosis and hypothyroidism at different stages of zebrafish Danio rerio (Teleostei; Cyprinidae) skeleton development

The effects and consequences of changes in thyroid hormones (THs) level are among the actively studied topics in teleost developmental and evolutionary biology. In most of the experimental models used, the altered hormonal status (either hypo- or hyperthyroidism) is a stable characteristic of the developing organism, and the observed phenotypic outcomes are the cumulative consequences of multiple TH-induced developmental changes. Meanwhile, the influence of the transient fluctuations of TH content on skeleton development has been much less studied. Here, we present experimental data on the developmental effects and phenotypic consequences of transient, pharmacologically induced thyrotoxicosis and hypothyroidism at different stages of ossified skeleton patterning in zebrafish. According to the results, the skeleton structures differed in TH sensitivity. Some showed a notable shift in the developmental timing and rate, while other demonstrated little or no response to changes in TH content. The developmental stages also differed in TH sensitivity. We identified a relatively short developmental period, during which changes in TH level significantly increased the developmental instability and plasticity, leading to phenotypic consequences comparable to those in fish with a persistent hypo- or hyperthyroidism. These findings allow this period to be considered as a critical developmental window.

Development of a Zebrafish Embryo-Based Test System for Thyroid Hormone System Disruption: 3Rs in Ecotoxicological Research

There is increasing concern regarding pollutants disrupting the vertebrate thyroid hormone (TH) system, which is crucial for development. Thus, identification of TH system-disrupting chemicals (THSDCs) is an important requirement in the Organisation for Economic Co-operation and Development (OECD) testing framework. The current OECD approach uses different model organisms for different endocrine modalities, leading to a high number of animal tests. Alternative models compatible with the 3Rs (replacement, reduction, refinement) principle are required. Zebrafish embryos, not protected by current European Union animal welfare legislation, represent a promising model. Studies show that zebrafish swim bladder inflation and eye development are affected by THSDCs, and the respective adverse outcome pathways (AOPs) have been established. The present study compared effects of four THSDCs with distinct molecular modes of action: Propylthiouracil (PTU), potassium perchlorate, iopanoic acid, and the TH triiodothyronine (T3) were tested with a protocol based on the OECD fish embryo toxicity test (FET). Effects were analyzed according to the AOP concept from molecular over morphological to behavioral levels: Analysis of thyroid- and eye-related gene expression revealed significant effects after PTU and T3 exposure. All substances caused changes in thyroid follicle morphology of a transgenic zebrafish line expressing fluorescence in thyrocytes. Impaired eye development and swimming activity were observed in all treatments, supporting the hypothesis that THSDCs cause adverse population-relevant changes. Findings thus confirm that the FET can be amended by TH system-related endpoints into an integrated protocol comprising molecular, morphological, and behavioral endpoints for environmental risk assessment of potential endocrine disruptors, which is compatible with the 3Rs principle. Environ Toxicol Chem 2024;00:1-18. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

From zero to ossified: Larval skeletal ontogeny of the Neotropical Cichlid fish Cichlasoma dimerus

The identification of skeletal elements, the analysis of their developmental sequence, and the time of their appearance during larval development are essential to broaden the knowledge of each fish species and to recognize skeletal abnormalities that may affect further fish performance. Therefore, this study aimed to provide a general description of the development of the entire skeleton highlighting its variability in Cichlasoma dimerus. Larvae of C. dimersus were stained with alcian blue and alizarin red from hatching to 25 days posthatching. Skeletogenesis began with the endoskeletal disk and some cartilage structures from the caudal fin and the splachnocranium, while the first bony structure observed was the cleithrum. When larvae reached the free-swimming and exogenous feeding stage, mostly bones from the jaws, the branchial arches, and the opercle series evidenced some degree of ossification, suggesting that the ossification sequence of C. dimerus adjusts to physiological demands such as feeding and ventilation. The caudal region was the most variable regarding meristic counts and evidenced higher incidence of bone deformities. In conclusion, this work provides an overview of C. dimerus skeletogenesis and lays the groundwork for further studies on diverse topics, like developmental plasticity, rearing conditions, or phylogenetic relationships.

Rapid hyperthyroidism-induced adaptation of salmonid fish in response to environmental pollution

The streams draining volcanic landscapes are often characterized by a complex series of factors that negatively affect hydrobionts and lead to declines in their populations. However, in a number of cases, a range of rapid adaptive changes ensure the resilience of hydrobiont populations. Here, we present both field and experimental data shedding light on the physiological basis of adaptation to heavy metal contamination in populations of Dolly Varden charr (Salvelinus malma) differing in duration of isolation in volcanic streams. The study reveals that isolated populations have a physiological phenotype that distinguishes them from populations inhabiting clean waters. They are characterized by a hyperthyroid status accompanied by an increased metabolic rate, elevated activity of antioxidant enzymes, decreased ionic conductivity of tissues and reduced stored energy reserves. Our experimental data reveal that hyperthyroidism is an adaptive characteristic enhancing the resistance to heavy metal contamination and shaping the evolution of these populations. The similarity of physiological, developmental and morphological changes in isolated populations suggests a common source and mechanisms underpinning this case of 'evolutionary rescue'. Thus, populations of S. malma trapped in volcanic streams represent a genuine case of rapid endocrine-driven adaptation to changing environmental stimuli.

Plasticity of cone photoreceptors in adult zebrafish revealed by thyroid hormone exposure

Vertebrate color vision is predominantly mediated by the presence of multiple cone photoreceptor subtypes that are each maximally sensitive to different wavelengths of light. Thyroid hormone (TH) has been shown to be essential in the spatiotemporal patterning of cone subtypes in many species, including cone subtypes that express opsins that are encoded by tandemly replicated genes. TH has been shown to differentially regulate the tandemly replicated lws opsin genes in zebrafish, and exogenous treatments alter the expression levels of these genes in larvae and juveniles. In this study, we sought to determine whether gene expression in cone photoreceptors remains plastic to TH treatment in adults. We used a transgenic lws reporter line, multiplexed fluorescence hybridization chain reaction in situ hybridization, and qPCR to examine the extent to which cone gene expression can be altered by TH in adults. Our studies revealed that opsin gene expression, and the expression of other photoreceptor genes, remains plastic to TH treatment in adult zebrafish. In addition to retinal plasticity, exogenous TH treatment alters skin pigmentation patterns in adult zebrafish after 5 days. Taken together, our results show a remarkable level of TH-sensitive plasticity in the adult zebrafish.

Heterochronic development of pelvic fins in zebrafish: possible involvement of temporal regulation of pitx1 expression

Anterior and posterior paired appendages of vertebrates are notable examples of heterochrony in the relative timing of their development. In teleosts, posterior paired appendages (pelvic fin buds) emerge much later than their anterior paired appendages (pectoral fin buds). Pelvic fin buds of zebrafish (Danio rerio) appear at 3 weeks post-fertilization (wpf) during the larva-to-juvenile transition (metamorphosis), whereas pectoral fin buds arise from the lateral plate mesoderm on the yolk surface at the embryonic stage. Here we explored the mechanism by which presumptive pelvic fin cells maintain their fate, which is determined at the embryonic stage, until the onset of metamorphosis. Expression analysis revealed that transcripts of pitx1, one of the key factors for the development of posterior paired appendages, became briefly detectable in the posterior lateral plate mesoderm at early embryonic stages. Further analysis indicated that the pelvic fin-specific pitx1 enhancer was in the poised state at the larval stage and is activated at the juvenile stage. We discuss the implications of these findings for the heterochronic development of pelvic fin buds.

The multi-level regulation of clownfish metamorphosis by thyroid hormones

Most marine organisms have a biphasic life cycle during which pelagic larvae transform into radically different juveniles. In vertebrates, the role of thyroid hormones (THs) in triggering this transition is well known, but how the morphological and physiological changes are integrated in a coherent way with the ecological transition remains poorly explored. To gain insight into this question, we performed an integrated analysis of metamorphosis of a marine teleost, the false clownfish (Amphiprion ocellaris). We show how THs coordinate a change in color vision as well as a major metabolic shift in energy production, highlighting how it orchestrates this transformation. By manipulating the activity of liver X regulator (LXR), a major regulator of metabolism, we also identify a tight link between metabolic changes and metamorphosis progression. Strikingly, we observed that these regulations are at play in the wild, explaining how hormones coordinate energy needs with available resources during the life cycle.

The Probiotic Phaeobacter inhibens Provokes Hypertrophic Growth via Activation of the IGF-1/Akt Pathway during the Process of Metamorphosis of Greater Amberjack (Seriola dumerili, Risso 1810)

Metamorphosis entails hormonally regulated morphological and physiological changes requiring high energy levels. Probiotics as feed supplements generate ameliorative effects on host nutrient digestion and absorption. Thereby, the aim of the present research was to investigate the impact of the probiotic Phaeobacter inhibens as a water additive on cellular signaling pathways in the metamorphosis of greater amberjack (Seriola dumerili). Activation of insulin-like growth factor type 1 receptor (IGF-1R), protein kinase B (Akt), mitogen-activated protein kinases (MAPKs) and AMP-activated protein kinase (AMPK), induction of heat shock proteins (Hsps), and programmed cell death were assessed through SDS-Page/immunoblot analysis, while energy metabolism was determined through enzymatic activities. According to the results, greater amberjack reared in P. inhibens-enriched water entered the metamorphic phase with greater body length, while protein synthesis was triggered to facilitate the hypertrophic growth as indicated by IGF-1/Akt activation and AMPK inhibition. Contrarily, MAPKs levels were reduced, whereas variations in Hsps response were evident in the probiotic treatment. Apoptosis and autophagy were mobilized potentially for the structural remodeling processes. Furthermore, the elevated enzymatic activities of intermediary metabolism highlighted the excess energy demands of metamorphosis. Collectively, the present findings demonstrate that P. inhibens may reinforce nutrient utilization, thus leading greater amberjack to an advanced growth and developmental state.

Determination of thyroid hormones in lake sturgeon (Acipenser fulvescens) tissues at different developmental stages using a sensitive liquid chromatography-mass spectrometry method

Thyroid hormones (TH) are known to play an important role in the growth and development of vertebrates. In fish species, TH regulates the larval-juvenile metamorphosis, and is crucial for development during early life stages. Monitoring the variations in TH levels at different life stages can provide insights into the regulation of metamorphosis and fish development. In this study, we developed an extremely sensitive method for the quantification of thyroxine (T4), triiodothyronine (T3), and reverse-triiodothyronine (rT3), in lake sturgeon (Acipenser fulvescens) tissues from eggs, free embryos, larvae, and juveniles. The target compounds were extracted by an enzymatic digestion method, followed by protein precipitation. Further cleanup was performed by liquid-liquid extraction (LLE) and solid phase extraction (SPE) using SampliQ OPT cartridges. The liquid-chromatography tandem mass spectrometry (LC-MS/MS) method used to quantify TH compounds showed remarkably high sensitivity with the limit of detection (LOD) and the limit of quantification (LOQ) ranging from < 1 pg/mL to 10 pg/mL and linearity in the range of 10-50,000 pg/mL. This method was validated for tissue samples across several early developmental stages and was checked for intra- and inter-day accuracy (78.3-111.2 %) and precision (0.1-4.9 %), matrix effect (75.4-134.1 %), and recovery (41.2-69.0 %). The method was successfully applied for the quantification and comparison of T4, T3 and rT3 in hatchery raised lake sturgeon samples collected at unique time points (i.e., days post fertilization dpf) including fertilized eggs (11 dpf), free embryos (14 dpf), larvae (22 dpf), juveniles (40 dpf) and older juveniles (74 dpf). With modifications, this method could be applied to other species important for agriculture or conservation.

Reversibility of Thyroid Hormone System-Disrupting Effects on Eye and Thyroid Follicle Development in Zebrafish (Danio rerio) Embryos

Early vertebrate development is partially regulated by thyroid hormones (THs). Environmental pollutants that interact with the TH system (TH system-disrupting chemicals [THSDCs]) can have massively disrupting effects on this essential phase. Eye development of fish is directly regulated by THs and can, therefore, be used as a thyroid-related endpoint in endocrine disruptor testing. To evaluate the effects of THSDC-induced eye malformations during early development, zebrafish (Danio rerio) embryos were exposed for 5 days postfertilization (dpf) to either propylthiouracil, a TH synthesis inhibitor, or tetrabromobisphenol A, which interacts with TH receptors. Subsequently, one half of the embryos were exposed further to the THSDCs until 8 dpf, while the other half of the embryos were raised in clean water for 3 days to check for reversibility of effects. Continued THSDC exposure altered eye size and pigmentation and induced changes in the cellular structure of the retina. This correlated with morphological alterations of thyroid follicles as revealed by use of a transgenic zebrafish line. Interestingly, effects were partly reversible after a recovery period as short as 3 days. Results are consistent with changes in TH levels measured in different tissues of the embryos, for example, in the eyes. The results show that eye development in zebrafish embryos is very sensitive to THSDC treatment but able to recover quickly from early exposure by effective repair mechanisms. Environ Toxicol Chem 2023;42:1276-1292. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

An integrative investigation of developmental toxicities induced by triphenyltin in a larval coral reef fish, Amphiprion ocellaris

Triphenyltin (TPT) is widely distributed on coastlines, which makes coral reef fish a potential target of TPT pollution. However, the negative effects of TPT on coral reef fish remain poorly understood. Therefore, in the present study, the larval coral reef fish Amphiprion ocellaris was used to investigate the developmental toxicities of TPT at environmentally relevant concentrations (0, 1, 10 and 100 ng/L). After TPT exposure for 14 d, the cumulative mortality increased, and growth was suppressed. In addition, TPT exposure inhibited the development of melanophores and xanthophores and delayed white strip formation, which might be responsible for the disruption of the genes (erbb3b, mitfa, kit, xdh, tyr, oca2, itk and trim33) related to pigmentation. TPT exposure also attenuated ossification of head skeletal elements and the vertebral column and inhibited the expression of genes (bmp2, bmp4 and sp7) related to skeletal development. The observed developmental toxicities on growth, pigmentation and skeleton development might be associated with the disruption of thyroid hormones and the genes related to thyroid hormone regulation (tshβ, thrα, thrβ, tg, tpo, dio2, and ttr). In addition, TPT exposure interfered with locomotor and shoaling behavior, and the related genes dbh, avp and avpr1aa. Taken together, our results suggest that TPT pollution might threaten the development of one of the most iconic coral reef fish, which might produce disastrous consequences on the health of coral reef ecosystems.

AOP Report: Thyroperoxidase Inhibition Leading to Altered Visual Function in Fish Via Altered Retinal Layer Structure

Thyroid hormones (THs) are involved in the regulation of many important physiological and developmental processes, including vertebrate eye development. Thyroid hormone system-disrupting chemicals (THSDCs) may have severe consequences, because proper functioning of the visual system is a key factor for survival in wildlife. However, the sequence of events leading from TH system disruption (THSD) to altered eye development in fish has not yet been fully described. The development of this adverse outcome pathway (AOP) was based on an intensive literature review of studies that focused on THSD and impacts on eye development, mainly in fish. In total, approximately 120 studies (up to the end of 2021) were used in the development of this AOP linking inhibition of the key enzyme for TH synthesis, thyroperoxidase (TPO), to effects on retinal layer structure and visual function in fish (AOP-Wiki, AOP 363). In a weight-of-evidence evaluation, the confidence levels were overall moderate, with ample studies showing the link between reduced TH levels and altered retinal layer structure. However, some uncertainties about the underlying mechanism(s) remain. Although the current weight-of-evidence evaluation is based on fish, the AOP is plausibly applicable to other vertebrate classes. Through the re-use of several building blocks, this AOP is connected to the AOPs leading from TPO and deiodinase inhibition to impaired swim bladder inflation in fish (AOPs 155-159), together forming an AOP network describing THSD in fish. This AOP network addresses the lack of thyroid-related endpoints in existing fish test guidelines for the evaluation of THSDCs. Environ Toxicol Chem 2022;41:2632-2648. © 2022 SETAC.

Harbours as unique environmental sites of multiple anthropogenic stressors on fish hormonal systems

Fish development and acclimation to environmental conditions are strongly mediated by the hormonal endocrine system. In environments contaminated by anthropogenic stressors, hormonal pathway alterations can be detrimental for growth, survival, fitness, and at a larger scale for population maintenance. In the context of increasingly contaminated marine environments worldwide, numerous laboratory studies have confirmed the effect of one or a combination of pollutants on fish hormonal systems. However, this has not been confirmed in situ. In this review, we explore the body of knowledge related to the influence of anthropogenic stressors disrupting fish endocrine systems, recent advances (focusing on thyroid hormones and stress hormones such as cortisol), and potential research perspectives. Through this review, we highlight how harbours can be used as "in situ laboratories" given the variety of anthropogenic stressors (such as plastic, chemical, sound, light pollution, and invasive species) that can be simultaneously investigated in harbours over long periods of time.

mRNA-miRNA sequencing reveals mechanisms of 2,2'-dipyridyl disulfide-induced thyroid disruption in Japanese flounder (Paralichthys olivaceus)

This study was conducted to evaluate the thyroid-disrupting effects of 2,2'-dipyridyl disulfide using Japanese flounder (Paralichthys olivaceus) as an animal model and to reveal the underlying mechanisms from the perspective of miRNA-mRNA interactions. The results indicated that 2,2'-dipyridyl disulfide exposure decelerated the metamorphic progress of P. olivaceus, suggesting its thyroid-disrupting property as an antagonist. Furthermore, radioimmunoassays, thyroid histological observation, real-time polymerase chain reaction, and mRNA sequencing showed that 2,2'-dipyridyl disulfide exposure exerted its thyroid-disrupting effects on larval and juvenile P. olivaceus by targeting multiple processes and pathways involved in the thyroid system, including peripheral metabolism of thyroid hormones, the thyroid hormone synthesis pathway, and the thyroid hormone/thyroid hormone receptor signaling pathway. In particular, global upregulation of the gene expression of three deiodinases caused decreases in thyroid hormone levels after 2,2'-dipyridyl disulfide exposure that are believed to be responsible for the inhibition of metamorphosis in P. olivaceus. Finally, miRNA sequencing suggested that several evolutionarily conserved miRNAs play important roles in the mechanism of 2,2'-dipyridyl disulfide-induced thyroid disruption. Specifically, overexpression of pny-miR-723a and pny-miR-216a resulted in upregulation of deiodinase 1 mRNA levels in the 2,2'-dipyridyl disulfide exposure group. This study provides the first evidence that 2,2'-dipyridyl disulfide has thyroid-disrupting properties and is also the first study remarking on the roles of miRNA-mRNA interactions in the action mechanisms of thyroid disruptors.

Direct development of the catfish pectoral fin - an alternative pectoral fin pattern of teleosts

BACKGROUND

Study of the teleosts' pectoral fin development touches on many crucial issues of evolutionary biology, from the formation of local adaptations to the tetrapod limbs' origin. Teleosts' pectoral fin is considered a rather developmentally and anatomically conservative structure. It displays larval and adult stages differing in the skeletal and soft tissues' composition. Larva-adult transition proceeds under the thyroid hormone (TH) control that defines pectoral fin ontogeny as an indirect development. However, the outstanding diversity of teleosts allows suggesting the existence of lineage specific developmental patterns.

RESULTS

We present a description of the North African catfish, Clarias gariepinus, pectoral fin development. It lacks a clear larval stage and directly develops the adult skeleton with the associated musculature and innervation. Interestingly, the development of catfish pectoral fin appears not to be under the TH dependence.

CONCLUSION

This catfish displays a direct pectoral fin developmental trajectory differing from the stereotyped teleost pattern. In the absence of the larval endoskeletal disk and TH control, the catfish's proximal radials arise in a manner somewhat similar to the metapterygial radials in basal actinopterygians and humerus in sarcopterygians. Thus, the catfish fin pattern seems homoplastic, arising by convergence with, or reversion to the ancestral developmental mechanisms. This article is protected by copyright. All rights reserved.

Thyroid hormone shapes craniofacial bones during postembryonic zebrafish development

Changing the shape of craniofacial bones can profoundly alter ecological function, and understanding how developmental conditions sculpt skeletal phenotypes can provide insight into evolutionary adaptations. Thyroid hormone (TH) stimulates metamorphosis and regulates skeletal morphogenesis across vertebrates. To assess the roles of this hormone in sculpting the craniofacial skeleton of a non-metamorphic vertebrate, we tested zebrafish for developmental periods of TH-induced craniofacial shape change. We analyzed shapes of specific bones that function in prey detection, capture and processing. We quantified these elements from late-larval through adult stages under three developmental TH profiles. Under wild-type conditions, each bone progressively grows allometrically into a mature morphology over the course of postembryonic development. In three of the four bones, TH was required to sculpt an adult shape: hypothyroidism inhibited aspects of shape change, and allowed some components of immature shape to be retained into adulthood. Excess developmental TH stimulated aspects of precocious shape change leading to abnormal morphologies in some bones. Skeletal features with functional importance showed high sensitivities to TH, including the transformator process of the tripus, the mandibular symphysis of the lower jaw, the scutiform lamina of the hyomandibula, and the anterior arm of the pharyngeal jaw. In all, we found that TH is necessary for shaping mature morphology of several essential skeletal elements; this requirement is particularly pronounced during larval development. Altered TH titer leads to abnormal morphologies with likely functional consequences, highlighting the potential of TH and downstream pathways as targets for evolutionary change.

Brains in Metamorphosis: Temporal Transcriptome Dynamics in Hatchery-Reared Flatfishes

Metamorphosis is a captivating process of change during which the morphology of the larva is completely reshaped to face the new challenges of adult life. In the case of fish, this process initiated in the brain has traditionally been considered to be a critical rearing point and despite the pioneering molecular work carried out in other flatfishes, the underlying molecular basis is still relatively poorly characterized. Turbot brain transcriptome of three developmental stages (pre-metamorphic, climax of metamorphosis and post-metamorphic) were analyzed to study the gene expression dynamics throughout the metamorphic process. A total of 1570 genes were differentially expressed in the three developmental stages and we found a specific pattern of gene expression at each stage. Unexpectedly, at the climax stage of metamorphosis, we found highly expressed genes related to the immune response, while the biological pathway enrichment analysis in pre-metamorphic and post-metamorphic were related to cell differentiation and oxygen carrier activity, respectively. In addition, our results confirm the importance of thyroid stimulating hormone, increasing its expression during metamorphosis. Based on our findings, we assume that immune system activation during the climax of metamorphosis stage could be related to processes of larval tissue inflammation, resorption and replacement, as occurs in other vertebrates.

Thyroid-Mediated Metabolic Differences Underlie Ecological Specialization of Extremophile Salmonids in the Arctic Lake El’gygytgyn

El’gygytgyn, the only “ancient lake” in the Arctic (3.6 MY), is a deep (176 m) and extremely cold (always ≤ 4°C) waterbody inhabited by unique salmonids, which colonized the ecosystem stepwise during the global fluctuations of the Quaternary climate. The descendant of the first-wave-invaders (long-finned charr) dwells in the deep waters and feeds on amphipods. The second-wave-invaders (smallmouth charr) consume copepods in the mid-waters. Recent third-wave-invaders (Boganida charr) are spread throughout the ecosystem and feed on insects when they are young shifting to piscivory at an older age. Here, we present the data on the charrs’ thyroid status and metabolic characteristics, confirming their ecological specialization. The long-finned charr exhibits an extremely low thyroid content, the substitution of carbohydrates for lipids in the cellular respiration, an increased hemoglobin level and a high antioxidant blood capacity. These traits are likely to be the legacy of anaerobic survival under perennial ice cover during several Quaternary glaciations. Moderate thyroid status and reduced metabolic rate of the smallmouth charr, along with an inactive lifestyle, could be regarded as a specialization to saving energy under the low food supply in the water column. The piscivorous Boganida charr could be sub-divided into shallow-water and deep-water groups. The former demonstrates a significantly elevated thyroid status and increased metabolism. The latter is characterized by a reduced thyroid level, metabolic rate, and lipid accumulation. Thus, the endemic El’gygytgyn charrs represent a wide spectrum of contrast physiological adaptation patterns essential to survive in sympatry under extremely cold conditions.

Larval metamorphosis is inhibited by methimazole and propylthiouracil that reveals possible hormonal action in the mussel Mytilus coruscus

Larval metamorphosis in bivalves is a key event for the larva-to-juvenile transformation. Previously we have identified a thyroid hormone receptor (TR) gene that is crucial for larvae to acquire “competence” for the metamorphic transition in the mussel Mytilus courscus (Mc). The mechanisms of thyroid signaling in bivalves are still largely unknown. In the present study, we molecularly characterized the full-length of two iodothyronine deiodinase genes (McDx and McDy). Phylogenetic analysis revealed that deiodinases of molluscs (McDy, CgDx and CgDy) and vertebrates (D2 and D3) shared a node representing an immediate common ancestor, which resembled vertebrates D1 and might suggest that McDy acquired specialized function from vertebrates D1. Anti-thyroid compounds, methimazole (MMI) and propylthiouracil (PTU), were used to investigate their effects on larval metamorphosis and juvenile development in M. coruscus. Both MMI and PTU significantly reduced larval metamorphosis in response to the metamorphosis inducer epinephrine. MMI led to shell growth retardation in a concentration-dependent manner in juveniles of M. coruscus after 4 weeks of exposure, whereas PTU had no effect on juvenile growth. It is hypothesized that exposure to MMI and PTU reduced the ability of pediveliger larvae for the metamorphic transition to respond to the inducer. The effect of MMI and PTU on larval metamorphosis and development is most likely through a hormonal signal in the mussel M. coruscus, with the implications for exploring the origins and evolution of metamorphosis.

Effects of thyroxine and propylthiouracil on feeding behavior and the expression of hypothalamic appetite-regulating peptides and thyroid function in goldfish (Carassius auratus)

There is poor evidence for an association between thyroidal state, feeding and appetite regulation in fish. We assessed how an altered thyroid state influences feeding behavior, food intake and expression of hypothalamic appetite-regulating peptides (Klotho-α and Klotho-β; orexin, OX; cholecystokinin, CCK; agouti-related peptide, AgRP; cannabinoid receptor 1, CB1) in goldfish. We also measured the expressions of hypothalamic, pituitary and liver transcripts that regulate the thyroid [thyrotropin-releasing hormone (TRH), thyrotropin-releasing hormone receptor (TRH-R) type 1, thyroid stimulating hormone beta (TSHβ), deiodinases (DIO2, DIO3), UDP-glucuronosyltransferase (UGT1A1), thyroid receptor alpha and beta (TRα, TRβ)], and circulating levels of total thyroxine (tT₄) and total triiodothyronine (tT₃). Goldfish were implanted with propylthiouracil (PTU) or T₄ osmotic pumps for 12 days. T₄- treatment increased feeding behavior but not food intake, increased central TSHβ and DIO2, and hepatic DIO2 transcript expression and increased central DIO3 mRNA. Under hyperthyroid conditions, hypothalamic Klotho and CCK expressions were downregulated, suggesting an increased metabolic state and a hypothalamic response to regulate energy balance. AgRP, OX and CB1 were not affected by T₄ treatment. PTU had no effect on any of the parameters examined, suggesting it is not a sensitive thyroid inhibitor in fish. Overall, we show that unlike in mammals, hyperthyroid conditions in goldfish do not lead to an increased desire or need to consume food, furthering evidence for a weak link between the thyroid and appetite.

Function and Evolution of Nuclear Receptors in Environmental-Dependent Postembryonic Development

Nuclear receptors (NRs) fulfill key roles in the coordination of postembryonal developmental transitions in animal species. They control the metamorphosis and sexual maturation in virtually all animals and by that the two main environmental-dependent developmental decision points. Sexual maturation and metamorphosis are controlled by steroid receptors and thyroid receptors, respectively in vertebrates, while both processes are orchestrated by the ecdysone receptor (EcR) in insects. The regulation of these processes depends on environmental factors like nutrition, temperature, or photoperiods and by that NRs form evolutionary conserved mediators of phenotypic plasticity. While the mechanism of action for metamorphosis and sexual maturation are well studied in model organisms, the evolution of these systems is not entirely understood and requires further investigation. We here review the current knowledge of NR involvement in metamorphosis and sexual maturation across the animal tree of life with special attention to environmental integration and evolution of the signaling mechanism. Furthermore, we compare commonalities and differences of the different signaling systems. Finally, we identify key gaps in our knowledge of NR evolution, which, if sufficiently investigated, would lead to an importantly improved understanding of the evolution of complex signaling systems, the evolution of life history decision points, and, ultimately, speciation events in the metazoan kingdom.

Thyroid hormones regulate the formation and environmental plasticity of white bars in clownfishes

Determining how plasticity of developmental traits responds to environmental conditions is a challenge that must combine evolutionary sciences, ecology, and developmental biology. During metamorphosis, fish alter their morphology and color pattern according to environmental cues. We observed that juvenile clownfish (Amphiprion percula) modulate the developmental timing of their adult white bar formation during metamorphosis depending on the sea anemone species in which they are recruited. We observed an earlier formation of white bars when clownfish developed with Stichodactyla gigantea (Sg) than with Heteractis magnifica (Hm). As these bars, composed of iridophores, form during metamorphosis, we hypothesized that timing of their development may be thyroid hormone (TH) dependent. We treated clownfish larvae with TH and found that white bars developed earlier than in control fish. We further observed higher TH levels, associated with rapid white bar formation, in juveniles recruited in Sg than in Hm, explaining the faster white bar formation. Transcriptomic analysis of Sg recruits revealed higher expression of duox, a dual oxidase implicated in TH production as compared to Hm recruits. Finally, we showed that duox is an essential regulator of iridophore pattern timing in zebrafish. Taken together, our results suggest that TH controls the timing of adult color pattern formation and that shifts in duox expression and TH levels are associated with ecological differences resulting in divergent ontogenetic trajectories in color pattern development.

Individual Serum Triiodothyronine and Thyroxine Levels in Seven Freshwater Fish Species

The thyroid hormones (THs) play an important role in the regulation of the rate of metabolism, affect the growth and function of different systems in the organism. The aim of this study was to assess serum concentration of total triiodothyronine (T3), total thyroxine (T4) as well as T3/T4 ratio in serum from healthy fresh water fish from Salmonidae, Acipenseridae, Cyprinidae, and Clariidae families to determine species-specific reference intervals. Mean concentrations of T3 and T4 levels varied significantly among fish. Finally, the test results show clear differences in the serum concentration of the T3 and T4 and give new insight into the thyroid hormones reference values in some commercial fresh water fish species.

Steroid hormones, dietary nutrients, and temporal progression of neurogenesis

Temporal patterning of neural progenitors, in which different factors are sequentially expressed, is an evolutionarily conserved strategy for generating neuronal diversity during development. In the Drosophila embryo, mechanisms that mediate temporal patterning of neural stem cells (neuroblasts) are largely cell-intrinsic. However, after embryogenesis, neuroblast temporal patterning relies on extrinsic cues as well, as freshly hatched larvae seek out nutrients and other key resources in varying natural environments. We recap current understanding of neuroblast-intrinsic temporal programs and discuss how neuroblast extrinsic cues integrate and coordinate with neuroblast intrinsic programs to control numbers and types of neurons produced. One key emerging extrinsic factor that impacts temporal patterning of neuroblasts and their daughters as well as termination of neurogenesis is the steroid hormone, ecdysone, a known regulator of large-scale developmental transitions in insects and arthropods. Lastly, we consider evolutionary conservation and discuss recent work on thyroid hormone signaling in early vertebrate brain development.

Natural toxic impact and thyroid signalling interplay orchestrates riverine adaptive divergence of salmonid fish

Adaptive radiation in fishes has been actively investigated over the last decades. Along with numerous well-studied cases of lacustrine radiation, some examples of riverine sympatric divergence have been recently discovered. In contrast to the lakes, the riverine conditions do not provide evident stability in the ecological gradients. Consequently, external factors triggering the radiation, as well as developmental mechanisms underpinning it, remain unclear. Herein, we present the comprehensive study of external and internal drivers of the riverine adaptive divergence of the salmonid fish Salvelinus malma. In the Kamchatka River, north-east Asia, this species splits in the reproductively isolated morphs that drastically differ in ecology and morphology: the benthivorous Dolly Varden (DV) and the piscivorous stone charr (SC). To understand why and how these morphs originated, we performed a series of field and experimental work, including common-garden rearing, comparative ontogenetic, physiological and endocrinological analyses, hormonal 'engineering' of phenotypes and acute toxicological tests. We revealed that the type of spawning ground acts as the decisive factor driving the radiation of S. malma. In contrast to DV spawning in the leaf krummholz zone, SC reproduces in the zone of coniferous forest, which litter has a toxic impact on developing fishes. SC enhances resistance to the toxicants via metabolism acceleration provided by the elevated thyroid hormone expenditure. These physiological changes lead to the multiple heterochronies resulting in a specific morphology and ecology of SC. Salvelinus malma represents a notable example of how the thyroid axis contributes to the generation of diverse phenotypic outcomes underlying the riverine sympatric divergence. Our findings, along with the paleoecology data concerning spruce forest distribution during the Pleistocene, provide an opportunity to reconstruct a scenario of S. malma divergence. Taken together, obtained results with the data of the role of thyroid hormones in the ontogeny and diversification of fishes contribute a resource to consider the thyroid axis as a prime director orchestrating the phenotypic plasticity promoting evolutionary diversification under the changing environmental conditions.

Misbalance of thyroid hormones after two weeks of exposure to artificial light at night in Eurasian perch Perca fluviatilis

Artificial light at night (ALAN) can affect the physiology and behavior of animals because it alters the natural rhythm of light and darkness. Thyroid hormones (TH) are partially regulated by the light information of photoperiod and are involved in metabolic adjustments to daily and seasonal changes in the environment, such as larval and juvenile development, somatic growth and reproduction. ALAN can change photoperiodic information and might thereby lead to changes in thyroid metabolism, but so far research on this topic is scarce. Therefore, we tested in two different experiments the effects of nocturnal illumination at a wide range of light intensities on TH in plasma of Eurasian perch (Perca fluviatilis). Total 3,3',5-triiodo-L-thyronine (T3) was significantly affected by ALAN and reduced at the highest tested intensity of 100 lx after only two weeks of exposure. Although total L-thyroxine (T4) was not significantly affected, the ratio of T3 to T4 tended to slightly decrease at 100 lx. In a second low-light experiment ALAN did not have clear effects on T3, T4 or the ratio of T3 to T4 at intensities between 0.01 lx and 1 lx. The results show first signs of endocrine disruption in thyroid metabolism after a relatively short ALAN exposure of two weeks under high-intensity streetlight conditions. Misbalanced thyroidal status can have serious implications for metabolic rates as well as developmental and reproductive processes.

Hierarchical analysis of ontogenetic time to describe heterochrony and taxonomy of developmental stages

Even though an accurate description of early life stages is available for some teleostean species in form of embryonic and post-embryonic developmental tables, there is poor overlap between species-specific staging vocabularies beyond the taxonomic family level. What is called "embryonic period", "larval period", "metamorphosis", or "juvenile" is anatomically different across teleostean families. This problem, already pointed out 50 years ago, challenges the consistency of developmental biology, embryology, systematics, and hampers an efficient aquaculture diversification. We propose a general solution by producing a proof-of-concept hierarchical analysis of ontogenetic time using a set of four freshwater species displaying strongly divergent reproductive traits. With a parsimony analysis of a matrix where "operational taxonomic units" are species at a given ontogenetic time segment and characters are organs or structures which are coded present or absent at this time, we show that the hierarchies obtained have both very high consistency and retention index, indicating that the ontogenetic time is correctly grasped through a hierarchical graph. This allows to formally detect developmental heterochronies and might provide a baseline to name early life stages for any set of species. The present method performs a phylogenetic segmentation of ontogenetic time, which can be correctly seen as depicting ontophylogenesis.

Gene clusters related to metamorphosis in Solea senegalensis are highly conserved

The flatfish, Solea senegalensis has considerable scientific interest and commercial value. The metamorphosis in this species occurs between 12 and 19 days after hatching and it takes about 1 week to complete. Eleven Bacterial Artificial Chromosomes (BAC) clones containing the various candidate genes involved in the process of metamorphosis: thyroxine 5 deiodinase 3 (dio3); forkhead box protein E4 (foxe4); melatonin receptor type 1C (mel1c); calsequestrin 1b (casq1b); thyrotropin subunit beta (tshβ); thyrotropin-releasing hormone receptor 1, 2, and 3 (trhr1, trhr2, trhr3); thyroid hormone receptor α a and b (thrαa, thrαb); and thyroid hormone receptor beta (thrβ) were analyzed by multiple Fluorescence in situ Hybridization (mFISH) and Next Generation Sequencing (NGS) techniques. The mFISH technique localized the 11 BAC clones on 12 different chromosome pairs because three of them, specifically the trhr1a, trhr2 and thrβ BAC clones, showed double signals. This signal duplication indicates a duplication of the genomic region inserted within the BAC clone, which provides evidence for the Teleost-Specific Whole Genome Duplication (TS-WGD). Micro-synteny and phylogenetic analysis showed that Cynoglossus semilaevis is the nearest species to S. senegalensis and that Danio rerio is the most distant one. The tshβ BAC clone was highly conserved as the genes belonging to this BAC were located on a single chromosome in all the species studied. These genes participate in proliferation, migration and cell-death, which are key processes during metamorphosis. Overall, micro-synteny analysis showed that most candidate genes are found in conserved genomic surroundings.

Hormones, developmental plasticity, and adaptive evolution: Endocrine flexibility as a catalyst for 'plasticity-first' phenotypic divergence

Explaining how populations adapt to environments is among the foremost objectives of evolutionary theory. Over generations, natural selection impels the phenotypic distribution of a population based on individual variation in phenotype and fitness. However, environmental conditions can also shape how individuals develop within their lifetime to influence which phenotypes are expressed in a population. It has been proposed that such environmentally-initiated phenotypic variation - also termed developmental plasticity - may enable adaptive evolution under some scenarios. As dynamic regulators of development and phenotypic expression, hormones are important physiological mediators of developmental plasticity. Patterns of hormone secretion, hormone transport, and the sensitivity of tissues to hormones can each be altered by environmental conditions, and understanding how endocrine regulation shapes phenotypic development in an ecologically-relevant context has much to contribute toward clarifying the role of plasticity in evolutionary adaptation. This article explores how the environmental sensitivity of endocrine regulation may facilitate 'plasticity-first' evolution by generating phenotypic variants that precede adaptation to altered or novel environments. Predictions arising from 'plasticity-first' evolution are examined in the context of thyroid hormone mediation of morphological plasticity in Cyprinodon pupfishes from the Death Valley region of California and Nevada, USA. This clade of extremophile fishes diversified morphologically over the last ~20,000 years, and observations that some populations experienced contemporary phenotypic differentiation under recent habitat change provide evidence that hormone-mediate plasticity preceded genetic assimilation of morphology in one of the region's species: the Devils Hole pupfish, Cyprinodon diabolis. This example illustrates how conceptualizing hormones not only as regulators of homeostasis, but also as developmental intermediaries between environment conditions and phenotypic variation at the individual-, population-, and species-levels can enrich our understanding of endocrine regulation both as a facilitator of phenotypic change under shifting environments, and as important proximate mechanisms that may initiate 'plasticity-first' evolutionary adaptation.

The Role of the Thyroid Axis in Fish

In all vertebrates, the thyroid axis is an endocrine feedback system that affects growth, differentiation, and reproduction, by sensing and translating central and peripheral signals to maintain homeostasis and a proper thyroidal set-point. Fish, the most diverse group of vertebrates, rely on this system for somatic growth, metamorphosis, reproductive events, and the ability to tolerate changing environments. The vast majority of the research on the thyroid axis pertains to mammals, in particular rodents, and although some progress has been made to understand the role of this endocrine axis in non-mammalian vertebrates, including amphibians and teleost fish, major gaps in our knowledge remain regarding other groups, such as elasmobranchs and cyclostomes. In this review, we discuss the roles of the thyroid axis in fish and its contributions to growth and development, metamorphosis, reproduction, osmoregulation, as well as feeding and nutrient metabolism. We also discuss how thyroid hormones have been/can be used in aquaculture, and potential threats to the thyroid system in this regard.

Functional morphogenesis from embryos to adults: Late development shapes trophic niche in coral reef damselfishes

The damselfishes are one of the dominant coral reef fish lineages. Their ecological diversification has involved repeated transitions between pelagic feeding using fast bites and benthic feeding using forceful bites. A highly-integrative approach that combined gene expression assays, shape analyses, and high-speed video analyses was used to examine the development of trophic morphology in embryonic, larval, juvenile, and adult damselfishes. The anatomical characters that distinguish pelagic-feeding and benthic-feeding species do not appear until after larval development. Neither patterns of embryonic jaw morphogenesis, larval skull shapes nor larval bite mechanics significantly distinguished damselfishes from different adult trophic guilds. Analyses of skull shape and feeding performance identified two important transitions in the trophic development of a single species (the orange clownfish; Amphiprion percula): (a) a pronounced transformation in feeding mechanics during metamorphosis; and (b) more protracted cranial remodeling over the course of juvenile development. The results of this study indicate that changes in postlarval morphogenesis have played an important role in damselfish evolution. This is likely to be true for other fish lineages, particularly if they consist of marine species, the majority of which have planktonic larvae with different functional requirements for feeding in comparison to their adult forms.