Conspicuous visual signals do not coevolve with increased body size in marine sea slugs

Phylogenetic analysis and genetic similarities of Phyllidia spp. by comparing the nucleotide sequence of 16S rRNA and cytochrome c genes

Phyllidiid nudibranchs are brightly colored gastropod molluscs, frequently encountered in coral reefs of the tropical Indo-Pacific. This study aimed to identify the phylogenetic similarities among the Phyllidia spp. The phylogenetic similarities among all the available Phyllidia spp. were studied by comparing the nucleotide sequence of 16s rRNA and cytochrome c genes (cox I). Sequences were retrieved from NCBI databases and aligned by using Geneious software. A phylogenetic tree was constructed for the retrieved sequences of Phyllidia spp. by using the neighbor-joining method on MEGA software and the pairwise distances were also calculated. The similarities among nucleotide sequences of 16s rRNA showed that the P. elegans, and P. haegeli had the highest similarities (99.92%) and the lowest similarities (99.14%) among P. haegeli and P. picta. While nucleotide sequences of cox I showed the highest similarities (99.90%) between P. elegans and P. ocellata, and the P. varicosa had the lowest similarities 99.74% with P. koehleri and P. larryi. The molecular phylogenetic analysis based on mitochondrial marker indicated a close relation between P. elegans and P. alyta in both cox I and 16s rRNA phylogenetic tree. The phylogenetic tree of 16s rRNA gene shows the P. ocellata is closely related to the clade of species P. exquisita. The available phylogenetic analysis could be useful in further studies of Phyllidiidae within Nudibranchia.

Seven new “cryptic” species of Discodorididae (Mollusca, Gastropoda, Nudibranchia) from New Caledonia

The study of a well-preserved collection of discodorid nudibranchs collected in Koumac, New Caledonia, revealed the presence of seven species new to science belonging to the genera Atagema, Jorunna, Rostanga, and Sclerodoris, although some of the generic assignments are tentative as the phylogeny of Discodorididae remains unresolved. Moreover, a poorly known species of Atagema originally described from New Caledonia is re-described and the presence of Sclerodoris tuberculata in New Caledonia is confirmed with molecular data. All the species described herein are highly cryptic on their food source and in the context of the present study the term “cryptic” is used to denote such species. This paper highlights the importance of comprehensive collecting efforts to identify and document well-camouflaged taxa.

The evolution of conspicuousness in frogs: When to signal toxicity?

Many organisms use conspicuous colour patterns to advertise their toxicity or unpalatability, a strategy known as aposematism. Despite the recognized benefits of this anti-predator tactic, not all chemically defended species exhibit warning coloration. Here, we use a comparative approach to investigate which factors predict the evolution of conspicuousness in frogs, a group in which conspicuous coloration and toxicity have evolved multiple times. We extracted colour information from dorsal and ventral photos of 594 frog species for which chemical defence information was available. Our results show that chemically defended and diurnal species have higher internal chromatic contrast, both ventrally and dorsally, than chemically undefended and/or nocturnal species. Among species that are chemically defended, conspicuous coloration is more likely to occur if species are diurnal. Our results also suggest that the evolution of conspicuous colour is more likely to occur in chemically defended prey with smaller body size. We discuss potential explanations for this association and suggest that prey profitability (related to body size) could be an important force driving the macroevolution of warning signals.

Body size in Batesian mimicry

A variety of traits is available for predators to distinguish unpalatable prey from palatable Batesian mimics. Among them, body size has received little attention as a possible mimetic trait. Size should influence predator behaviour if it shows variation between models and mimics, is detectable by the predator in question, and is not overshadowed by other traits more salient to the predator. Simple predictions within mimetic populations are that perfect mimics receive the lowest predation rate. However, prey body size is typically tightly linked to the nutritional yield and handling time for a successful predator, as well as likely being correlated with a model’s levels of defence. In certain circumstances, these confounding factors might mean that (a) selection pressures on a mimic’s size either side of the model’s phenotype are not symmetrical, (b) the optimal body size for a mimic is not necessarily equal to that of the model, and/or (c) for predators, attacking better mimics of a model’s body size more readily is adaptive. I discuss promising avenues for improving our understanding of body size as a mimetic trait, including the importance of treatments that range in both directions from the model’s size. Further work is required to understand how body size ranks in saliency against other mimetic traits such as pattern. Comparative studies could investigate whether mimics are limited to resembling only models that are already similar in size.

Phyllidiidae (Nudibranchia, Heterobranchia, Gastropoda): an integrative taxonomic approach including chemical analyses

Members of the widely distributed and common nudibranch family Phyllidiidae are often easily spotted in the marine environment because of their conspicuous colours and obvious presence on the reef. They are interesting with regard to their defensive chemical compounds that may lead to new drug discoveries. Despite their abundance, the family is also well known for its taxonomic problems and the difficulties in species identification due to very similarly coloured species and lack of morphological characters. In this study, phyllidiid species were analysed using an integrative approach. Molecular analysis of the mitochondrial genes 16S and CO1 was utilised, running phylogenetic analyses, species delimitation tests, and haplotype network analyses. Additionally, for the first time, external morphological characters were analysed, museum material was re-analysed, and chemical profiles were applied for characterising species. The analyses are based on sequences of 598 specimens collected in Indonesia by the team, with the addition of published sequences available on GenBank. This study comprises 11 species of Phyllidia, seven species of Phyllidiopsis, and at least 14 species of Phyllidiella. Moreover, 11 species belonging to these three genera are probably new to science, Phyllidiopsis pipeki is synonymised with P. krempfi, and Phyllidiella albonigra is resurrected. Some of the external colouration previously used for species identification is shown to not be valid, but alternative characters are provided for most species. Chemical analyses led to species characterisation in a few examples, indicating that these species use particular sponge species as food; however, many species show a broad array of compounds and are therefore characterised more by their composition or profile than by distinct or unique compounds.

A molecular phylogeny of Thuridilla Bergh, 1872 sea slugs (Gastropoda, Sacoglossa) reveals a case of flamboyant and cryptic radiation in the marine realm

The genus Thuridilla Bergh, 1872 comprises mostly tropical sap-sucking sea slugs species with flamboyantly coloured forms. However, the potential for cryptic or pseudocryptic species masked by convergent or polymorphic colour patterns has not been tested using molecular characters. In this study, we sampled 20 of the 23 recognized worldwide species and performed the most comprehensive molecular phylogenetic analysis of the genus to date using a multi-locus approach combining two mitochondrial (cytochrome c oxidase subunit I, 16S rRNA) and two nuclear (Histone H3, 28S rRNA) genes using maximum likelihood, maximum-parsimony and Bayesian criteria. Three molecular species delimitation methods (ABGD, GMYC, bPTP) and the morphology of radular teeth were additionally used to aid in species delimitation. Our analyses supported 35 species within Thuridilla, of which more than one-third (13) are part of a single radiation here named the Thuridilla gracilis (Risbec, 1928) species-complex. This complex includes T. gracilis, T. splendens (Baba, 1949), T. bayeri (Er. Marcus, 1965), and T. ratna (Er. Marcus, 1965), plus nine additional undescribed species. All 13 species are distinguishable by radular characters, external morphology and their DNA. The detection of this radiation led diversity of Thuridilla to be underestimated by about 25% and provides a new comparative system for studying the role of colour patterns in marine diversification.

Metabolome of the Phyllidiella pustulosa Species Complex (Nudibranchia, Heterobranchia, Gastropoda) Reveals Rare Dichloroimidic Sesquiterpene Derivatives from a Phylogenetically Distinct and Undescribed Clade

Phyllidiid nudibranchs are brightly colored gastropod mollusks, frequently encountered in coral reefs of the tropical Indo-Pacific. The lack of a protective shell is suggested to be compensated by toxic secondary metabolites that are sequestered from specific prey sponges. Our ongoing reconstruction of phyllidiid phylogeny using molecular data of more than 700 specimens, based on published data and newly collected specimens in various seasons and localities around North Sulawesi (Indonesia), demonstrates that Phyllidiella pustulosa is a species complex with at least seven well-supported clades. A metabolomic analysis of 52 specimens from all seven clades of P. pustulosa was performed. Secondary metabolite profiles were found to correlate with the phylogenetic study and not the prevailing food sponges as expected. GNPS molecular networking revealed a unique chemotype in clade 6. Detailed chemical analysis of a specimen from this chemically and genetically distinct P. pustulosa clade led to the identification of seven new sesquiterpenoids with a rare dichloroimidic moiety (1 and 4) and derivatives thereof (2, 3, 5-7). Our findings suggest that P. pustulosa clades should be raised to the species level.

Warning signals promote morphological diversification in fossorial uropeltid snakes (Squamata: Uropeltidae)

Many species possess warning colourations that signal unprofitability to predators. Warning colourations are also thought to provide prey with a ‘predator-free space’ and promote niche expansion. However, how such strategies release a species from environmental constraints and facilitate niche expansion is not clearly understood. Fossoriality in reptiles imposes several morphological limits on head and body size to facilitate burrowing underground, but many fossorial snakes live close to the surface and occasionally move above ground, exposing them to predators. In such cases, evolving antipredator defences that reduce predation on the surface could potentially relax the morphological constraints associated with fossoriality and promote morphological diversification. Fossorial uropeltid snakes possess varying degrees of conspicuous warning colourations that reduce avian predation when active above ground. We predicted that species with more conspicuous colourations will exhibit more robust body forms and show faster rates of morphological evolution because constraints imposed by fossoriality are relaxed. Using a comparative phylogenetic approach on the genus Uropeltis, we show that more conspicuous species tend to have more robust morphologies and have faster rates of head-shape evolution. Overall, we find that the evolution of warning colourations in Uropeltis can facilitate niche expansion by influencing rates of morphological diversification.

Differences in signal contrast and camouflage among different colour variations of a stomatopod crustacean, Neogonodactylus oerstedii

Animal colouration is often a trade-off between background matching for camouflage from predators, and conspicuousness for communication with con- or heterospecifics. Stomatopods are marine crustaceans known to use colour signals during courtship and contests, while their overall body colouration may provide camouflage. However, we have little understanding of how stomatopods perceive these signals in their environment or whether overall body coloration does provide camouflage from predators. Neogonodactylus oerstedii assess meral spot colour during contests, and meral spot colour varies depending on local habitat. By calculating quantum catch for N. oerstedii's 12 photoreceptors associated with chromatic vision, we found that variation in meral spot total reflectance does not function to increase signal contrast in the local habitat. Neogonodactylus oerstedii also show between-habitat variation in dorsal body colouration. We used visual models to predict a trichromatic fish predator's perception of these colour variations. Our results suggest that sandy and green stomatopods are camouflaged from a typical fish predator in rubble fields and seagrass beds, respectively. To our knowledge, this is the first study to investigate signal contrast and camouflage in a stomatopod. These results provide new insight into the function and evolution of colouration in a species with a complex visual system.

From cryptic to colorful: Evolutionary decoupling of larval and adult color in butterflies

Many animals undergo complete metamorphosis, where larval forms change abruptly in adulthood. Color change during ontogeny is common, but there is little understanding of evolutionary patterns in these changes. Here, we use data on larval and adult color for 246 butterfly species (61% of all species in Australia) to test whether the evolution of color is coupled between life stages. We show that adults are more variable in color across species than caterpillars and that male adult color has lower phylogenetic signal. These results suggest that sexual selection is driving color diversity in male adult butterflies at a broad scale. Moreover, color similarities between species at the larval stage do not predict color similarities at the adult stage, indicating that color evolution is decoupled between young and adult forms. Most species transition from cryptic coloration as caterpillars to conspicuous coloration as adults, but even species with conspicuous caterpillars change to different conspicuous colors as adults. The use of high‐contrast coloration is correlated with body size in caterpillars but not adults. Taken together, our results suggest a change in the relative importance of different selective pressures at different life stages, resulting in the evolutionary decoupling of coloration through ontogeny.

Colours and colour vision in reef fishes: Past, present and future research directions

Many fishes, both freshwater or marine, have colour vision that may outperform humans. As a result, to understand the behavioural tasks that vision enables; including mate choice, feeding, agonistic behaviour and camouflage, we need to see the world through a fish's eye. This includes quantifying the variable light environment underwater and its various influences on vision. As well as rapid loss of light with depth, light attenuation underwater limits visual interaction to metres at most and in many instances, less than a metre. We also need to characterize visual sensitivities, fish colours and behaviours relative to both these factors. An increasingly large set of techniques over the past few years, including improved photography, submersible spectrophotometers and genetic sequencing, have taken us from intelligent guesswork to something closer to sensible hypotheses. This contribution to the special edition on the Ecology of Fish Senses under a shifting environment first reviews our knowledge of fish colour vision and visual ecology, past, present and very recent, and then goes on to examine how climate change may impinge on fish visual capability. The review is limited to mostly colour vision and to mostly reef fishes. This ignores a large body of work, both from other marine environments and freshwater systems, but the reef contains examples of many of the challenges to vision from the aquatic environment. It is also a concentrate of life, perhaps the most specious and complex on earth, suffering now catastrophically from the consequences of our lack of action on climate change. A clear course of action to prevent destruction of this habitat is the need to spend more time in it, in the study of it and sharing it with those not fortunate enough to see coral reefs first-hand. Sir David Attenborough on The Great Barrier Reef: "Do we really care so little about the Earth upon which we live that we don't wish to protect one of its greatest wonders from the consequences of our behaviours?"

Phylogenetic comparative analysis supports aposematic colouration-body size association in millipede assassins (Hemiptera: Reduviidae: Ectrichodiinae)

The diversity of colour patterns and its importance in interactions with the environment make colouration in animals an intriguing research focus. Aposematic colouration is positively correlated with body size in certain groups of animals, suggesting that warning colours are more effective or that crypsis is harder to achieve in larger animals. Surprisingly, this relationship has not been recovered in studies investigating insects, which may have been confounded by a focus on aposematic taxa that are also gregarious. Millipede assassin bugs (Hemiptera: Reduviidae: Ectrichodiinae) comprise species with cryptic and aposematic colour patterns across a range of body sizes, are typically solitary as adults and are thus an excellent model for investigating a possible association between colouration and body size. Here, we use a comprehensive phylogeny for Ectrichodiinae, ancestral state reconstruction of colouration, and phylogenetic comparative methods to test for a colouration-body size association. The ancestor of Ectrichodiinae is reconstructed as cryptically coloured, with multiple subsequent transitions between aposematic and cryptic colouration. Aposematic colouration is positively associated with male body length and supports the hypothesis that selection on Ectrichodiinae body size may influence evolutionary transitions between aposematic and cryptic colouration or alternatively that selection for aposematic colouration influences body size evolution.

Color Patterning in Hard Ticks (Acari: Ixodidae)

Among the hard ticks (Acari: Ixodidae), many species in the section Metastriata have intricate ornamentation on the scutum that is often used as a taxonomic character. However, the biological function(s) of this ornamentation remains unknown. Here, we summarize the main functions of color patterns recognized in the animal kingdom-thermoregulation, aposematism, camouflage, aggregation, mate recognition, and sexual signaling-and evaluate the potential of each of these to explain ornamentation in hard ticks. We also note the challenges and uncertainties involved in interpreting ornamentation in ticks as well as potential approaches for future research.

Systematics of the genus Halgerda Bergh, 1880 (Heterobranchia : Nudibranchia) of Mozambique with descriptions of six new species

The species of the genus Halgerda Bergh, 1880, are restricted to the Indo-Pacific; some being common inhabitants of reefs off the coast of Mozambique. These species have been relatively well studied morphologically, but few molecular data are available. During a seven-year period surveying the reefs of Mozambique, 11 Halgerda spp. were collected, six of which are described here. We provide details on their morphology, anatomy, novel genetic markers and additional information about their colour variation. The new species described herein are Halgerda leopardalis, sp. nov., H. mozambiquensis, sp. nov., H. jennyae, sp. nov., H. meringuecitrea, sp. nov., H. nuarroensis, sp. nov. and H. indotessellata, sp. nov., the last of which was found to be a pseudocryptic species of H. tessellata. Moreover, we identified two species complexes, one composed mainly of specimens from the Western Indian Ocean and another with specimens mostly from the Pacific Ocean and Western Australia.

Disruptive colouration in reef fish: does matching the background reduce predation risk?

Animals use disruptive colouration to prevent detection or recognition by potential predators or prey. Highly contrasting elements within colour patterns, including vertical or horizontal bars, are thought to be effective at distracting attention away from body form and reducing detection likelihood. However, it is unclear whether such patterns need to be a good match to the spatial characteristics of the background to gain cryptic benefits. We tested this hypothesis using the iconic vertically barred humbug damselfish, Dascyllus aruanus (Linneaus 1758), a small reef fish that lives among the finger-like projections of branching coral colonies. Using behavioural experiments, we demonstrated that the spatial frequency of the humbug pattern does not need to exactly match the spatial frequency of the coral background to reduce the likelihood of being attacked by two typical reef fish predators: slingjaw wrasse, Epibulus insidiator (Pallas 1770), and coral trout, Plectropomus leopardus (Lacépède 1802). Indeed, backgrounds with a slightly higher spatial frequency than the humbug body pattern provided more protection from predation than well-matched backgrounds. These results were consistent for both predator species, despite differences in their mode of foraging and visual acuity, which was measured using anatomical techniques. We also showed that a slight mismatch in the orientation of the vertical bars did not increase the chances of detection. However, the likelihood of attack did increase significantly when the bars were perpendicular to the background. Our results provide evidence that fish camouflage is more complex than it initially appears, with likely many factors influencing the detection likelihood of prey by relevant predators.

Phylogenetic relationships within the Phyllidiidae (Opisthobranchia, Nudibranchia)

The Phyllidiidae (Gastropoda, Heterobranchia, Nudibranchia) is a family of colourful nudibranchs found on Indo-Pacific coral reefs. Despite the abundant and widespread occurrence of many species, their phylogenetic relationships are not well known. The present study is the first contribution to fill the gap in our knowledge on their phylogeny by combining morphological and molecular data. For that purpose 99 specimens belonging to 16 species were collected at two localities in Indonesia. They were photographed and used to make a phylogeny reconstruction based on newly obtained

cytochrome oxidase subunit

(COI) sequences as well as sequence data from GenBank. All mitochondrial 16S sequence data available from GenBank were used in a separate phylogeny reconstruction to obtain information for species we did not collect. COI data allowed the distinction of the genera and species, whereas the 16S data gave a mixed result with respect to the genera Phyllidia and Phyllidiella. Specimens which could be ascribed to species level based on their external morphology and colour patterns showed low variation in COI sequences, but there were two exceptions: three specimens identified as Phyllidiacf.babai represent two to three different species, while Phyllidiellapustulosa showed highly supported subclades. The barcoding marker COI also confirms that the species boundaries in morphologically highly variable species such as Phyllidiaelegans, Phyllidiavaricosa, and Phyllidiopsiskrempfi, are correct as presently understood. In the COI as well as the 16S cladogram Phyllidiopsiscardinalis was located separately from all other Phyllidiidae, whereas Phyllidiopsisfissuratus was positioned alone from the Phyllidiella species by COI data only. Future studies on phyllidiid systematics should continue to combine morphological information with DNA sequences to obtain a clearer insight in their phylogeny.

Choose Your Weaponry: Selective Storage of a Single Toxic Compound, Latrunculin A, by Closely Related Nudibranch Molluscs

Natural products play an invaluable role as a starting point in the drug discovery process, and plants and animals use many interesting biologically active natural products as a chemical defense mechanism against predators. Among marine organisms, many nudibranch gastropods are known to derive defensive metabolites from the sponges they eat. Here we investigated the putative sequestration of the toxic compound latrunculin A--a 16-membered macrolide that prevents actin polymerization within cellular processes--which has been identified from sponge sources, by five closely related nudibranch molluscs of the genus Chromodoris. Only latrunculin A was present in the rim of the mantle of these species, where storage reservoirs containing secondary metabolites are located, whilst a variety of secondary metabolites were found in their viscera. The species studied thus selectively accumulate latrunculin A in the part of the mantle that is more exposed to potential predators. This study also demonstrates that latrunculin-containing sponges are not their sole food source. Latrunculin A was found to be several times more potent than other compounds present in these species of nudibranchs when tested by in vitro and in vivo toxicity assays. Anti-feedant assays also indicated that latrunculin A was unpalatable to rock pool shrimps, in a dose-dependent manner. These findings led us to propose that this group of nudibranchs has evolved means both to protect themselves from the toxicity of latrunculin A, and to accumulate this compound in the mantle rim for defensive purposes. The precise mechanism by which the nudibranchs sequester such a potent compound from sponges without disrupting their own key physiological processes is unclear, but this work paves the way for future studies in this direction. Finally, the possible occurrence of both visual and chemosensory Müllerian mimicry in the studied species is discussed.

From crypsis to mimicry: changes in colour and the configuration of the visual system during ontogenetic habitat transitions in a coral reef fish

Animals often change their habitat throughout ontogeny; yet, the triggers for habitat transitions and how these correlate with developmental changes – e.g. physiological, morphological, and behavioural – remain largely unknown. Here, we investigated how ontogenetic changes in body colouration and of the visual system relate to habitat transitions in a coral-reef fish. Adult dusky dottybacks, Pseudochromis fuscus, are aggressive mimics that change colour to imitate various fishes in their surroundings; however, little is known about the early life stages of this fish. Using a developmental time-series in combination with the examination of wild caught specimens we uncover that dottybacks change colour twice during development: (i) nearly translucent cryptic pelagic larvae change to a grey camouflage colouration when settling on coral reefs; and (ii) juveniles change to mimic yellow or brown coloured fishes when reaching a size capable of consuming juvenile fish prey. Moreover, microspectrophotometric (MSP) and quantitative real time PCR (qRT-PCR) experiments show developmental changes of the dottyback visual system, including the use of a novel adult specific visual gene (RH2 opsin). This gene is likely to be coexpressed with other visual pigments to form broad spectral sensitivities that cover the medium-wavelength part of the visible spectrum. Surprisingly, the visual modifications precede changes in habitat and colour, possibly because dottybacks need to first acquire the appropriate visual performance before transitioning into novel life stages.