Algivore or phototroph? Plakobranchus ocellatus (Gastropoda) continuously acquires kleptoplasts and nutrition from multiple algal species in nature

Habitat shapes the lipidome of the tropical photosynthetic sea slug Elysia crispata

Sacoglossan sea slugs have attracted considerable scientific attention due to their capacity to retain functional macroalgal chloroplasts inside their cells. This endosymbiotic association is nutritionally relevant for these organisms and represents an interesting research issue for biotechnological applications. The Caribbean species Elysia crispata can integrate chloroplasts from different macroalgal species. The lipidome of chloroplasts includes lipid classes unique to these photosynthetic organelles. Specialized lipids, such as the glycolipids MGDG, DGDG, and SQDG, are essential for maintaining the integrity of both the thylakoid membranes and the overall chloroplast membrane structure. Additionally, lipids are a diverse group of biomolecules playing essential roles at nutritional and physiological levels. A combined approach using LC-HR-MS and MS/MS was employed to determine the polar lipid profile of the photosynthetic sea slug E. crispata from two habitats in the north-western tropical Atlantic (Sistema Arrecifal Veracruzano and Mahahual) and two different feeding conditions (fed and after 1 week of starvation). Significant differences were identified in the abundance of structural and signalling phospholipids (PC, PI, PG, PS, CL) suggesting different nutritional states between populations. The composition of glycolipids demonstrated a clear separation by habitat, but not by feeding conditions. The lower abundance of glycolipids in the Mahahual samples suggests a lower density of chloroplasts in their tissues compared to Veracruz individuals. These results corroborate that 1 week of starvation is insufficient to initiate the degradation of plastid membranes. This study confirms the advantages of using lipidomics as a tool to enhance our knowledge of the ecology of marine invertebrates.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-025-00281-1.

Optical Properties of Body Mucus Secreted from Coral Reef Sea Slugs: Measurement of Refractive Indices and Relative Absorption Spectra

Sea slugs are always covered in a mucus layer that has various functions including chemical defense that often involves aposematism and mimicry. Therefore, it is necessary for sea slugs to exhibit their body colors and patterns exactly, and the optical properties of mucus should support this requirement. We examined body mucus from heterobranch sea slugs collected in the Okinawan coral reefs. The refractive indices of mucus from 32 species ranged from 1.3371 to 1.3854 and were similar or slightly greater than the refractive index of seawater (ca. 1.34), indicating that light reflectance on the mucus layer is generally small. Moreover, dissolution of mucus into seawater would form a gradient of refractive indices and enhance the reduction of reflectance. We also obtained relative absorption spectra of the mucus from 32 species. In the range of visible light, absorption spectra of mucus suggest that the mucus layer is almost transparent and is not likely to interfere with the body colors. The presence of absorption peaks and/or shoulders in the UV (ultraviolet) range (280-400 nm) indicates that the mucus layer potentially serves as a sunscreen that absorbs UV radiation in 23 species, whereas prominent UV absorption was not found in the other 9 species. In a kleptoplasty sacoglossan Plakobranchus ocellatus, the refractive indices and presence or absence of UV-absorption showed that the optical properties of the mucus varied to some extent but did not show seasonal fluctuation. The UV-absorption in the mucus may also protect kleptoplasts in kleptoplasty sacoglossans. The present results support the importance of mucus as a functional optical layer for the shell-less life of sea slugs.

A novel kleptoplastidic symbiosis revealed in the marine centrohelid Meringosphaera with evidence of genetic integration

Plastid symbioses between heterotrophic hosts and algae are widespread and abundant in surface oceans. They are critically important both for extant ecological systems and for understanding the evolution of plastids. Kleptoplastidy, where the plastids of prey are temporarily retained and continuously re-acquired, provides opportunities to study the transitional states of plastid establishment. Here, we investigated the poorly studied marine centrohelid Meringosphaera and its previously unidentified symbionts using culture-independent methods from environmental samples. Investigations of the 18S rDNA from single-cell assembled genomes (SAGs) revealed uncharacterized genetic diversity within Meringosphaera that likely represents multiple species. We found that Meringosphaera harbors plastids of Dictyochophyceae origin (stramenopiles), for which we recovered six full plastid genomes and found evidence of two distinct subgroups that are congruent with host identity. Environmental monitoring by qPCR and catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) revealed seasonal dynamics of both host and plastid. In particular, we did not detect the plastids for 6 months of the year, which, combined with the lack of plastids in some SAGs, suggests that the plastids are temporary and the relationship is kleptoplastidic. Importantly, we found evidence of genetic integration of the kleptoplasts as we identified host-encoded plastid-associated genes, with evolutionary origins likely from the plastid source as well as from other alga sources. This is only the second case where host-encoded kleptoplast-targeted genes have been predicted in an ancestrally plastid-lacking group. Our results provide evidence for gene transfers and protein re-targeting as relatively early events in the evolution of plastid symbioses.

mTORC1 regulates phagosome digestion of symbiotic bacteria for intracellular nutritional symbiosis in a deep-sea mussel

Phagocytosis is one of the methods used to acquire symbiotic bacteria to establish intracellular symbiosis. A deep-sea mussel, Bathymodiolus japonicus, acquires its symbiont from the environment by phagocytosis of gill epithelial cells and receives nutrients from them. However, the manner by which mussels retain the symbiont without phagosome digestion remains unknown. Here, we show that controlling the mechanistic target of rapamycin complex 1 (mTORC1) in mussels leads to retaining symbionts in gill cells. The symbiont is essential for the host mussel nutrition; however, depleting the symbiont's energy source triggers the phagosome digestion of symbionts. Meanwhile, the inhibition of mTORC1 by rapamycin prevented the digestion of the resident symbionts and of the engulfed exogenous dead symbionts in gill cells. This indicates that mTORC1 promotes phagosome digestion of symbionts under reduced nutrient supply from the symbiont. The regulation mechanism of phagosome digestion by mTORC1 through nutrient signaling with symbionts is key for maintaining animal-microbe intracellular nutritional symbiosis.

A new era of isotope ecology: Nitrogen isotope ratio of amino acids as an approach for unraveling modern and ancient food web

Food web research is rapidly expanding through study of natural fractional abundance of 15N in individual amino acids. This paper overviews the principles of this isotope approach, and from my perspective, reanalyzes applications, and further extends the discussion. It applies kinetic isotope effects that enriches 15N in certain amino acids associated with the metabolic processes, which was clearly demonstrated by observations of both natural ecosystem and laboratory experiments. In trophic processes 'trophic amino acids' such as glutamic acid that significantly enrich 15N, whereas 'source amino acids' such as phenylalanine and methionine show little 15N enrichment. Through various applications conducted over the years, the principles of the method have shown to operate well and disentangle complex food webs and relevant problems. Applications include food chain length estimate, nitrogen resource assessment, tracking fish migration, and reconstruction of paleodiet. With this approach, considerations of a wide range of classical issues have been reinvigorated, while in the same time, new challenging frontiers are emerging.

Kleptoplasty: Getting away with stolen chloroplasts

Kleptoplasty, the process by which a host organism sequesters and retains algal chloroplasts, is relatively common in protists. The origin of the plastid varies, as do the length of time it is retained in the host and the functionality of the association. In metazoa, the capacity for long-term (several weeks to months) maintenance of photosynthetically active chloroplasts is a unique characteristic of a handful of sacoglossan sea slugs. This capability has earned these slugs the epithets "crawling leaves" and "solar-powered sea slugs." This Unsolved Mystery explores the basis of chloroplast maintenance and function and attempts to clarify contradictory results in the published literature. We address some of the mysteries of this remarkable association. Why are functional chloroplasts retained? And how is the function of stolen chloroplasts maintained without the support of the algal nucleus?

Sea Slug Mucus Production Is Supported by Photosynthesis of Stolen Chloroplasts

A handful of sea slugs of the order Sacoglossa are able to steal chloroplasts-kleptoplasts-from their algal food sources and maintain them functionally for periods ranging from several weeks to a few months. In this study, we investigated the role of kleptoplast photosynthesis on mucus production by the tropical sea slug Elysia crispata. Animals reared for 5 weeks in quasi dark (5 μmol photons m^-2 s^-1) showed similar growth to those under regular light (60-90 μmol photons m^-2 s^-1), showing that kleptoplast photosynthesis was not relevant for growth when sea slugs were fed ad libitum. However, when subjected to short-term desiccation stress, animals reared under regular light produced significantly more mucus. Furthermore, the carbohydrate content of secreted mucus was significantly lower in slugs limited in the photosynthetic activity of their kleptoplasts by quasi-dark conditions. This study indicates that photosynthesis supports the synthesis of protective mucus in kleptoplast-bearing sea slugs.

Chemoautotrophy, symbiosis and sedimented diatoms support high biomass of benthic molluscs in the Namibian shelf

The molluscs Lucinoma capensis, Lembulus bicuspidatus and Nassarius vinctus are highly abundant in Namibian oxygen minimum zone sediments. To understand which nutritional strategies allow them to reach such impressive abundances in this extreme habitat we investigated their trophic diversity, including a chemosymbiosis in L. capensis, focussing on nitrogen biochemical pathways of the symbionts. We combined results of bulk nitrogen and carbon (δ13C and δ15N) and of compound-specific isotope analyses of amino acid nitrogen (AAs-δ15NPhe and δ15NGlu), with 16S rRNA gene sequencing of L. capensis tissues and also with exploratory results of ammonium, nitrate and nitrite turnover. The trophic position (TP) of the bivalve L. capensis is placed between autotrophy and mixotrophy, consistent with its proposed symbiosis with sulfur-oxidizing Candidatus Thiodiazotropha sp. symbionts. The symbionts are here revealed to perform nitrate reduction and ammonium uptake, with clear indications of ammonium host-symbionts recycling, but surprisingly unable to fix nitrogen. The TP of the bivalve L. bicuspidatus is placed in between mixotrophy and herbivory. The TP of the gastropod N. vinctus reflected omnivory. Multiple lines of evidences in combination with current ecosystem knowledge point to sedimented diatoms as important components of L. bicuspidatus and N. vinctus' diet, likely supplemented at times with chemoautotrophic bacteria. This study highlights the importance of benthic-pelagic coupling that fosters the dietary base for macrozoobenthos in the OMZ. It further unveils that, in contrast to all shallow water lucinid symbionts, deeper water lucinid symbionts rely on ammonium assimilation rather than dinitrogen fixation to obtain nitrogen for growth.

Seasonality and Longevity of the Functional Chloroplasts Retained by the Sacoglossan Sea Slug Plakobranchus ocellatus van Hasselt, 1824 Inhabiting A Subtropical Back Reef Off Okinawa-jima Island, Japan

Plakobranchus ocellatus is a sacoglossan sea slug that feeds on multiple algal species and retains chloroplasts as kleptoplasts for several months. The seasonal differences in the photosynthetic properties of kleptoplasts were examined in sacoglossans collected from a subtropical back reef off of Okinawa-jima (26°21'55"N 127°44'10"E) in 2017-2018. The effective quantum yield of photosystem II in kleptoplasts indicated that stronger ambient light causes more stress in kleptoplasts. The maximum quantum yields (QY) at 20°C, 30°C, and 40°C indicated that kleptoplasts were more functional in photosynthesis in winter than in spring or summer, whereas kleptoplasts may have the highest tolerance to high temperatures in summer. In the long-starvation experiment (LSE), the relative ratio of body weight (relW) linearly decreased and the sacoglossans died within 2 months in the total dark condition, whereas in the LSE with illumination, the animals survived up to 5 months. The time course for the decrease in the relative ratio of the QY (relQY) in the LSE indicated that the photosynthetic function was almost normal for 2 months, regardless of the presence or absence of illumination, after which time relQY gradually decreased to zero. In the field, P. ocellatus continuously took up new kleptoplasts that have suitable properties of photosynthetic ability for each season. In a subtropical environment, in which water temperatures vary from below 20°C to above 30°C, seasonal changes could cause a temporary shortage of algal food and affect the photosynthetic activity of P. ocellatus kleptoplast. Our results, however, indicated the kleptoplasts of P. ocellatus functioned normally for several months and maintained the presence of this sacoglossan in a subtropical environment throughout the year.

An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters

Improved access to field survey infrastructure throughout South-East Asia has allowed for a greater intensity of biodiversity surveys than ever before. The rocky bottoms and coral reef habitats across the region have been shown to support some of the highest sea slug biodiversity on the planet, with ever increasing records. During the past ten years, intensive SCUBA surveys have been carried out at Koh Tao, in the Gulf of Thailand, which have yielded remarkable findings in sea slug biology and ecology. In this work a brief history of sea slug biodiversity research from Thailand is covered and a complete inventory of sea slugs from Koh Tao, Gulf of Thailand is provided. This inventory is based on surveys from 2012 to 2020, with previously unreported findings since 2016. Habitat specificity and species-specific ecology are reported where available with a focused comparison of coral reef habitats and deeper soft-sediment habitats. The findings contribute 90 new species records for Thai waters (92 for the Gulf of Thailand) and report a remarkable consistency in the proportional diversity found to be exclusive to one habitat type or another. Additionally, taxonomic remarks are provided for species documented from Koh Tao that have not been discussed in past literature from Thailand, and a summary of previous records in the Indo-West Pacific is given.

Chloroplast acquisition without the gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus

Some sea slugs sequester chloroplasts from algal food in their intestinal cells and photosynthesize for months. This phenomenon, kleptoplasty, poses a question of how the chloroplast retains its activity without the algal nucleus. There have been debates on the horizontal transfer of algal genes to the animal nucleus. To settle the arguments, this study reported the genome of a kleptoplastic sea slug, Plakobranchus ocellatus, and found no evidence of photosynthetic genes encoded on the nucleus. Nevertheless, it was confirmed that light illumination prolongs the life of mollusk under starvation. These data presented a paradigm that a complex adaptive trait, as typified by photosynthesis, can be transferred between eukaryotic kingdoms by a unique organelle transmission without nuclear gene transfer. Our phylogenomic analysis showed that genes for proteolysis and immunity undergo gene expansion and are up-regulated in chloroplast-enriched tissue, suggesting that these molluskan genes are involved in the phenotype acquisition without horizontal gene transfer.

On the Plakobranchidae (Gastropoda, Sacoglossa) from soft sediment habitats of Koh Tao, Gulf of Thailand, with descriptions of two new species

Research in recent years have provided rapid advances in biogeographic and taxonomic documentation of sea slugs around the world. However, efforts are lacking in surveying most coastlines and habitats in South-East Asia. Recent studies from the Gulf of Thailand have indicated that a wealth of unexplored sea slug diversity and ecology may be gained from an investigation of soft sediment habitats beyond the reef slopes. Additionally, the waters of Koh Tao have been found to host regionally high levels of sea slug diversity with several species awaiting taxonomic clarification. In this work the initial findings of an expanded survey effort from the waters around Koh Tao are provided, with the identity of two soft sediment-associated sacoglossan species in the family Plakobranchidae being investigated. By integrating morphological and molecular analyses, the species Plakobranchusnoctisstellatussp. nov. and Elysiaaowthaisp. nov. are described and species complexes surrounding Plakobranchusocellatus van Hasselt, 1824 and Elysiajaponica Eliot, 1913 are discussed. The topics of morphological variability and the cryptic species problem are also discussed.

Finding the Sweet Spot: Sub-Ambient Light Increases Fitness and Kleptoplast Survival in the Sea Slug Plakobranchus cf. ianthobaptus Gould, 1852

Sacoglossans, or "sap-sucking" sea slugs, are primarily algivorous, with many taxa exhibiting kleptoplasty, the feeding and retaining of photosynthetically active chloroplasts from algae. The Plakobranchus species complex exhibits some of the longest kleptoplast retention and survival times under starvation conditions, but the contributions of these kleptoplasts to their survival and overall fitness have been widely debated. In this study we assessed the effects of starvation and light on the fitness of Plakobranchus cf. ianthobaptus and its kleptoplasts by placing starved individuals in eight daily average light treatments, ranging from near dark (2 µmol photon m^-2 s^-1) to ambient light (470 µmol photon m^-2 s^-1). Slug weight was used as a metric of fitness, and kleptoplast photosynthetic activity was determined via maximum quantum yield (F_v/F_m) by pulse-amplitude modulated fluorometry as a proxy for kleptoplast health. Plakobranchus individuals in near-dark and high light treatments (>160 µmol photon m^-2 s^-1) experienced significantly greater weight loss than those in low light (65 µmol photon m^-2 s^-1) and moderate light treatments (95-135 µmol photon m^-2 s^-1). Additionally, individuals in high light treatments experienced a rapid decline in kleptoplast photosynthetic activity, while all other treatments experienced minimal decline. This relationship between kleptoplast degradation and weight loss suggests an important link between fitness and kleptoplasty. Given the significant negative effects of ambient conditions, regular refreshment and replenishment of kleptoplasts or physiological or behavioral adjustments are likely employed for the benefits of kleptoplasty to be maintained.

Kleptoprotein bioluminescence: Parapriacanthus fish obtain luciferase from ostracod prey

Through their diet, animals can obtain substances essential for imparting special characteristics, such as toxins in monarch butterflies and luminescent substances in jellyfishes. These substances are typically small molecules because they are less likely to be digested and may be hard for the consumer to biosynthesize. Here, we report that Parapriacanthus ransonneti, a bioluminescent fish, obtains not only its luciferin but also its luciferase enzyme from bioluminescent ostracod prey. The enzyme purified from the fish's light organs was identical to the luciferase of Cypridina noctiluca, a bioluminescent ostracod that they feed upon. Experiments where fish were fed with a related ostracod, Vargula hilgendorfii, demonstrated the specific uptake of the luciferase to the fish's light organs. This "kleptoprotein" system allows an organism to use novel functional proteins that are not encoded in its genome and provides an evolutionary alternative to DNA-based molecular evolution.

Algal Sources of Sequestered Chloroplasts in the Sacoglossan Sea Slug Elysia crispata Vary by Location and Ecotype

Sacoglossan sea slugs feed by suctorially consuming siphonaceous green algae. Most sacoglossan species are feeding specialists, but the Caribbean coral reef-dwelling Elysia crispata is polyphagous and sequesters chloroplasts from multiple algal species into cells lining its digestive diverticulum for use in photosynthesis. We have used sequences of the chloroplast-encoded rbcL gene to compare the chloroplast donor algae in five populations of E. crispata from various Caribbean locations. We found that E. crispata utilizes more algal species than was previously known, including some algae previously not reported as present in the region. In addition, slugs from each location had unique chloroplast arrays with little overlap, except that all locations had slugs feeding on algae within the genus Bryopsis. This variation in diet between locations suggests that the slugs may be exhibiting local adaptation in their dietary choices, and it highlights ecological differences between the Caribbean-wide reef-dwelling ecotypes and the mangrove lagoon ecotypes found in the Florida Keys.

Compound-specific isotope analysis of benthic foraminifer amino acids suggests microhabitat variability in rocky-shore environments

The abundance and biomass of benthic foraminifera are high in intertidal rocky-shore habitats. However, the availability of food to support their high biomass has been poorly studied in these habitats compared to those at seafloor covered by sediments. Previous field and laboratory observations have suggested that there is diversity in the food preferences and modes of life among rocky-shore benthic foraminifera. In this study, we used the stable nitrogen isotopic composition of amino acids to estimate the trophic position, trophic niche, and feeding strategy of individual foraminifera species. We also characterized the configuration and structure of the endobiotic microalgae in foraminifera using transmission electron microscopy, and we identified the origin of endobionts based on nucleotide sequences. Our results demonstrated a large variation in the trophic positions of different foraminifera from the same habitat, a reflection of endobiotic features and the different modes of life and food preferences of the foraminifera. Foraminifera did not rely solely on exogenous food sources. Some species effectively used organic matter derived from endobionts in the cell cytoplasm. The high biomass and species density of benthic foraminifera found in intertidal rocky-shore habitats are thus probably maintained by the use of multiple nitrogen resources and by microhabitat segregation among species as a consequence.

Updating Plakobranchus cf. ianthobapsus (Gastropoda, Sacoglossa) host use: Diverse algal-animal interactions revealed by NGS with implications for invasive species management

Sacoglossa, the "sap sucking" sea slugs, are highly specialized herbivores and the only metazoans that exhibit kleptoplasty, the sequestration and retention of chloroplasts from algae. Plakobranchus is one of the most generalistic herbivores within this order, with as many as 12 reported "algal host" (i.e. kleptoplast source) species. However, kleptoplast diversity studies conducted on Plakobranchus to date most likely underestimated the full diversity of kleptoplast sources within the studied populations due to limitations of the molecular techniques employed. Here, we apply a high throughput sequencing technique to assess kleptoplast diversity of Plakobranchus cf. ianthobapsus' from 10 sites across the Main Hawaiian Islands during winter and summer seasons. In so doing, we effectively used P. cf. ianthobapsus as a novel sampling tool to explore diminutive algal communities, including the current distribution of the invasive alga "Avrainvillea amadelpha." Our results show that P. cf. ianthobapsus sequesters chloroplasts from 23 algal species from across the siphonous green algal order Bryopsidales. We identified "Avrainvillea amadelpha" and Codium edule as new host species for P. cf. ianthobapusus, but their rarity among the data suggests they were most likely less preferential as hosts and were possibly utilized due to low abundance or unavailability of more preferable species, and therefore a response to starvation risk. Additionally, the identification of the highly invasive siphonous green alga "A. amadelpha" as a kleptoplast source provides new fine-scale range and distribution data for this problematic species. Overall kleptoplast diversity does not differ among sites, except in a coral-dominated, (i.e. not algal dominated) environment, suggesting that siphonous algal assemblages are common in algal-dominated ecosystems in the Hawaiian Islands. Diversity dissimilarity among seasons was recovered from the majority of sites sampled, supporting the need for seasonal data collection in algal diversity assessments. This case study using metabarcoding of sacoglossan kleptoplasts provides deeper insights into these plant-animal interactions with a better understanding of host use than previous studies using traditional molecular methods and illustrates how algal diversity studies on the scale of plastids can have implications for understanding algal community structure and invasive species dynamics.

How does temperature affect functional kleptoplasty? Comparing populations of the solar-powered sister-species Elysia timida Risso, 1818 and Elysia cornigera Nuttall, 1989 (Gastropoda: Sacoglossa)

Background

Despite widespread interest in solar-powered sea slugs (Sacoglossa: Gastropoda), relatively little is know about how they actually perform functional kleptoplasty. Sister-taxa Elysia timida and E. cornigera provide an ideal model system for investigating this phenomenon, since they feed on the same algal genus and only E. timida is capable of long-term kleptoplasty. Recent research has explored factors regarding functional kleptoplasty in E. timida, including their starvation longevity, digestive activity, autophagal response and photosynthetic efficiency under two different temperature conditions (18 °C and 21 °C). These studies revealed the trends E. timida displays regarding each factor during starvation as well as influences temperature has on some aspects of functional kleptoplasty. This study examines E. cornigera regarding each of these factors in an attempt to elucidate differences between each species that could explain their differing kleptoplastic abilities. Since both species naturally occur in 25 °C seawater (E. timida peak summer temperature, E. cornigera low winter temperature), each species was acclimatized to 25 °C to facilitate comparison and determine if these species exhibit physiological differences to starvation when under the same environmental conditions.

Results

When comparing the different E. timida temperature treatments, it becomes clear that increased temperatures compromise E. timida’s kleptoplastic abilities. Specimens acclimatized to 25 °C revealed shorter starvation longevities surviving an average 42.4 days compared to the 95.9 day average observed in specimens exposed to 18 °C. Each temperature treatment displayed a significantly different decrease throughout the starvation period in both, the rate of photosynthetic efficiency and in the decreasing functional kleptoplast abundance. Lysosomal abundances are assessed here as indicators of different aspects of metabolic activity, which could be correlated to temperature. E. cornigera, also acclimatized to 25 °C did not display significantly similar patterns as any of the E. timida temperature treatments, having fewer incorporated kleptoplasts, a higher lysosomal response to starvation, a faster decrease in photosynthetic efficiency and a lower starvation longevity.

Conclusions

These results confirm that each species has different physiological reactions to starvation and kleptoplast retention, even under the same conditions. While temperature affects aspects of functional kleptoplasty, it is likely not responsible for the differences in kleptoplastic abilities seen in these species.

Electronic supplementary material

The online version of this article (10.1186/s12983-018-0264-y) contains supplementary material, which is available to authorized users.

Kleptoplast photosynthesis is nutritionally relevant in the sea slug Elysia viridis

Several sacoglossan sea slug species feed on macroalgae and incorporate chloroplasts into tubular cells of their digestive diverticula. We investigated the role of the “stolen” chloroplasts (kleptoplasts) in the nutrition of the sea slug Elysia viridis and assessed how their abundance, distribution and photosynthetic activity were affected by light and starvation. Elysia viridis individuals feeding on the macroalga Codium tomentosum were compared with starved specimens kept in dark and low light conditions. A combination of variable Chl a fluorescence and hyperspectral imaging, and HPLC pigment analysis was used to evaluate the spatial and temporal variability of photopigments and of the photosynthetic capacity of kleptoplasts. We show increased loss of weight and body length in dark-starved E. viridis as compared to low light-starved sea slugs. A more pronounced decrease in kleptoplast abundance and lower photosynthetic electron transport rates were observed in dark-starved sea slugs than in low light-starved animals. This study presents strong evidence of the importance of kleptoplast photosynthesis for the nutrition of E. viridis in periods of food scarcity. Deprived of photosynthates, E. viridis could accelerate the breakdown of kleptoplasts in the dark to satisfy its’ energy requirements.

Molecular determination of kleptoplast origins from the sea slug Plakobranchus ocellatus (Sacoglossa, Gastropoda) reveals cryptic bryopsidalean (Chlorophyta) diversity in the Hawaiian Islands

The sacoglossan sea slug species complex Plakobranchus ocellatus is a common algivore throughout the tropical Pacific, including the Hawaiian Islands. Plakobranchus ocellatus is kleptoplastic-it sequesters and retains algal chloroplasts-a characteristic that can be exploited to molecularly characterize diminutive bryopsidalean algae that are typically difficult to locate, collect, and identify. Previous DNA barcode analyses of both P. ocellatus and its kleptoplasts have been conducted primarily in the western Pacific and have only minimally sampled the most eastern populations in the Hawaiian Islands. Using two chloroplast markers, rbcL and tufA, kleptoplast samples from an Oahu population of P. ocellatus were amplified and cloned to identify their algal sources. Plakobranchus ocellatus sequester chloroplasts from up to 11 bryopsidalean algal species, all but one being diminutive in thallus size. Notably, eight of the detected algal species were new records to the Hawaiian Islands. A sequestration preference study demonstrated that the O'ahu population of P. ocellatus preferentially sequesters chloroplasts from diminutive, epilithic taxa. Using coxI barcoding of P. ocellatus, we showed the O'ahu population to be part of a clade that includes sequences from the neighboring island Maui, Australia, and the Philippines. The use of P. ocellatus as a novel sampling tool allows the exploration of the green algal community diversity and composition at a fine scale.

Population Dynamics of the Sea Slug Plakobranchus ocellatus (Opisthobranch: Sacoglossa: Elysioidea) on a Subtropical Coral Reef off Okinawa-jima Island, Ryukyu Archipelago, Japan

Daisuke Tanamura and Euichi Hirose (2016) Plakobranchus ocellatus is a sacoglossan sea slug that can retain functional chloroplast from its algal food. This species feed on multiple species of siphonous green algae and can survive several months without food by utilizing retained chloroplasts in its digestive gland (kleptoplasty). While the population dynamics of opisthobranchs are often influenced by the seasonal fluctuation of the abundance of food resources, the fluctuation of food availability would not be a crucial factor to restrict the occurrence of P. ocellatus. We monitored the population density of P. ocellatus for 20 months on a subtropical coral reef where the water temperature fluctuated from 17°C to 32°C, in order to examine whether the population density, distribution pattern of individuals, and size distribution of P. ocellatus are stable or seasonally change. The present results showed that P. ocellatus appeared all year round in the study site, while the population density changed seasonally. The population density decreased in cold (≤ 21°C) and hot (≥ 27°C) periods, and densities in the months of intermediate temperature range (< 21°C, > 25°C) were significantly higher than the densities in other months (Student's t-test, P < 0.0001). Accordingly, population density is probably influenced by water temperature. Morisita's Iδ indicated that the sea slugs were distributed in random patterns (13 months) or clumped patterns (7 months). Our field observations indicated that the sea slugs do not feed in daytime, and probably feed at night. Whereas P. ocellatus individuals of less than 10 mm were rarely recorded in the monitoring area, a decrease of the average body length and increase in population density in April - May suggest active recruitment of small individuals in this period.

Molecular trophic markers in marine food webs and their potential use for coral ecology

Notable advances in ecological genomics have been driven by high-throughput sequencing technology and taxonomically broad sequence repositories that allow us to accurately assess species interactions with great taxonomic resolution. The use of DNA as a marker for ingested food is particularly relevant to address predator-prey interactions and disentangle complex marine food webs. DNA-based methods benefit from reductionist molecular approaches to address ecosystem scale processes, such as community structure and energy flow across trophic levels, among others. Here we review how molecular trophic markers have been used to better understand trophic interactions in the marine environment and their advantages and limitations. We focus on animal groups where research has been focused, such as marine mammals, seabirds, fishes, pelagic invertebrates and benthic invertebrates, and use case studies to illustrate how DNA-based methods unraveled food-web interactions. The potential of molecular trophic markers for disentangling the complex trophic ecology of corals is also discussed.

Diet quality influences isotopic discrimination among amino acids in an aquatic vertebrate

Stable nitrogen isotopic composition of amino acids (δ (15)NAA) has recently been employed as a powerful tool in ecological food web studies, particularly for estimating the trophic position (TP) of animal species in food webs. However, the validity of these estimates depends on the consistency of the trophic discrimination factor (TDF; - Δδ (15)NAA at each shift of trophic level) among a suite of amino acids within the tissues of consumer species. In this study, we determined the TDF values of amino acids in tadpoles (the Japanese toad, Bufo japonicus) reared exclusively on one of three diets that differed in nutritional quality. The diets were commercial fish-food pellets (plant and animal biomass), bloodworms (animal biomass), and boiled white rice (plant carbohydrate), representing a balanced, protein-rich, and protein-poor diet, respectively. The TDF values of two "source amino acids" (Src-AAs), methionine and phenylalanine, were close to zero (0.3-0.5‰) among the three diets, typifying the values reported in the literature (∼0.5‰ and ∼0.4‰, respectively). However, TDF values of "trophic amino acids" (Tr-AAs) including alanine, valine, leucine, isoleucine, and glutamic acid varied by diet: for example, the glutamic acid TDF was similar to the standard value (∼8.0‰) when tadpoles were fed either the commercial pellets (8.0‰) or bloodworms (7.9‰), but when they were fed boiled rice, the TDF was significantly reduced (0.6‰). These results suggest that a profound lack of dietary protein may alter the TDF values of glutamic acid (and other Tr-AAs and glycine) within consumer species, but not the two Src-AAs (i.e., methionine and phenylalanine). Knowledge of how a nutritionally poor diet can influence the TDF of Tr- and Src-AAs will allow amino acid isotopic analyses to better estimate TP among free-roaming animals.

High-resolution food webs based on nitrogen isotopic composition of amino acids

Food webs are known to have myriad trophic links between resource and consumer species. While herbivores have well-understood trophic tendencies, the difficulties associated with characterizing the trophic positions of higher-order consumers have remained a major problem in food web ecology. To better understand trophic linkages in food webs, analysis of the stable nitrogen isotopic composition of amino acids has been introduced as a potential means of providing accurate trophic position estimates. In the present study, we employ this method to estimate the trophic positions of 200 free-roaming organisms, representing 39 species in coastal marine (a stony shore) and 38 species in terrestrial (a fruit farm) environments. Based on the trophic positions from the isotopic composition of amino acids, we are able to resolve the trophic structure of these complex food webs. Our approach reveals a high degree of trophic omnivory (i.e., noninteger trophic positions) among carnivorous species such as marine fish and terrestrial hornets.This information not only clarifies the trophic tendencies of species within their respective communities, but also suggests that trophic omnivory may be common in these webs.

Abundance and size distribution of the sacoglossan Elysia viridis on co-occurring algal hosts on the Swedish west coast

Sacoglossans are specialized marine herbivores that tend to have a close evolutionary relationship with their macroalgal hosts, but the widely distributed species Elysia viridis can associate with several algal species. However, most previous investigations on the field abundance and size distribution of E. viridis have focussed on Codium spp. in the British Isles, and algae from this genus are considered superior hosts for E. viridis. In the present study, we investigated the abundance and size distribution of E. viridis on 6 potential host algae with differing morphologies (the septate species Cladophora sericea, Cladophora rupestris, Chaetomorpha melagonium, and Ceramium virgatum, as well as the siphonaceous species Codium fragile and Bryopsis sp.) at 2 sites on the Swedish west coast over the course of a year. In spring, slugs were almost absent from all algal hosts. In summer and autumn, E. viridis consistently occurred on several of the algal species at both sites. The highest number of small E. viridis were found on C. sericea, intermediate numbers of significantly larger E. viridis were found on C. rupestris, while fewer, intermediate sized animals were found on C. fragile. Throughout the study period, only a few E. viridis individuals were found on C. melagonium, Bryopsis sp., and C. virgatum. Our results indicate that E. viridis is an annual species in Sweden, capable of exploiting co-occurring congeneric and intergeneric algal hosts with differing morphologies. These results corroborate previous findings that E. viridis can exploit several different algal species, but does not indicate that C. fragile is a superior host.

Photophysiology of kleptoplasts: photosynthetic use of light by chloroplasts living in animal cells

Kleptoplasty is a remarkable type of photosynthetic association, resulting from the maintenance of functional chloroplasts--the 'kleptoplasts'--in the tissues of a non-photosynthetic host. It represents a biologically unique condition for chloroplast and photosynthesis functioning, occurring in different phylogenetic lineages, namely dinoflagellates, ciliates, foraminiferans and, most interestingly, a single taxon of metazoans, the sacoglossan sea slugs. In the case of sea slugs, chloroplasts from macroalgae are often maintained as intracellular organelles in cells of these marine gastropods, structurally intact and photosynthetically competent for extended periods of time. Kleptoplasty has long attracted interest owing to the longevity of functional kleptoplasts in the absence of the original algal nucleus and the limited number of proteins encoded by the chloroplast genome. This review updates the state-of-the-art on kleptoplast photophysiology, focusing on the comparative analysis of the responses to light of the chloroplasts when in their original, macroalgal cells, and when sequestered in animal cells and functioning as kleptoplasts. It covers fundamental but ecologically relevant aspects of kleptoplast light responses, such as the occurrence of photoacclimation in hospite, operation of photoprotective processes and susceptibility to photoinhibition. Emphasis is given to host-mediated processes unique to kleptoplastic associations, reviewing current hypotheses on behavioural photoprotection and host-mediated enhancement of photosynthetic performance, and identifying current gaps in sacoglossan kleptoplast photophysiology research.

Identification of sequestered chloroplasts in photosynthetic and non-photosynthetic sacoglossan sea slugs (Mollusca, Gastropoda)

Background

Sacoglossan sea slugs are well known for their unique ability among metazoans to incorporate functional chloroplasts (kleptoplasty) in digestive glandular cells, enabling the slugs to use these as energy source when starved for weeks and months. However, members assigned to the shelled Oxynoacea and Limapontioidea (often with dorsal processes) are in general not able to keep the incorporated chloroplasts functional. Since obviously no algal genes are present within three (out of six known) species with chloroplast retention of several months, other factors enabling functional kleptoplasty have to be considered. Certainly, the origin of the chloroplasts is important, however, food source of most of the about 300 described species is not known so far. Therefore, a deduction of specific algal food source as a factor to perform functional kleptoplasty was still missing.

Results

We investigated the food sources of 26 sacoglossan species, freshly collected from the field, by applying the chloroplast marker genes tufA and rbcL and compared our results with literature data of species known for their retention capability. For the majority of the investigated species, especially for the genus Thuridilla, we were able to identify food sources for the first time. Furthermore, published data based on feeding observations were confirmed and enlarged by the molecular methods. We also found that certain chloroplasts are most likely essential for establishing functional kleptoplasty.

Conclusions

Applying DNA-Barcoding appeared to be very efficient and allowed a detailed insight into sacoglossan food sources. We favor rbcL for future analyses, but tufA might be used additionally in ambiguous cases. We narrowed down the algal species that seem to be essential for long-term-functional photosynthesis: Halimeda, Caulerpa, Penicillus, Avrainvillea, Acetabularia and Vaucheria. None of these were found in Thuridilla, the only plakobranchoidean genus without long-term retention forms. The chloroplast type, however, does not solely determine functional kleptoplasty; members of no-retention genera, such as Cylindrobulla or Volvatella, feed on the same algae as e.g., the long-term-retention forms Plakobranchus ocellatus or Elysia crispata, respectively. Evolutionary benefits of functional kleptoplasty are still questionable, since a polyphagous life style would render slugs more independent of specific food sources and their abundance.

Resolving phenotypic plasticity and species designation in the morphologically challenging Caulerpa racemosa-peltata complex (Chlorophyta, Caulerpaceae)

Although recent molecular studies have indicated the presence of a number of distinct species within the Caulerpa racemosa-peltata complex, due to the difficulties presented by high levels of phenotypic plasticity and the large number of synonyms, infra-specific taxa, and names of uncertain affinity, taxonomic proposals are yet to be made. In this study, we aimed to resolve the taxonomy of the complex and provide an example of how historical nomenclature can best be integrated into molecular based taxonomies. We accomplished this by first determining the number of genetic species within our globally sampled data set through a combination of phylogenetic and species-delimitation approaches of partial elongation factor TU and RUBISCO large subunit gene sequences. Guided by these results, comparative morphological examinations were then undertaken to gauge the extent of phenotypic plasticity within each species, as well as any morphological overlap between them. Our results revealed the presence of 11 distinct species within the complex, five of which showed high levels of phenotypic plasticity and partial overlap with other species. On the basis of observations of a large number of specimens, including type specimens/descriptions, and geographic inferences, we were able to confidently designate names for the lineages. Caulerpa peltata, C. imbricata and C. racemosa vars. laetevirens, occidentalis and turbinata were found to represent environmentally induced forms of a single species, for which the earlier-described C. chemnitzia, previously regarded as a synonym of C. racemosa var. turbinata, is reinstated. C. cylindracea, C. lamourouxii, C. macrodisca, C. nummularia and C. oligophylla are also reinstated and two new species, C. macra stat. nov. and C. megadisca sp. nov., are proposed.

Chloroplast incorporation and long-term photosynthetic performance through the life cycle in laboratory cultures of Elysia timida (Sacoglossa, Heterobranchia)

Introduction

The Mediterranean sacoglossan Elysia timida is one of the few sea slug species with the ability to sequester chloroplasts from its food algae and to subsequently store them in a functional state in the digestive gland cells for more than a month, during which time the plastids retain high photosynthetic activity (= long-term retention). Adult E. timida have been described to feed on the unicellular alga Acetabularia acetabulum in their natural environment. The suitability of E. timida as a laboratory model culture system including its food source was studied.

Results

In contrast to the literature reporting that juvenile E. timida feed on Cladophora dalmatica first, and later on switch to the adult diet A. acetabulum, the juveniles in this study fed directly on A. acetabulum (young, non-calcified stalks); they did not feed on the various Cladophora spp. (collected from the sea or laboratory culture) offered. This could possibly hint to cryptic speciation with no clear morphological differences, but incipient ecological differentiation. Transmission electron microscopy of chloroplasts from A. acetabulum after initial intake by juvenile E. timida showed different states of degradation — in conglomerations or singularly — and fragments of phagosome membranes, but differed from kleptoplast images of C. dalmatica in juvenile E. timida from the literature. Based on the finding that the whole life cycle of E. timida can be completed with A. acetabulum as the sole food source, a laboratory culture system was established. An experiment with PAM-fluorometry showed that cultured E. timida are also able to store chloroplasts in long-term retention from Acetabularia peniculus, which stems from the Indo-Pacific and is not abundant in the natural environment of E. timida. Variations between three experiment groups indicated potential influences of temperature on photosynthetic capacities.

Conclusions

E. timida is a viable laboratory model system to study photosynthesis in incorporated chloroplasts (kleptoplasts). Capacities of chloroplast incorporation in E. timida were investigated in a closed laboratory culture system with two different chloroplast donors and over extended time periods about threefold longer than previously reported.

Plastid-bearing sea slugs fix CO2 in the light but do not require photosynthesis to survive

Several sacoglossan sea slugs (Plakobranchoidea) feed upon plastids of large unicellular algae. Four species--called long-term retention (LtR) species--are known to sequester ingested plastids within specialized cells of the digestive gland. There, the stolen plastids (kleptoplasts) remain photosynthetically active for several months, during which time LtR species can survive without additional food uptake. Kleptoplast longevity has long been puzzling, because the slugs do not sequester algal nuclei that could support photosystem maintenance. It is widely assumed that the slugs survive starvation by means of kleptoplast photosynthesis, yet direct evidence to support that view is lacking. We show that two LtR plakobranchids, Elysia timida and Plakobranchus ocellatus, incorporate (14)CO2 into acid-stable products 60- and 64-fold more rapidly in the light than in the dark, respectively. Despite this light-dependent CO2 fixation ability, light is, surprisingly, not essential for the slugs to survive starvation. LtR animals survived several months of starvation (i) in complete darkness and (ii) in the light in the presence of the photosynthesis inhibitor monolinuron, all while not losing weight faster than the control animals. Contrary to current views, sacoglossan kleptoplasts seem to be slowly digested food reserves, not a source of solar power.

Crawling leaves: photosynthesis in sacoglossan sea slugs

Some species of sacoglossan sea slugs can maintain functional chloroplasts from specific algal food sources in the cells of their digestive diverticula. These 'stolen' chloroplasts (kleptoplasts) can survive in the absence of the plant cell and continue to photosynthesize, in some cases for as long as one year. Within the Metazoa, this phenomenon (kleptoplasty) seems to have only evolved among sacoglossan sea slugs. Known for over a century, the mechanisms of interaction between the foreign organelle and its host animal cell are just now starting to be unravelled. In the study of sacoglossan sea slugs as photosynthetic systems, it is important to understand their relationship with light. This work reviews the state of knowledge on autotrophy as a nutritional source for sacoglossans and the strategies they have developed to avoid excessive light, with emphasis to the behavioural and physiological mechanisms suggested to be involved in the photoprotection of kleptoplasts. A special focus is given to the advantages and drawbacks of using pulse amplitude modulated fluorometry in photobiological studies addressing sacoglossan sea slugs. Finally, the classification of photosynthetic sacoglossan sea slugs according to their ability to retain functional kleptoplasts and the importance of laboratory culturing of these organisms are briefly discussed.

Integrative species delimitation in photosynthetic sea slugs reveals twenty candidate species in three nominal taxa studied for drug discovery, plastid symbiosis or biological control

DNA barcoding can highlight taxa in which conventional taxonomy underestimates species richness, identifying mitochondrial lineages that may correspond to unrecognized species. However, key assumptions of barcoding remain untested for many groups of soft-bodied marine invertebrates with poorly resolved taxonomy. Here, we applied an integrative approach for species delimitation to herbivorous sea slugs in clade Sacoglossa, in which unrecognized diversity may complicate studies of drug discovery, plastid endosymbiosis, and biological control. Using the mitochondrial barcoding COI gene and the nuclear histone 3 gene, we tested the hypothesis that three widely distributed "species" each comprised a complex of independently evolving lineages. Morphological and reproductive characters were then used to evaluate whether each lineage was distinguishable as a candidate species. The "circumtropical" Elysia ornata comprised a Caribbean species and four Indo-Pacific candidate species that are potential sources of kahalalides, anti-cancer compounds. The "monotypic" and highly photosynthetic Plakobranchus ocellatus, used for over 60 years to study chloroplast symbiosis, comprised 10 candidate species. Finally, six candidate species were distinguished in the Elysia tomentosa complex, including potential biological control agents for invasive green algae (Caulerpa spp.). We show that a candidate species approach developed for vertebrates effectively categorizes cryptic diversity in marine invertebrates, and that integrating threshold COI distances with non-molecular character data can delimit species even when common assumptions of DNA barcoding are violated.

Genome analysis of Elysia chlorotica Egg DNA provides no evidence for horizontal gene transfer into the germ line of this Kleptoplastic Mollusc

The sea slug Elysia chlorotica offers a unique opportunity to study the evolution of a novel function (photosynthesis) in a complex multicellular host. Elysia chlorotica harvests plastids (absent of nuclei) from its heterokont algal prey, Vaucheria litorea. The “stolen” plastids are maintained for several months in cells of the digestive tract and are essential for animal development. The basis of long-term maintenance of photosynthesis in this sea slug was thought to be explained by extensive horizontal gene transfer (HGT) from the nucleus of the alga to the animal nucleus, followed by expression of algal genes in the gut to provide essential plastid-destined proteins. Early studies of target genes and proteins supported the HGT hypothesis, but more recent genome-wide data provide conflicting results. Here, we generated significant genome data from the E. chlorotica germ line (egg DNA) and from V. litorea to test the HGT hypothesis. Our comprehensive analyses fail to provide evidence for alga-derived HGT into the germ line of the sea slug. Polymerase chain reaction analyses of genomic DNA and cDNA from different individual E. chlorotica suggest, however, that algal nuclear genes (or gene fragments) are present in the adult slug. We suggest that these nucleic acids may derive from and/or reside in extrachromosomal DNAs that are made available to the animal through contact with the alga. These data resolve a long-standing issue and suggest that HGT is not the primary reason underlying long-term maintenance of photosynthesis in E. chlorotica. Therefore, sea slug photosynthesis is sustained in as yet unexplained ways that do not appear to endanger the animal germ line through the introduction of dozens of foreign genes.

What remains after 2 months of starvation? Analysis of sequestered algae in a photosynthetic slug, Plakobranchus ocellatus (Sacoglossa, Opisthobranchia), by barcoding

The sacoglossan sea slug, Plakobranchus ocellatus, is a so-called long-term retention form that incorporates chloroplasts for several months and thus is able to starve while maintaining photosynthetic activity. Little is known regarding the taxonomy and food sources of this sacoglossan, but it is suggested that P. ocellatus is a species complex and feeds on a broad variety of Ulvophyceae. In particular, we analysed specimens from the Philippines and starved them under various light conditions (high light, low light and darkness) and identified the species of algal food sources depending on starvation time and light treatment by means of DNA-barcoding using for the first time the combination of two algal chloroplast markers, rbcL and tufA. Comparison of available CO1 and 16S sequences of specimens from various localities indicate a species complex with likely four distinct clades, but food analyses do not indicate an ecological separation of the investigated clades into differing foraging strategies. The combined results from both algal markers suggest that, in general, P. ocellatus has a broad food spectrum, including members of the genera Halimeda, Caulerpa, Udotea, Acetabularia and further unidentified algae, with an emphasis on H. macroloba. Independent of the duration of starvation and light exposure, this algal species and a further unidentified Halimeda species seem to be the main food source of P. ocellatus from the Philippines. It is shown here that at least two (or possibly three) barcode markers are required to cover the entire food spectrum in future analyses of Sacoglossa.

The number, speed, and impact of plastid endosymbioses in eukaryotic evolution

Plastids (chloroplasts) have long been recognized to have originated by endosymbiosis of a cyanobacterium, but their subsequent evolutionary history has proved complex because they have also moved between eukaryotes during additional rounds of secondary and tertiary endosymbioses. Much of this history has been revealed by genomic analyses, but some debates remain unresolved, in particular those relating to secondary red plastids of the chromalveolates, especially cryptomonads. Here, I examine several fundamental questions and assumptions about endosymbiosis and plastid evolution, including the number of endosymbiotic events needed to explain plastid diversity, whether the genetic contribution of the endosymbionts to the host genome goes far beyond plastid-targeted genes, and whether organelle origins are best viewed as a singular transition involving one symbiont or as a gradual transition involving a long line of transient food/symbionts. I also discuss a possible link between transporters and the evolution of protein targeting in organelle integration.