Symbiotic Crab Research Articles

Shallow-water Trapezioidea (Decapoda: Brachyura) from the South China Sea, including two new records, and notes on Tetralia glaberrima Herbst, 1790.

A collection of coral symbiotic crabs (Trapezioidea) from the South China Sea reports a total of 24 species, including 12 species of Trapeziidae (including one new record: Trapezia plana Ward, 1941), four species of Domeciidae, and eight species of Tetraliidae (including one new record: Tetralia brengelae Trautwein, 2007). Live color and mitochondrial cytochrome oxidase I (COI) sequence data of all species are provided.

The Population Structure of the Symbiotic Crab, Trapezia septata (Decapoda, Trapeziidae), at the Early Stages of Development of a Coral Colonies

The Population Structure of the Symbiotic Crab, Trapezia septata (Decapoda, Trapeziidae), at the Early Stages of Development of a Coral Colonies

Phylomitogenomics elucidates the evolution of symbiosis in Thoracotremata (Decapoda: Cryptochiridae, Pinnotheridae, Varunidae).

Thoracotremata belong to the large group of "true" crabs (infraorder Brachyura), and they exhibit a wide range of physiological and morphological adaptations to living in terrestrial, freshwater and marine habitats. Moreover, the clade comprises various symbiotic taxa (Aphanodactylidae, Cryptochiridae, Pinnotheridae, some Varunidae) that are specialised in living with invertebrate hosts, but the evolutionary history of these symbiotic crabs is still partially unresolved. Here we assembled and characterised the complete mitochondrial genomes (hereafter mitogenomes) of three gall crab species (Cryptochiridae): Kroppcarcinus siderastreicola, Opecarcinus hypostegus and Troglocarcinus corallicola. A phylogenetic tree of the Thoracotremata was reconstructed using 13 protein-coding genes and two ribosomal RNA genes retrieved from three new gall crab mitogenomes and a further 72 available thoracotreme mitogenomes. Furthermore, we applied a comparative analysis to characterise mitochondrial gene order arrangement, and performed a selection analysis to test for selective pressure of the protein-coding genes in symbiotic Cryptochiridae, Pinnotheridae, and Varunidae (Asthenognathus inaequipes and Tritodynamia horvathi). The results of the phylogenetic reconstruction confirm the monophyly of Cryptochiridae, which clustered separately from the Pinnotheridae. The latter clustered at the base of the tree with robust branch values. The symbiotic varunids A. inaequipes and T. horvathi clustered together in a clade with free-living Varunidae species, highlighting that symbiosis in the Thoracotremata evolved independently on multiple occasions. Different gene orders were detected in symbionts and free-living species when compared with the ancestral brachyuran gene order. Lastly, the selective pressure analysis detected two positively selected sites in the nad6 gene of Cryptochiridae, but the evidence for positive selection in Pinnotheridae and A. inaequipes and T. horvathi was weak. Adaptive evolution of mitochondrial protein-coding genes is perhaps related to the presumably higher energetic demands of a symbiotic lifestyle.

THE POPULATION STRUCTURE OF THE SYMBIOTIC CRAB, <i>TRAPEZIA</i> <i>SEPTATA</i> (DECAPODA, TRAPEZIIDAE) AT THE EARLY STAGES OF CORAL COLONY DEVELOPMENT

Symbiotic invertebrates play crucial roles in coral reef ecosystems due to their extremely high abundance and biomass, and by maintaining coral fitness and protecting them from predation. Among these invertebrates, symbiotic crabs of the family Trapeziidae are especially important for branching corals of the family Pocilloporidae. In this research, we studied the spatial arrangement and parameters of the populations of the symbiotic crab, Trapezia septata at the early stage of Pocillopora verrucosa colony formation. The study was conducted based on symbionts sampled from small coral colonies exposed for three months at a coral nursery in the Nha Trang Bay, Vietnam. As much as 59.5% of the study colonies are shown to be inhabited by the T. septata crab. The percentage of inhabited colonies grows proportionately to their volume. The male to female ratio is 1.4, being skewed to males. The number of males and females is increased with the volume of inhabited colonies, as is the average size of individuals. Individual colonies are commonly inhabited by a single crab or heterosexual pairs. Sexual dimorphism was found, females being larger than males. A positive relationship between the sizes of males and females in pairs was revealed, as in 73% pairs females were larger than males. The size of the clutch correlated positively to the size of the female. We hypothesize that the predominance of males in the population, as well as the individuals located among the hosts one by one, is the result of intersexual competition

Biological features of symbiotic pea crabs of the genus Pinnixa sensu lato (Decapoda: Brachyura: Pinnotheridae) from Vostok Bay of the Sea of Japan

Biological features of symbiotic pea crabs of the genus Pinnixa sensu lato (Decapoda: Brachyura: Pinnotheridae) from Vostok Bay of the Sea of Japan

A Review of the Ecomorphology of Pinnotherine Pea Crabs (Brachyura: Pinnotheridae), with an Updated List of Symbiont-Host Associations

Almost all pea crab species in the subfamily Pinnotherinae (Decapoda: Brachyura: Pinnotheridae) are considered obligatory endo- or ectosymbionts, living in a mutualistic or parasitic relationship with a wide variety of invertebrate hosts, including bivalves, gastropods, echinoids, holothurians, and ascidians. While the subfamily is regarded as one of the most morphologically adapted groups of symbiotic crabs, the functionality of these adaptations in relation to their lifestyles has not been reviewed before. Available information on the ecomorphological adaptations of various pinnotherine crab species and their functionality was compiled in order to clarify their ecological diversity. These include the size, shape, and ornamentations of the carapace, the frontal appendages and mouthparts, the cheliped morphology, the ambulatory legs, and the reproductive anatomy and larval characters. The phylogenetic relevance of the adaptations is also reviewed and suggestions for future studies are made. Based on an updated list of all known pinnotherine symbiont–host associations and the available phylogenetic reconstructions, it is concluded that, due to convergent evolution, unrelated species with a similar host interaction might display the same morphological adaptations.

Geographical ecology of the symbiotic crabs Libinia ferreirae and Libinia spinosa: Distributional patterns in south and south‐eastern coast of Brazil

AbstractThe Brazilian coast is home to two species of Majoid crabs: Libinia ferreirae and Libinia spinosa, with similar habits, as they live in symbiosis with the jellyfish Lychnorhiza lucerna during their juvenile stage and dwell on the bottom of the ocean near the coast when adults. This study investigated the spatial and temporal distribution of L. ferreirae and L. spinosa during different ontogenic phases on the south and south‐eastern coast of Brazil, between latitudes 22º and 27ºS. Our results showed that both species have different distributions during their development in the three studied regions. Libinia ferreirae were more abundant associated with jellyfish, and L. spinosa were more abundant in the free‐living phase. Environmental factors, especially temperature and salinity, for associated and free‐living crabs, respectively, are the main factors that need to be supported for the establishment of these specimens. We were able to identify that the presence or absence of hosts (given the ecological interactions between the species) is determinant in finding the associated crabs. Geographical variations (morphology of the regions, marine currents and the strength of the tide) are linked directly to the movement of the jellyfish hosts to the coast, influencing the abundance of L. fereirae and L. spinosa populations and affecting their distribution patterns. We can infer that both species are connected with other populations because of the association with the jellyfish, crabs could be passively transported along the south and south‐eastern coast of Brazil, maintaining the connection between metapopulations.

A New Record of Symbiotic Crab (Harrovia elegans de Man, 1887) from the Gulf of Mannar, India

The present study deals with the first record of symbiotic crab Harrovia elegans de Man 1887 (Crustacea: Decapoda: Pilumnidae) collected from Tuticorin and Mandapam regions of Gulf of Mannar during February 2019. This crab species affiliates to family Pilumnidae, under the Decapoda infra-order Brachyura and is for the first time reported from the Indian mainland. The present specimens (One adult male, one adult female and a sub-adult) were collected from a depth of 5 m and showing symbiotic association with the crinoids, Comanthus wahlbergii. Although widely distributed in the Indo-Pacific region, this is the first report of this species from the Indian mainland.

A new distribution record of the Pentagonal Sea Urchin Crab Echinoecus pentagonus (A. Milne-Edwards, 1879) (Decapoda: Brachyura: Pilumnidae) from the Andaman Islands, India

First time, a Sea Urchin Crab, Echinoecus pentagonus (A. Milne-Edwards, 1879) is recorded from the rocky intertidal region of the Andaman Islands. It is a symbiotic crab live with Sea Urchin. This species is recorded from the Lakshadweep and Nicobar Islands but still there is no record from mainland India. A detail description of the species with standard photographs and line diagrams is provided.

On the pea crabs found in the chiton Tonicia chilensis (Frembly, 1827) (Mollusca, Polyplacophora: Chitonidae) identified as "Orthotheres sp." by Melzer Schwabe (2008), and its reassignment to Calyptraeotheres Campos, 1990 (Crustacea: Pinnotheridae).

Crabs of the family Pinnotheridae have been considered a phylogenetically heterogeneous group and taxonomically problematic (Palacios-Theil 2009, 2016; Tsang et al. 2018). The lack of knowledge of sexual dimorphism and morphological variation throughout the life history for many species have complicated its taxonomy and has resulted in errors in its classification (Campos 1989, 1993, 2016). Consequently, if the taxonomy of these symbiotic crabs had been based on juveniles and undeveloped character states, the problems get even more serious (Campos 1989, 1993). Melzer Schwabe (2008) studied three juvenile crabs living in the chiton Tonicia chilensis (Frembly, 1827) (Polyplacophora: Chitonidae) collected in Muelle Dichato, Chile, and they identified them as the putative invasive stage of a species of Orthotheres Sakai, 1969. The crabs are of the typical juvenile pinnotherid form with a masculine habitus, including a suborbicular carapace, large eyes and a slender pleon (= abdomen) with the telson well defined (Ocampo et al. 2017). Unpublished observations on juveniles and adults of several species of pinnotherid crabs by the author, for example, Juxtafabia muliniarum (Rathbun, 1918), Dissodactylus lockingtoni Glassell, 1935, D. xantusi Glassell, 1936, Calyptraeotheres granti (Glassell, 1933), Austinotheres angelicus (Lockington, 1877) and Tumidotheres margarita (Smith, 1870) have revealed that the third maxilliped exhibit little morphological variation through the post-larval stages of development (from juvenile to adult). This appendage, including the partial or total fusion of the ischium and merus (Fig. 1A-G; Fig 2A, C-G), has been considered a valuable feature for distinguishing genera in the family Pinnotheridae (Bürger 1895; Rathbun 1918; Manning 1993; Ahyong Ng 2007; Campos 2009). According to Melzer Schwabe (2008) the ischium and merus of the third maxilliped are completely fused, the carpus is larger than the propodus and the small dactylus is subterminally inserted on the propodus (Fig 1C), and as such the juveniles studied are possibly members of the genus Orthotheres Sakai, 1969. A detailed re-examination of the third maxilliped, chela of pereiopod 1 (cheliped) and pleon, however, has shown otherwise. Campos (1989) discussed the taxonomy of Orthotheres and treated all the species he recognised as belonging to this genus; characterising its members by its diagnostic third maxilliped palp structure. Ng Ho (2014) rediagnosed the genus based on the type species, O. turboe Sakai, 1969, from Japan, and restricted it for three Indo-West Pacific species; commenting that the American species as well as some Indo-West Pacific taxa will need to referred to other genera when a full revision is done. This revision is now in progress by the author, PKL Ng and ST Ahyong. Until the revision is complete, the genus is treated here in the broader sense of Campos (1989) and Geiger Martin (1999).

Host selection and preferences of coral symbiotic crab Tetralia rubridactyla

Coral symbiotic crabs provide considerable benefits to their host corals. A comprehensive understanding of the association between these crabs and their hosts could help clarify the relationship, interaction, and importance of symbionts with coral reefs as hosts. In this study, the coral symbiotic crab Tetralia rubridactyla was test for host preference and fidelity. Five oceanic objects were provided to the crabs: common host corals (Acropora hyacinthus and A. digitifera), uncommon host corals (Pocillopora damicornis and Stylophora pistillata), and dead coral skeletons. The crabs were collected from the 2 source host corals A. hyacinthus and A. digitifera and subjected to an experiment comprising 7 treatments. Each treatment included 2 stages of no-choice and choice conditions to estimate the expected selection frequencies. The results revealed that the crabs chose any available object under the no-choice condition, and exhibited various preferences under the choice condition. Moreover, T. rubridactyla exhibited significantly higher frequencies to inhabit Acropora corals (p<0.01, χ2 test), than dead coral skeletons and uncommon host corals. In all the treatments, the preferences of the crabs from the 2 source hosts were similar. Present results demonstrated T. rubridactyla host selection conditioning as follows: (1) Under the no-choice condition, inhabit any choice object for shelter; (2) under the choice condition, if without a common host, randomly inhabit any uncommon choice object as a host; and (3) under the choice condition, if a common host is available, selecting the common host is the first priority because it could provide food and space. This study revealed that T. rubridactyla express neither fidelity nor preference between A. hyacinthus and A. digitifera. Thus, these results also suggested that the distribution of T. rubridactyla on Acropora corals in the reef is affected by an abundance of corals rather than the preferences of coral species.

Trace fossil evidence of coral-inhabiting crabs (Cryptochiridae) and its implications for growth and paleobiogeography.

Members of the Cryptochiridae are small, fragile, symbiotic crabs that live in domiciles in modern corals. Despite their worldwide occurrence with over 50 species known today, their fossil record is unknown. We provide the first unambiguous evidence of cryptochirids in the fossil record through their crescentic pits, typical for certain cryptochirids, in Western Atlantic fossil corals, while the Eocene genus Montemagrechirus is excluded from the Cryptochiridae and referred to Montemagrechiridae fam. nov. Nine Pleistocene corals with crescentic pits originate from Florida (USA), and single specimens with pits come from the late Pleistocene of Cuba and the late Pliocene of Florida, all of which are measured for growth analyses. These pits represent trace fossils named Galacticus duerri igen. nov., isp. nov. A study of modern cryptochirid domicile shape (crescentic pit, circular-oval pit, or a true gall) shows that species within crab genera tend to inhabit the same pit shape. Crescentic pits in corals occur not only in the Western Atlantic today, but also in the Indo-West Pacific and in the Eastern Pacific. Thus, examination of Cenozoic fossil coral collections from these regions should yield further examples of cryptochirid pits, which would help to constrain the antiquity of this cryptic crab family.

Anomuran and Brachyuran Symbiotic Crabs in Coastal Areas between the Southern Ryukyu arc and the Coral Triangle.

Parinya Limviriyakul, Li-Chun Tseng, Jiang-Shiou Hwang, and Tung-Wei Shih (2016) Symbiotic anomuran and brachyuran crabs were identified after extensive surveys of reef zones, especially from sponges, scleractinian corals, crinoids, and sea urchins, in the coastal areas of the southern East China Sea. Twenty-nine species belonging to 17 genera, 9 families, and 2 infraorders were identified (3 were identified to the generic level). More crabs belonged to the infraorder Brachyura (82.8%) than to Anomura (17.2%). Two anomuran symbionts (Allogalathea elegans and Petrolisthes virgatus) and 5 brachyuran symbionts (Tetralia glaberrima, Tetralia rubridactyla, Trapezia cymodoce, Trapezia septata, and Cymo melanodactylus) are common in this area. Two species of Anomura (Lauriea simulata, Petrolisthes virgatus) and 3 of Brachyura (Gonatonotus nasutus, Tetralia aurantistellata and Tetralia nigrolineata) were identified for the first time from waters adjacent to Taiwan. These records represent the northernmost recorded of L. simulata and T. aurantistellata. The occurrence of P. virgatus is the second in the western Pacific Ocean. This study revealed the geospatial distribution of symbiotic crabs, which connects the region from the southern Ryukyu arc to the Coral Triangle, and provides the supporting taxonomic account of symbiotic anomuran and brachyuran crab fauna inhabiting the reef zone in northern Taiwan.

Population distribution, sexual dimorphism, and reproductive parameters in the crab Pinnixa valdiviensis Rathbun, 1907 (Decapoda: Pinnotheridae), a symbiont of the ghost shrimp Callichirus garthi (Retamal, 1975) in the southeastern Pacific

Pinnixa valdiviensis Rathbun, 1907 inhabits burrows of the ghost shrimp Callichirus garthi (Retamal, 1975) in the south-eastern Pacific. We described the host-use pattern and sexual dimorphism of P. valdiviensis to test for monogamy considering that the few other studied species of Pinnixa White inhabit their respective hosts as male-female pairs. Against expectations, P. valdiviensis lived either solitarily, or in small groups of up to four individuals; only a few burrows were inhabited by two crabs (either male-female or female-female pairs). The observations above argue against the notion that P. valdiviensis is monogamous. Furthermore, that a high frequency of solitary females were found brooding embryos and that the population sex ratio was skewed toward females suggests that males might roam among hosts in search of receptive females. We argue in favour of additional long-term (seasonal, >1 year) descriptive and experimental studies to reveal the mating tactics used by both males and females of P. valdiviensis. We also examined individual-level reproductive parameters in the studied species as little is known about reproduction in symbiotic crabs. Fecundity varied between 383 and 1052 eggs crab−1 with a mean ± SD of 774 ± 160 eggs crab−1 and increased significantly with female body size. Embryo volume varied between 0.0169 and 0.0443 with a mean ± SD of 0.0222 ± 0.0063 mm3 and did not vary with female body size. Reproductive output (RO) represented a mean ± SD of 5.74% ± 0.86% of crab body dry weight and increased proportionally with crab body weight. The RO in P. valdiviensis is lower than that reported for other free-living and symbiotic crabs.

Maternal investment in egg production: Substrate- and population-specific effects on offspring performance of the symbiotic crab Pachycheles monilifer (Anomura: Porcellanidae)

The porcelain crab Pachycheles monilifer is a non-obligate symbiotic species, living in association with marine invertebrates. In view of the importance of this relationship for the life cycle, this study investigated the reproductive performance of this crab in relation to its hosts. The crabs were collected in symbiosis with bryozoan and polychaete colonies on the southeastern coast of Brazil. Only the initial egg stages were used to estimate the fecundity and reproductive output (RO). To obtain the RO, the female and the egg-mass dry weight were used. Additionally, the genetic distance between the crab populations from the two substrates was evaluated, using partial sequences of the mitochondrial genes 16S and COI, in order to determine the intra- and interspecific genetic variation. The crabs found in bryozoans (CL, carapace length=4.6±1.3mm) were larger than those living among polychaetes (CL=4.1±1.1mm). The fecundity and RO of crabs inhabiting the bryozoans increased with carapace length, but in the females inhabiting the polychaetes only the fecundity was correlated with the female body size. In the polychaete colonies, the space available for the symbiotic porcelain crab is more restricted since it is a closed habitat, which could limit the size of its inquilines and consequently the reproductive fitness, whereas the females associated with polychaete colonies have a lower fecundity and RO. Our molecular results confirmed the existence of low genetic divergence and of potential gene flow between the two populations. Taken together, this information suggests that the differences in reproductive traits of these crab populations were due to the difference in hosts.

Juvenile Trapezia spp. crabs can increase juvenile host coral survival by protection from predation

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 515:151-159 (2014) - DOI: https://doi.org/10.3354/meps10970 Juvenile Trapezia spp. crabs can increase juvenile host coral survival by protection from predation H. Rouzé1,2,*, G. Lecellier1,2,3, S. C. Mills2,4, S. Planes1,2, V. Berteaux-Lecellier1,2, H. Stewart1,5 1CRIOBE USR 3278 CNRS-EPHE-UPVD, BP 1013, Moorea, 98729 French Polynesia 2Laboratoire d’Excellence ‘CORAIL’, 58 avenue Paul Alduy, 66860 Perpignan, France 3Université de Versailles-Saint Quentin, 55 Avenue de Paris, Versailles Cedex, France 4CRIOBE USR 3278 CNRS-EPHE-UPVD, 58 Avenue Paul Alduy, 66860 Perpignan Cedex, France 5Department of Fisheries and Oceans Canada, 4160 Marine Drive, West Vancouver, British Columbia V7V 1N6, Canada *Corresponding author: heloise.rouze@gmail.com ABSTRACT:Adult crabs are known to play critical roles in the survival of their adult coral hosts, but little is known of the mutualism between juvenile crabs (≤0.5 cm) and their juvenile hosts. Field and laboratory experiments both demonstrated that the presence of juvenile crabs of the genus Trapezia in young host Pocillopora corals (2 to 3 cm diameter) increased coral survival by 32% and reduced consumption by the corallivorous seastar Acanthaster planci. These experiments also showed that juvenile Trapezia were not effective at deterring predation by another common predatory seastar, Culcita novaeguineae. Finally, our work highlights that the defensive ability of symbiotic crabs may be genus-specific, as juvenile Tetralia spp. crabs, obligate symbionts of Acropora spp., displayed no protection against either A. planci or C. novaeguineae. KEY WORDS: Juvenile · Trapeziid · Corals · Acanthaster planci · Culcita novaeguineae · Mutualism · Predation · Defence Full text in pdf format PreviousNextCite this article as: Rouzé H, Lecellier G, Mills SC, Planes S, Berteaux-Lecellier V, Stewart H (2014) Juvenile Trapezia spp. crabs can increase juvenile host coral survival by protection from predation. Mar Ecol Prog Ser 515:151-159. https://doi.org/10.3354/meps10970 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 515. Online publication date: November 18, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

Finding of the pea crab Pinnaxodes mutuensis Sakai, 1939 (Crustacea: Decapoda: Pinnotheridae) in an unusual host in Busse Lagoon, southern Sakhalin

The symbiotic crab Pinnaxodes mutuensis Sakai, 1939 was found in the mantle cavity of the mud-burrowing bivalve Mya arenaria Linnaeus, 1758 (the family Myidae) in Busse Lagoon at southern Sakhalin Island. Along the Russian coasts, this species has been previously found in the mantle cavity of epifaunal bivalves of the family Mytilidae [Modiolus difficilis (Deshayes in Maillard, 1863), Modiolus modiolus (Linnaeus, 1758), Mytilus edulis Linnaeus, 1758, Crenomytilus grayanus (Dunker, 1853)] and the family Ostreidae [Crassostrea gigas (Thunberg, 1793)]. New data on the morphology and ecology of this crab are presented.

Parasitic castration in slipper limpets infested by the symbiotic crab Calyptraeotheres garthi

Two ill-explored hypotheses might explain host castration by parasitic pea crabs. The ‘energy drain’ hypothesis states that castration is caused by host-derived nutrient consumption of parasites that ultimately diminishes host-energy intake. The ‘steric interference’ hypothesis states that castration occurs when parasites physically inhibit host reproduction. This study evaluated whether Calyptraeotheres garthi, a pea crab from the southwestern Atlantic, is a parasitic castrator and explored whether the two hypotheses above explain castration in the limpet Crepidula cachimilla. None of three studied limpet species brooded embryos during the reproductive season when infested by mature female pea crabs. Also, limpets of C. cachimilla infested by C. garthi did not reproduce during a 90-day experimental period while crab-free limpets did spawn embryos during this period. Limpets resumed reproduction soon after pea crabs were experimentally removed from their brooding chamber. Thus, C. garthi does castrate limpets, and castration is reversible. Pea crabs ‘steal’ food from limpets, and infested limpets did not modify their feeding behavior to counteract nutrient loss. Thus, infested limpets are expected to ingest less food which provides partial support for the ‘energy drain’ hypothesis. However, the limpet’s body condition increased or was not affected by pea crabs during the breeding season which argues against the same hypothesis. Furthermore, that limpets promptly recovered reproductive activity once pea crabs were experimentally removed, that castration was not induced by the smallest pea crabs in the population (that fill only partially the brooding chamber), and that parasitized limpets did exhibit fully mature ovaries, support the ‘steric interference’ hypothesis explaining parasitic castration.

Austinotheres angelicus (Lockington, 1877): the correct name for the symbiotic crab Juxtafabia muliniarum sensu Cabrera Pena et al. (2001) (Crustacea, Brachyura, Pinnotheridae)

The taxonomic identity of the symbiotic crabs Juxtafabia muliniarum sensu Cabrera-Pena et al. (2001) was revised. The morphology of these specimens, collecte...

The diet of the Harlequin crab Lissocarcinus orbicularis, an obligate symbiont of sea cucumbers (holothuroids) belonging to the genera Thelenota, Bohadschia and Holothuria

The present paper characterizes, for the first time, the diet of the Harlequin crab Lissocarcinus orbicularis, an obligate symbiotic crab that associates with sea cucumbers (holothuroids) belonging to the genera Thelenota, Bohadschia and Holothuria. These tropical holothuroids host a rich symbiotic community in the Indo-West Pacific Ocean of which the Harlequin crab is the best known. The diet of L. orbicularis was characterized by analyzing the microscopic, molecular and isotopic signatures obtained from its gastric content. The presence of sea cucumber ossicles in the gastric mills of the crabs suggests that symbionts eat the superficial integument of their host and this was supported by the fact that Holothuroid DNA was detected in the stomach of L. orbicularis after DGGE and sequencing of the 18S rDNA gene. The stable isotopic δ13C and δ15N values of crab tissues were compared with diverse potential food sources including three holothuroids, three algae, one sea grass as well as the organic matter contained in the water column, in the sediment, and the second most abundant symbiont, the polychaete Gastrolepidia clavigera. The low δ15N values of crabs suggests that the crabs do not exclusively feed on sea cucumber tissue but assimilate diverse food sources such as sea grasses and organic matter contained in sediment that have similar δ13C values. There were no differences between the feeding of males and females but there was a positive correlation between the carapace length and the stable isotopic values indicating a shift of the food source as crabs grow larger.