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Prior to the present study, 11 genera and 16 species of pinnotherid crabs were known from Australia, of which two species of the genus Arcotheres were recorded: Arcotheres boninensis and A. similis. The current study extends our knowledge of Australian Arcotheres in a species new to science, significant range extensions for A. boninensis in Australia, and the first national record of A. exiguus. The new species (A. eclecticus) is closest to Arcotheres palaensis , but differs chiefly in features of the carapace. Arcotheres boninensis was formerly known from Western Australia and is now known to span all of northern Australia. To date, A. exiguus has been recorded from the Philippines westwards to Singapore, Malaysia and India; first records from Australia and Hong Kong represent the southern- and northernmost records of the species, respectively. The Australian pinnotherid fauna now stands at 11 genera and 18 species, of which four species belong to Arcotheres. A key to the Australian species of Arcotheres is provided. [ABSTRACT FROM AUTHOR]

The pea crab, Pinnotheres onychodactylus Tesch, 1918, is only known on the basis of the types collected from the Moluccas, Indonesia. A re-examination of the type specimens shows that the species is actually a junior subjective synonym of Magnotheres globosus (Hombron & Jacquinot, 1846), a widely distributed Indo-West Pacific species associated with horse mussels of the family Pinnidae. [ABSTRACT FROM AUTHOR]

Background. 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. Methods. 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). Results. 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. [ABSTRACT FROM AUTHOR]

Examination of pea crabs, Plenotheres coarctatus (Bürger, 1895) (Decapoda: Pinnotheridae), from mangrove clams, Geloina coaxans (Gmelin, 1791) (Bivalvia: Cyrenidae), collected in Vietnam led to the discovery of a new genus and species of bopyrid parasite. Herein we describe Rhizophoracepon magnagibbus, new genus, new species, females of which are found in the left or right branchial chamber of the pea crab hosts and are distinguished from other pinnotherid bopyrids based on rounded coxal plates with crenulate surfaces and small thin projections, lack of dorsolateral bosses, tergal projections or mid-dorsal projections on the pereomeres, and five pairs of short, subequal digitate biramous pleopods and uniramous lateral plates and uniramous uropods. Males of R. magnagibbus are distinguished based on their slightly tapered pleon with the pleomeres splayed out laterally, mid-ventral tubercles on pleomeres 1-4, and broad, widely separated lobes on the pleotelson. These findings expand on our knowledge of the diversity of these poorly known hyperparasites from pinnotherid crabs, which are symbionts of commercially important bivalves. [ABSTRACT FROM AUTHOR]

The type species of Abyssotheres Manning & Galil, 2000, Pinnoteres abyssicola Alcock & Anderson, 1899, was re-examined to ascertain the veracity of several character states which had been used by the original authors to establish the genus. Komatsu & Ohtsuka (2009) had questioned whether the palp of the third maxilliped only had two articles, with the dactylus absent as reported by Manning & Galil (2000). The type female of the species actually has a dactylus on the third maxilliped palp. In addition, despite earlier statements that the first and second ambulatory legs have the longest dactyli, this is not the case, with the second and third legs possessing the longest dactyli. In addition, A. acesticola Komatsu & Ohtsuka, 2009, is here reported from Tungsha Island in the South China Sea, for the first time. [ABSTRACT FROM AUTHOR]

Many marine crustaceans form symbiotic relationships, yet there has been little work quantifying morphological adaptations in species specialized on different hosts. Here we examine morphological adaptations of symbiotic pea crabs (Pinnotheridae) to different host phyla. Multiple authors have noted that crabs living with burrowing hosts (burrowing shrimps and annelids) have different carapace shapes than species living with non-burrowing hosts (bivalves, gastropods, echinoderms, and ascidians), but this hypothesis has never been tested. Using digital analyses of taxonomic images, we calculated carapace aspect ratio of 149 pinnotherid species, and used phylogenetic ANOVA to test whether aspect ratio differed among species living with different hosts. Pea crab species living with burrowing hosts had significantly larger carapace aspect ratios (wide carapaces) than species living with non-burrowing hosts (round or square carapaces, or otherwise non-wider than long). Convergent evolution of morphological features in species with specialized host use may be a common—but understudied—pattern in many marine groups. [ABSTRACT FROM AUTHOR]

Brachyuran crabs of the family Pinnotheridae De Haan are a classic example of symbiotic lifestyle. Here, we described the host‐use pattern, relative growth and sexual dimorphism of Austinixa patagoniensis (Rathbun 1918), a symbiont of the ghost shrimp Callichirus major (Say 1818). We use these analyses to test the A. patagoniensis mating system considering that monogamous species inhabit their respective host as male–female pairs and have a low degree of sexual dimorphism. Austinixa patagoniensis showed a high social plasticity, with individuals inhabiting galleries in solitary, in pairs, in trios or in groups of up to eight crabs. While it is true that 38% of the burrows sampled harbored pairs of crabs, the body size of paired crabs from the opposite sex was poorly correlated. This suggests that heterosexual pairing is short term with frequent shifts among burrows by male and/or female crabs, as reported before for other symbiotic decapods in which the body size of paired crabs is poorly correlated. A considerable proportion of the ovigerous females (34%) were found living solitarily, which suggest that males abandon these females shortly after insemination. The sexual dimorphism in terms of body and cheliped size was evident in this species, a condition that argues in favor of male–male competition in A. patagoniensis. The observations above argue against the notion that A. patagoniensis is monogamous. Lastly, our results revealed that body size and chelipeds of males are not under sexual selection by females, thus reinforcing the idea that male–male competition is the mechanism behind sexual selection in A. patagoniensis. [ABSTRACT FROM AUTHOR]

Increasing attention to the systematics of Southeast Asian pinnotherid crabs has required the reevaluation of many species described in older works in order to correctly recognise and characterise new species. The pinnotherids from Thailand are known on the basis of only four studies, the most important being that by Mary Rathbun in 1909, who described seven new species from the Gulf of Thailand: Pinnotheres buergeri, P. kamensis, P. kutensis, P. lanensis, P. nigrans, P. quadratus, and P. siamensis. The seven species are reviewed and figured based on type material. Pinnotheres buergeri, currently assigned to Viridotheres, is a senior synonym of P. kutensis and P. siamensis, both of which are earlier juvenile stages of the same species. Pinnotheres kamensis, also based on a juvenile holotype, is a junior synonym of Arcotheres similis (Bürger, 1895). Pinnotheres lanensis and P. quadratus are each valid species of Arcotheres, and mature at a much smaller size than known congeners. Pinnotheres nigrans is a junior synonym of A. purpureus Alcock, 1900. The re-assessment of P. nigrans required a study of A. purpureus and its close congener, A. boninensis (Stimpson, 1858), both of which are redescribed, the latter based on a newly designated neotype. [ABSTRACT FROM AUTHOR]

Pea crab is a small crustacean known as kleptoparasite and belongs to the family Pinnotheridae, inhabit bivalves, gastropods, and other species. Nonetheless, pea crab is one of the unfamiliar species to the Malaysia citizen and the information is limited. This study aimed to determine the distribution of pea crab from four bivalves species (mud clam, oyster, hard clam and cockle) in Setiu Wetlands, Terengganu. Sampling was conducted twice, in October 2018 and February 2019, in which mud clam, oyster, cockle and hard clam were collected. In this study, the distribution of pea crab was observed by collecting randomly fifty samples of each bivalve at four sites and checked for any presence of pea crab. The size shell length of mud clams measured was 40 to 75 mm, oysters were 30 to 80 mm, marine clams and cockles were 20 to 40 mm. Pea crab presence was analysed and identified for the genus identification based on the microscopic and macroscopic methods. The result showed that, pea crab only present in the mud clam but none in other bivalves. Fifteen pea crabs were isolated from the first sampling (mean=0.3) with 11 females, whilst 13 pea crabs from the second sampling (mean=0.26) with 10 femalesidentified. The size of the pea crab measured was between 10 mm to 15 mm and belongs to the genus of Arcotheres sp. However, there were other none pea crab species were found in the oyster. On the other hand, the correlation between the carapace width of pea crab and mud clam shell length were analysed using linear regression (r²=0.0699, p=0.341) from first sampling, while (r²=0.2885, p=0.058) from second sampling. The size range of mud clams without pea crab was 35 to 60 mm shell length. [ABSTRACT FROM AUTHOR]

The “solar-powered” Elysia timida (Risso, 1818) is an endemic Mediterranean sacoglossan living in rocky substrates at shallow water. During a scuba dive, one E. timida was photographed and collected. The observation revealed the presence of a small crustacean of the Pinnotheridae family. It was not possible to assign the crustacean to a genus, but it shares typical ecological and external morphological features with other Pinnotherinae species. This is the first report of a pea crab hosted by a non-shelled Heterobranchia and the first case of a symbiotic association between crustaceans and marine Heterobranchia reported in the Mediterranean Sea.

Fluctuations in abundance of dominant species can cause competitive release of resources with consequences on community structure and functioning. In the present study, changes in the intertidal macroinfauna community of an exposed sandy beach were evaluated during two contrasting periods characterized by low and high densities of the yellow clam Amarilladesma mactroides. The increase in clam abundance and biomass was associated with a significant decrease in abundance of the rest of the community. In particular, a decline was observed for the pea crab Austinixa patagoniensis, a commensal species that lives in the burrows of the shrimp Sergio mirim. Our study demonstrates that fluctuations in clam abundance lead to long-term changes in community structure, suggesting the presence of competitive interactions. The environmental stability over the two periods strengthens the hypothesis that the competition between species is crucial for shaping the ecological community. Stable isotope analysis allows discarding trophic competition as mechanism of exclusion. Image maps reveal complementary distribution of species, showing the relevance of the spatial competition, which is mediated by changes in abundance of a third species. Indeed, high densities of A. mactroides reduce the available area for the establishment of the S. mirim burrows, limiting the foraging behavior of its commensal, the pea crab. Such an interaction drives density-dependent exclusion of the pea crab from the intertidal zone following the establishment of the yellow clam population. This study illustrates that spatial competition triggered by the increase of a bed-forming species can have community-wide consequences in exposed sandy beaches. [ABSTRACT FROM AUTHOR]

Pinna nobilis, commonly known as the fan mussel or pen shell, is an endemic species in the Mediterranean Sea. The fan mussel population has recently been significantly endangered along the Mediterranean coast, mainly due to diseases. However, it has a critical role in the ecological system of the throughout the coasts of the Mediterranean. Therefore, it is important to investigate the interactions between P. nobilis and their enemies, parasites, symbiont in the ecological environment. Pea crabs are small crustacean and symbionts in a variety of invertebrates. They inhabit the mantle cavities of bivalve. The association between the pea crab, Nepinnotheres pinnotheres (Decapoda), and P. nobilis (Bivalvia) from the Aegean Sea, Turkey was examined in this study. The crab samples were collected off Urla Karantina Island, Izmir Bay, Aegean Sea. The biometric characteristics of bivalve and crab in this coexistence were analyzed. 80% of P. nobilis was occupied by N. pinnotheres. The weight of pea crabs was recorded between 0.01 g and 3.87 g. [ABSTRACT FROM AUTHOR]

The holotype of the pinnotherid, Pinnotheres socius Lanchester, 1902, is extant and deposited in the University Museum of Zoology, Cambridge. The taxonomy of this pea crab is treated in this note and the specimen is illustrated by photographs and line drawings of the third maxilliped and pereiopods. Although the ovigerous type specimen is damaged, it is sufficiently intact to identify the specimen and synonymize the species with Nepinnotheres cardii (Bürger, 1895), described from the Philippines. [ABSTRACT FROM AUTHOR]

Currently, there are five species of pea crabs in European waters. In the context of the AFROBIV project, in which the effect of the African pea crab Afropinnotheres monodi on bivalve species of commercial interest is being studied, a sampling campaign has been carried out on bivalve species of the coasts of southwestern Europe. The results of this campaign are used in the present study to evaluate the current distribution of A. monodi and its degree of infestation in its various hosts, together with the distribution of native European pinnotherid species. Previously collected information has also been reviewed and included with the present results to establish a baseline so as to enable future changes in distribution and host use of European pinnotherid species to be compared and quantified. To enable the more accurate identification of species and detection of new species in the area, three genetic markers (16S, Cox1, and H3) for the current five European pinnotherid species have also been compiled from Genbank or obtained in the present work. The results showed the high abundance of A. monodi in a significant portion of the area studied. In contrast, few specimens of the native Pinnotheres pectunculi and P. pisum were collected. The northern limit of the current distribution of A. monodi extends to the Sado estuary (Portugal), and the species has been recorded for the first time in the Mediterranean (Alboran Sea). New records of P. pectunculi in the Alboran Sea confirm the presence of this species in the Mediterranean, and considerably expand the known distribution of the species, until now restricted to the coasts of Brittany. [ABSTRACT FROM AUTHOR]

Background: 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., Methods: 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 )., Results: 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., Competing Interests: The authors declare there are no competing interests., (©2023 Xu et al.)

The Indo-West Pacific pinnotherid crab genus, Ostracotheres H. Milne Edwards, 1853, was recognised for species with a simple, unadorned carapace without marginal lamellae, and a spatulate maxilliped 3 propodus with a rounded or subtruncate apex. Prior to the present study, the genus included: O. tridacnae (Red Sea; type species), O. affinis (Mauritius, Djibouti), O. cynthiae Nobili, 1906a (Red Sea); O. spondyli Nobili, 1905 (Persian Gulf); O. subglobosus (Baker, 1907) (southern Australia); O. holothuriensis Baker, 1907 (southern Australia); O. subquadratus Sakai, 1939 (Japan); and O. tomentipes Takeda & Konishi, 1994 (Japan). Reconsideration of species assigned to Ostracotheres shows they differ considerably in other aspects such as carapace physiognomy, pereopod article proportions, and gonopod structure (where known), suggesting the genus is not monophyletic. Thus, Ostracotheres is revised based on type and other material. Ostracotheres sensu stricto is restricted to O. tridacnae and O. cynthiae. Ostracotheres tridacnae is confirmed as a senior synonym of O. savignyi H. Milne Edwards, 1853, and appears to be restricted to the Red Sea as a commensal of tridacnine clams; records of O. tridacnae from southeast Africa are referred to Afropinnotheres dofleini (Lenz in Lenz & Strunck, 1914). Similarly, O. cynthiae appears to be restricted to the Red Sea as a commensal of ascidians; the single record of O. cynthiae from Indonesia requires confirmation but its published account is not consistent with O. cynthiae sensu stricto. A new genus, Austrotheres, is proposed for O. holothuriensis and a new species from eastern Australia. Ostracotheres subglobosus and O. subquadratus, considered by some to be conspecific, are shown to be separate valid species, with the identity of the latter fixed by a neotype selection; both are transferred to a new genus, Discorsotheres, together with O. spondyli and a new species from the tropical western Pacific. Ostracotheres affinis and O. tomentipes are referred to a new genus Latatheres, with the identity of the former fixed by a neotype selection. Keys to the species of the genera studied herein and to pinnotherid genera with a 2-segmented maxilliped 3 palp are provided. [ABSTRACT FROM AUTHOR]

Some members of the Pinnotheridae family exhibit close symbiotic relationships with bivalves, which may cause severe damage to the mollusk. The condition index (CI) is a tool that serves to evaluate the effect of biotic and abiotic factors on the morphology and physiology of mollusks, including those caused by symbiotic organisms. In the present study, we evaluated the effects of the infestation of the pinnotherid crab Austinotheres angelicus on the oyster Saccostrea palmula through monthly sampling (April to November 2012) in Punta Morales, Pacific coast of Costa Rica. A total of 245 oysters were analyzed, of which 38% were infested by the pinnotherid crab. Most of the oysters analyzed (92%) varied in depth between 1.5 and 15.0 mm; 89% of the infested oysters were in this depth range. The prevalence was positively correlated with the depth of the oyster. Our results revealed that crab infestation could reduce the CI by 11 to 33%. The obtained findings suggest that the infestation by A. angelicus provokes alterations of the flesh production of the oyster, especially when the association is with a pair of crabs including an ovigerous female. The association of A. angelicus with S. palmula is best described as facultative parasitism for male crabs and obligatory parasitism for females.

The endosymbiont-host relationship between a pea crab and its bivalve host has been studied previously using bivalve's multi-physiological parameters. The present study is first of its kind that uses hemocyte's immune functionality to evaluate the symbiotic relationship between bivalve Paphia malabarica and its symbiont crab, Pinnotheres vicajii . The sampling was carried out at two stations (differed in nutrient concentration, productivity, and bacterial abundance) located along the southwest coast of India. The results showed lower immunocompetence in the case of bivalves infested with pea crabs. The lowering of hemocyte population was the most prominent and perhaps the initial reaction in response to focal inflammation or wound inflicted by the pea crabs. A significant lowering of phagocytosis, lysozyme, and esterase activity was also observed, indicating the detrimental impact of crab infestation. Interestingly, no significant correlation was found between parasitism and condition index despite the lower immunocompetence, validating the effectiveness of using hematological parameters in comparison to other life-history traits. This indicates that the immune parameters being directly related to the organism's metabolic state can elucidate the early effect of stressors, therefore, proving to be a better proxy for understanding a host-pathogen relationship. Disparity found in bivalve's immunocompetence between the sampling sites could be linked to either variation in environmental parameters, the size of the symbiont or the infection level. Our preliminary results provide a direction towards examining immunodepression in parasitized bivalves by considering ex-situ controlled parasitism induction along with varying environmental conditions. In view of this, efforts must be directed towards minimizing pea crab's outbreaks, especially in cultivated shellfish farms. Future studies should elucidate molecular mechanisms involved in the immunocompetence of P. malabarica , hosting the parasitic pea crabs. [ABSTRACT FROM AUTHOR]

Se determinaron las tallas, frecuencia de infección y fecundidad de Juxtafabia muliniarum de la cavidad paleal de Saccostrea palmula de mayo de 1998 a mayo de 1999, la frecuencia de infestación fue de un 18.52 % en una muestra de 540 ostras. De los 136 cangrejos, el 36 % fueron parejas, el 60 % hembras solas y el 4 % machos solos. El promedio de la longitud del caparazón fue de 5.6 + 0.74 mm (ámbito entre 4.0 y 7.6 mm) para hembras y 2.71 + 0.60 mm (ámbito entre 1.6 y 4.0 mm) para machos. El peso promedio fue de 0.180 + 0.084 g (ámbito entre 0.06 y 0.4 g) para hembras y 0.011 + 0.003 g (ámbito entre 0.01 y 0.02 g) para machos. La fecundidad de J. muliniarum se rigió por la ecuación F = 3904.6 Ln (Lc) -4651.1. En todos los meses se encontraron hembras ovígeras. La relación longitud del caparazón-peso total para machos y hembras se rigió porla ecuación Pt = 6 X 10-4 Lc3.2122. La proporción sexual fue de 1.0 macho: 2.04 hembras. Se registra a S. palmula como huésped de J. muliniarumSize and fecundity observations of pea crab (Juxtafabia muliniarum) from the paleal cavity of!he oyster Saccostrea palmula were made from May 1998 to May 1999. Infestation frequency was 18.52 % in a sample of 540 oysters. Of 136 pea crabs, 36 % were couples, 60 % were single females and 4 % were single males. The mean caparace length of J. muliniarum was 5.6 + 0.74 mm (range 4.0 to 7.6 mm) for females and 2.7 + 0.60 mm (range 1.6 to 4.0 mm) for males. The mean weight was 0.180 + 0.084 g (range 0.06 to 0.4g) for females and 0.011 + 0.003 g (range 0.01 to 0.02 g) for males. Ovigerous females (43.75 % of all females) were found in all months. The caparace length - fecundity relationship was F = 3904.6 Ln (Lc) -4651.1. The caparace length-weight relationship was P = 6 X 10-4 Lc3.2122. The mean sex-ratio was 1.0 male: 2.4 females. Saccostrea palmula infected only by females was the dominant group (60.78 %). This mollusk is a new host record for the crab

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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 min3 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Austinixa aidae inhabits burrows of the ghost shrimp Callichirus major at Perequê-açu beach, Ubatuba, Brazil. We described the host-use pattern and sexual dimorphism of A. aidae to test for monogamy given the generality of this mating system in the subfamily Pinnothereliinae (family Pinnotheridae) to which A. aidae belongs. Against expectations, A. aidae lives as solitary individuals within burrows more frequently than expected by chance alone. Additional observations suggested that A. aidae exhibits a polygynandrous mating system with males moving among burrows in search of receptive females. First, only 21% of the burrows harboured heterosexual pairs of crabs and the body size of paired crabs was poorly correlated. This suggests pair instability and frequent shifts among burrows by male and/or female crabs, as reported before for other symbiotic crustaceans in which the body size of paired crabs is poorly correlated. Second, males paired with females that were sexually receptive (without embryos) or that have been receptive recently (carrying early embryos) were found more frequently than expected by chance alone. The above agrees with that reported for species in which sexual pairing does not last long. Third, sexual dimorphism in terms of claw size and coloration was evident. Claws were larger in males than in females, a condition that argues in favour of male–male competition in A. aidae. In addition, the body coloration of males was more similar to the sand grains of the beach than that of females. This sex-specific coloration suggests that males are ‘better adapted' than females to roam on the surface of the beach in search of burrows because their coloration should diminish the risk of detection by predators. Experiments are needed to reveal the details of the polygynandrous mating system herein inferred for A. aidae and to understand those conditions favouring particular reproductive strategies in symbiotic decapod crustaceans. [ABSTRACT FROM PUBLISHER]