Morphological diversity in true and false crabs reveals the plesiomorphy of the megalopa phase.

Brachyura and Anomala (or Anomura), also referred to as true and false crabs, form the species-rich and globally abundant group of Meiura, an ingroup of Decapoda. The evolutionary success of both groups is sometimes attributed to the process of carcinization (evolving a crab-like body), but might also be connected to the megalopa, a specific transitional larval phase. We investigate these questions, using outline analysis of the shields (carapaces) of more than 1500 meiuran crabs. We compare the morphological diversity of different developmental phases of major ingroups of true and false crabs. We find that morphological diversity of adults is larger in false crabs than in true crabs, indicating that taxonomic diversity and morphological diversity are not necessarily linked. The increasing morphological disparity of adults of true and false crabs with increasing phylogenetic distance furthermore indicates diverging evolution of the shield morphology of adult representatives of Meiura. Larvae of true crabs also show larger diversity than their adult counterparts, highlighting the importance of larvae for biodiversity studies. The megalopa phase of Meiura appears to be plesiomorphic, as it overlaps between true and false crabs and shows little diversity. Causes may be common evolutionary constraints on a developmental phase specialized for transitioning.

Morphological and molecular analysis of the tropical hermit crab Calcinus vachoni (Decapoda, Diogenidae) and its potential association with colonial anemone.

Calcinus is the colorful hermit crab genus belonging to the family Diogenidae and is often found in coral reefs of the tropical Indo-West Pacific region, including southern Honshu, Japan, which is the northern limit of their occurrence. In the present study, we found C. vachoni for the first time in the intertidal zone of Jeju Island-the southernmost island of South Korea. We examined their morphology and provided a diagnosis of their morphological details with illustrations. In addition, the Korean Calcinus population was genetically characterized using mtDNA cox1 sequences and by placing them into three previously reported regional haplogroups. The phylogenetic tree from maximum likelihood analysis revealed that Korean C. vachoni is assigned to the C. vachoni haplogroups exclusively, one of the three well-supported mitochondrial haplogroups with distinct geographic ranges (i.e., C. vachoni, C. aff. vachoni Cooks, and C. aff. vachoni Mascarenes). This result provides new information on the species distribution of C.vachoni, extending their geographic range further north into the southern coast of Korea. In this study, we also first report the potential association of C. vachoni with their co-occurring colonial anemone species Palythoa aff. mutuki and dead coral head of Pocillopora species based on our on-site observation and a public coral collection database of Calcinus species. However, their ecological association with co-occurring coral species is putatively assumed for now and therefore has to be validated by compelling evidence from further field observation and experimental studies (i.e., whether the presence/absence of colonial anemones affects the behavior and survival of the hermit crabs).

New records and species of deep-sea squat lobsters (Galatheoidea, Munidopsidae) from the Hawaiian Archipelago: an integrative approach using micro-CT and barcodes.

The Hawaiian Archipelago remains extensively under-sampled for many marine invertebrate taxa, including squat lobsters. During the last few years, several deep-sea expeditions carried out in the Pacific Ocean have conducted opportunistic collections of specimens and image data from the vicinity of Hawai'i. Here we describe a new species: Munidopsis hawaii sp. nov. and provide new records for Munidopsidae in the Archipelago and its associations. We illustrate and describe the new species using an integrative approach including micro-CT 3D imaging. Phylogenetic analyses of the species collected from seamounts from Hawai'i indicate that the new species represents a divergent lineage compared to morphologically similar species such as M. dispar and M. papanui. We also study the genetic distances for the species recorded in Hawai'i and other populations of the same species in the adjacent West Pacific. Three species are now known in the Hawaiian region. We also compiled identifications from images captured with ROVs in the area. These observations suggest that munidopsid species are common in the deep sea of Hawaiian waters below 1,000 m.

Porcellanid crab Pachycheles stevensii parasitized by rhizocephalans Lernaeodiscus spp.: population biology in the northwestern Sea of Japan.

We examined the population and reproductive characteristics of the anomuran crab Pachycheles stevensii Stimpson, 1858 (Decapoda: Porcellanidae) heavily infested by 2 rhizocephalans of the genus Lernaeodiscus in the northern part of the species range (Peter the Great Bay, Sea of Japan) in 2020-2021. The prevalence of infestation reached 32.3%, and L. rybakovi proved to be much more numerous than L. kasyanovi. The sex structure of the P. stevensii population was represented by males (38.8%), females (50.6%), and modified specimens (10.6%), which were extremely feminized males. The latter had a broadened abdomen, smaller chelae, shortened gonopod, and 3 additional pairs of female-type pleopods. Thus, the initial sex ratio in the P. stevensii population approximated the expected 1:1, as in most porcellanid crabs. No significant modifications of the secondary sex characters of the female hosts were observed. The size structures of males and females did not significantly differ. P. stevensii produced 1 clutch yr-1. Oviposition and incubation lasted from summer to early fall; recruitment was noted in the fall. There was a correlation between the reproductive cycles of the host and parasites: rhizocephalans with mature externae were found from June-October. A large number of ovigerous female P. stevensii simultaneously carried parasitic externae of L. rybakovi; fecundity was considerably higher in non-infested females.

Functional morphology of the primary olfactory centers in the brain of the hermit crab Coenobita clypeatus (Anomala, Coenobitidae).

Terrestrial hermit crabs of the genus Coenobita display strong behavioral responses to volatile odors and are attracted by chemical cues of various potential food sources. Several aspects of their sense of aerial olfaction have been explored in recent years including behavioral aspects and structure of their peripheral and central olfactory pathway. Here, we use classical histological methods and immunohistochemistry against the neuropeptides orcokinin and allatostatin as well as synaptic proteins and serotonin to provide insights into the functional organization of their primary olfactory centers in the brain, the paired olfactory lobes. Our results show that orcokinin is present in the axons of olfactory sensory neurons, which target the olfactory lobe. Orcokinin is also present in a population of local olfactory interneurons, which may relay lateral inhibition across the array of olfactory glomeruli within the lobes. Extensive lateral connections of the glomeruli were also visualized using the histological silver impregnation method according to Holmes-Blest. This technique also revealed the structural organization of the output pathway of the olfactory system, the olfactory projection neurons, the axons of which target the lateral protocerebrum. Within the lobes, the course of their axons seems to be reorganized in an axon-sorting zone before they exit the system. Together with previous results, we combine our findings into a model on the functional organization of the olfactory system in these animals.

Specialized structures on the border between rhizocephalan parasites and their host's nervous system reveal potential sites for host-parasite interactions.

Rhizocephalan barnacles are a unique group of endoparasitic crustaceans. In their extreme adaptation to endoparasitism, rhizocephalan adults have lost almost all features of their free-living relatives but acquired an outstanding degree of control over the body of their hosts (mostly decapods). The subtle influence exercised by rhizocephalans on the physiology, morphology and behaviour of their hosts is a vivid example of the most intimate host-parasite interactions but their mechanisms are very poorly known. In this study we examined the morphology and the adaptive ultrastructure of the organs invading the nervous system of the host in two rhizocephalan species from the families Peltogastridae, (Peltogaster paguri) and Peltogasterellidae (Peltogasterella gracilis). We found two essentially different types of structures involved in interactions of these two rhizocephalans with the nervous system of their hosts: modified rhizocephalan rootlets lying inside the ganglia and the neural fibres of the host enlacing the trophic rootlets of the parasites. We suggest that both these structures may be highly specialized tools allowing the parasite to interact with the host on the humoral level via neuromediators, hormones, attractants and trophic factors.

Mushroom bodies in Reptantia reflect a major transition in crustacean brain evolution.

Brain centers possessing a suite of neuroanatomical characters that define mushroom bodies of dicondylic insects have been identified in mantis shrimps, which are basal malacostracan crustaceans. Recent studies of the caridean shrimp Lebbeus groenlandicus further demonstrate the existence of mushroom bodies in Malacostraca. Nevertheless, received opinion promulgates the hypothesis that domed centers called hemiellipsoid bodies typifying reptantian crustaceans, such as lobsters and crayfish, represent the malacostracan cerebral ground pattern. Here, we provide evidence from the marine hermit crab Pagurus hirsutiusculus that refutes this view. P. hirsutiusculus, which is a member of the infraorder Anomura, reveals a chimeric morphology that incorporates features of a domed hemiellipsoid body and a columnar mushroom body. These attributes indicate that a mushroom body morphology is the ancestral ground pattern, from which the domed hemiellipsoid body derives and that the "standard" reptantian hemiellipsoid bodies that typify Astacidea and Achelata are extreme examples of divergence from this ground pattern. This interpretation is underpinned by comparing the lateral protocerebrum of Pagurus with that of the crayfish Procambarus clarkii and Orconectes immunis, members of the reptantian infraorder Astacidea.

MicroCT imaging applied to description of a new species of Pagurus Fabricius, 1775 (Crustacea: Decapoda: Anomura: Paguridae), with selection of three-dimensional type data.

A new species of hermit crab, Pagurus fraserorum n. sp. (family Paguridae) is described from rocky subtidal reefs off KwaZulu-Natal, South Africa, and illustrated using both conventional drawings and colour photographs, and via three-dimensional (3D) X-ray micro-computed tomography (µCT). Because of the limitation µCT has in detecting very fine and soft structures, a novel approach of manually drawing setation and spinulation onto the two-dimensional images of the 3D visualizations was used to illustrate the pereopods. In addition, an interactive figure and rotation movie clips in the supplement section complement the species description, and the 3D raw data of the 3D type data are downloadable from the Gigascience Database repository. The new species is the sixth species of Pagurus Fabricius, 1775 reported from South Africa and is closely allied to the Indo-Pacific P. boriaustraliensis Morgen, 1990 and P. pitagsaleei McLaughlin, 2002, from which it differs by its shorter ocular peduncles, by the armature of the carpus of the right cheliped, and also in colouration. This study presents the first description of a hermit crab in which a majority of taxonomic details are illustrated through 3D volume-rendered illustrations. In addition, colour photographs and COI molecular barcodes are provided, and the latter compared to COI sequences of specimens from Western Australia previously identified as P. boriaustraliensis and of specimens of P. pitagsaleei from Taiwan, as well as to three additional South African members of the genus. The South African taxon was confirmed to be genetically distinct from all species tested.

A new yeti crab phylogeny: Vent origins with indications of regional extinction in the East Pacific.

The recent discovery of two new species of kiwaid squat lobsters on hydrothermal vents in the Pacific Ocean and in the Pacific sector of the Southern Ocean has prompted a re-analysis of Kiwaid biogeographical history. Using a larger alignment with more fossil calibrated nodes than previously, we consider the precise relationship between Kiwaidae, Chirostylidae and Eumunididae within Chirostyloidea (Decapoda: Anomura) to be still unresolved at present. Additionally, the placement of both new species within a new "Bristly" clade along with the seep-associated Kiwa puravida is most parsimoniously interpreted as supporting a vent origin for the family, rather than a seep-to-vent progression. Fossil-calibrated divergence analysis indicates an origin for the clade around the Eocene-Oligocene boundary in the eastern Pacific ~33-38 Ma, coincident with a lowering of bottom temperatures and increased ventilation in the Pacific deep sea. Likewise, the mid-Miocene (~10-16 Ma) rapid radiation of the new Bristly clade also coincides with a similar cooling event in the tropical East Pacific. The distribution, diversity, tree topology and divergence timing of Kiwaidae in the East Pacific is most consistent with a pattern of extinctions, recolonisations and radiations along fast-spreading ridges in this region and may have been punctuated by large-scale fluctuations in deep-water ventilation and temperature during the Cenozoic; further affecting the viability of Kiwaidae populations along portions of mid-ocean ridge.

First Finding of Representatives of the Eccrinida Order in the Digestive Tract of King Crab Specie from the Barents Sea.

This is the first report on the finding of large intra-intestinal symbionts belonging to the order Eccrinida in crab-like decapod crustaceans of the family Lithodidae, Paralithodes camtschaticus and Lithodes maja, inhabiting the Barents Sea. Studies have been regularly conducted since the early 2000s in Kola Bay and in one of the inlets of the Eastern Murman Coast. Since 2005, Eccrinida representatives (presumably, a new species of the genus Arundinula) have been found in the guts of the red king crab. A brief description of the endosymbionts and data on their frequency of occurrence are presented. The possible reasons for the distribution of eccrinids in the crab-like decapod crustaceans of the Barents Sea are discussed.

A new species of hermit crab, Diogenes heteropsammicola (Crustacea, Decapoda, Anomura, Diogenidae), replaces a mutualistic sipunculan in a walking coral symbiosis.

Symbiont shift is rare in obligate mutualisms because both the partners are reciprocally dependent on and specialized to each other. In the obligate accommodation-transportation mutualism between walking corals and sipunculans, however, an unusual saltatory symbiont shift was discovered. In shallow waters of southern Japan, an undescribed hermit crab species was found living in corallums of solitary scleractinian corals of the genera Heterocyathus and Heteropsammia, replacing the usual sipunculan symbiont. We described the hermit crab as a new species Diogenes heteropsammicola (Decapoda, Anomura, Diogenidae), and explored its association with the walking corals. This hermit crab species obligately inhabits the coiled cavity of the corals, and was easily distinguished from other congeneric species by the exceedingly slender chelipeds and ambulatory legs, and the symmetrical telson. Observations of behavior in aquaria showed that the new hermit crab, like the sipunculan, carries the host coral and prevents the coral from being buried. This is an interesting case in which an organism phylogenetically distant from Sipuncula takes over the symbiotic role in association with a walking coral. The hermit crab species is unique in that its lodging is a living solitary coral that grows with the hermit crab in an accommodation-transportation mutualism.

Facultative Parasitism by the Bivalve Kurtiella pedroana in the Mole Crab Emerita analoga.

Evolutionary transitions to parasitism are rare. In this study, we documented a potential step toward parasitism in the commensal clam Kurtiella pedroana (Bivalvia: Galeommatoidea). Galeommatoideans are known commensals of various invertebrates, including crustaceans. Emerita analoga (Decapoda: Hippidae) is an abundant intertidal mole crab inhabiting Pacific coast beaches in North and South America. Mole crabs collected from Monterey Bay, California, were measured and examined externally and internally for associated molluscs. Out of the 520 mole crabs, 37 large female individuals harbored 49 clams (prevalence of 7.11% and mean intensity of 1.3). Forty-one ectocommensal clams were attached by their byssal threads to the inside of the gill chambers or to the lateroventral surfaces. However, our key finding was 8 clams that lacked byssal threads and were living in the hemocoel of 6 crabs. These internal clams were smaller than the ectocommensals. Because these internal clams lacked access to their normal food, we hypothesize they might have fed on hemolymph as would a parasite. Clam larvae have no obvious exit from the hemocoel, implying that endoparasitism is a dead-end for K. pedroana. Regardless, facultative parasitism in a free-living or an ectocommensal is uncommon and suggests a pathway to parasitism.

A tale of two antennules: the performance of crab odour-capture organs in air and water.

Odour capture is an important part of olfaction, where dissolved chemical cues (odours) are brought into contact with chemosensory structures. Antennule flicking by marine crabs is an example of discrete odour capture (sniffing) where an array of chemosensory hairs is waved through the water to create a flow-no flow pattern based on a narrow range of speeds, diameters of and spacings between hairs. Changing the speed of movement and spacing of hairs at this scale to manipulate flow represents a complicated fluid dynamics problem. In this study, we use numerical simulation of the advection and diffusion of a chemical gradient to reveal how morphological differences of the hair arrays affect odour capture. Specifically, we simulate odour capture by a marine crab (Callinectes sapidus) and a terrestrial crab (Coenobita rugosus) in both air and water to compare performance. We find that the antennule morphologies of each species are adaptions to capturing odours in their native habitats. Sniffing is an important part of odour capture for marine crabs in water where the diffusivity of odorant molecules is low and flow through the array is necessary. On the other hand, flow within the hair array diminishes odour-capture performance in air where diffusivities are high. This study highlights some of the adaptations necessary to transition from water to air.

A new species of the genus Diogenes Dana, 1851 (Crustacea: Decapoda: Anomura: Diogenidae) from Gujarat, northwestern India.

A new species of diogenid hermit crab, Diogenes chhapgari n. sp., is described on the basis of material collected during the crustacean faunal survey along the coastal areas of the Gujarat State, northwestern India. The new species is referred to the D. edwardsii (De Haan, 1849) species group and appears closest to D. singaporensis Rahayu, 2015. However, D. chhapgari n. sp. is distinguished from this species by the armature on the fourth segment of the antennal peduncle and carpus of the left cheliped.

Morphological and molecular data reveal the cryptic diversity among populations of Aegla paulensis (Decapoda, Anomura, Aeglidae), with descriptions of four new species and comments on dispersal routes and conservation status.

The taxonomy of the Brazilian aeglid species Aegla paulensis Schmitt, 1942 from two disjunct hydrographic basins is revised using morphological and molecular data. Results show that six disjunctive populations of Aegla paulensis form a species complex. Aegla paulensis sensu stricto is redescribed and Aegla rosanae Campos Jr., 1998 is revalidated. The four remaining populations previously assigned to Aegla paulensis are now recognized as different species, namely Aegla  vanini n. sp., Aegla japi n. sp., Aegla jaragua n. sp. and Aegla jundiai n. sp. All new species are described and illustrated and are well supported by both morphological and molecular data. Aegla lancinhas Bond-Buckup & Buckup in Santos et al., 2015, which until recently was confounded with Aegla paulensis s. str., is supported as a valid species. A key to all members of the A. paulensis species complex is provided, and their phylogenetic and biogeographic relationships to other closely related species is discussed.

A review of the genus Spiropagurus Stimpson, 1858 (Crustacea: Decapoda: Paguridae) from the China seas.

Three species of the genus Spiropagurus Stimpson, 1858, are reported in the present paper, including a new species, S. monospinosus sp. nov. This new species is easily recognized by the carpus of the cheliped bearing only one row of spines, whereas two rows are present in the other two species from the China seas. A key to all the species of Spiropagurus is provided.

A new species of the hermit crab genus Alainopaguroides McLaughlin, 1997 (Crustacea: Decapoda: Anomura: Paguridae) from the South China Sea.

A new species of the pagurid hermit crab, Alainopaguroides sinensis sp. nov., is described and illustrated on the basis of a single ovigerous female specimen collected from the South China Sea. It is immediately distinguished from other known congeneric species by the possession of a subdistal spine on the lateral margin of the basal segment of the antennular peduncle, although the most similar congener is A. andamanensis McLauglin, 2002. This is the first record of the genus from the South China Sea. An identification key to the species of the genus Alainopaguroides McLaughlin, 1997 is provided.

A new species of the hermit crab genus Areopaguristes Rahayu & McLaughlin (Crustacea, Decapoda, Anomura, Paguroidea, Diogenidae) from the Mexican Pacific.

A new species of hermit crab, Areopaguristes espetacioni, family Diogenidae, is described and illustrated in detail, including the color pattern in live/fresh specimens. This is the fifth species of Areopaguristes reported from the eastern Pacific. Given the morphological similarity between Areopaguristes espetacioni n. sp. and A. tudgei Lemaitre & Felder, 2012, these can be considered sister species. The presence of a long rostrum, the antennal flagella with short setae, chelipeds and ambulatories legs with dense plumose setae, and telson with calcareous teeth on the posterior margin allow to separate Areopaguristes espetacioni n. sp. from all other species in the genus previously described for the region. A key for Areopaguristes species from the eastern tropical Pacific is provided.

A new species of Munidopsis Whiteaves, 1874 (Crustacea: Decapoda: Anomura) from the Northwest Indian Ocean Ridge.

A new species, Munidopsis militaris n. sp., from the Carlsberg Ridge, Northwest Indian Ocean Ridge is described herein. The species belongs to a group of species having a pair of epigastric spines, mesial and lateral eye-spines, abdominal tergites unarmed, five or six spines on the lateral margin of the carapace, and a denticulate carina on the distolateral margin of the P1 fixed finger. It can be distinguished from its relatives by the spinous lateral margin of the palm and dorsal carinae on the P2-4 propodus. The Munidopsis fauna of the Indian Ocean Ridge is seldom reported on; this new species is the sixth member of this genus found inhabiting the Indian Ocean Ridge.

A new species of hermit crab of the genus Pagurus Fabricius, 1775 (Crustacea: Anomura: Paguridae) from the southern Caribbean off Venezuela.

A male specimen of a new species of the heterogeneous genus Pagurus Fabricius, 1775, collected in 1968 off the Caribbean coast of Venezuela, was discovered among the vast crustacean collections of the Smithsonian Institution. This new species herein described and illustrated is named P. scopaopsis, and is characterized primarily by the presence of: a brush-like setation pattern on the dactyl of the left third pereopod, dense small tubercles on the dorsal surfaces of the dactyl and fixed finger of the right chela, and a raised longitudinal ridge armed with spines on the palm and fixed finger of the left chela.