Agonistic behaviors are crucial and common among animals due to their importance in securing an individual’s fitness, and neural signaling molecules are known to mediate these behaviors. Stenopus, a genus of shrimp-like decapod crustaceans characterized by a pair of enlarged pereiopods, exhibits prominent agonistic behaviors when encountering conspecifics of the same sex owing to its monogamous social structure. These shrimps represent another potentially excellent model organism for investigating the neural signaling basis of agonistic behaviors in crustaceans aside from traditional models. Yet, their underpinning molecular aspects have never been studied. Using S. hispidus and S. cyanoscelis as representatives, the present study is the first that systematically examines the genetics of agonistic behaviors in Stenopus. Three organs, including (1) antennae + antennules, (2) central nervous system, and (3) eyestalk ganglia, were RNA-sequenced to identify the differentially expressed genes (DEGs) and pathways potentially conserved in winners and losers of Stenopus after fighting interactions. Our results suggested that Stenopus agonistic interactions might be systemic activities involving the simultaneous modulation and interplay of multiple signaling cascades, organismal systems, and metabolic pathways. In particular, winners and losers typically exhibited enriched gene ontologies involved in neural signaling, and sensory and behavioral processes. Regarding enriched pathways, while those related to glycan biosynthesis and metabolism were enriched in winners, cholesterol metabolism and one-carbon pool by folate were enriched in losers. These different sets of pathways suggested that while fighting interactions in Stenopus were injurious to both combatants, the damage in losers appeared to be more traumatic. Furthermore, four neural signaling systems, including dopamine, acetylcholine, octopamine, and glutamate, were identified as potentially major mediators of agonistic behaviors and fighting interactions in both Stenopus species, with the first two appearing to be relatively more important. A comparison of the neural signaling systems involved in mediating aggression among pan-crustaceans suggested that Stenopus appeared to stand out by its seemingly major reliance on dopamine and acetylcholine, as opposed to the primarily serotonin-based regulation of aggression observed in most examined pan-crustaceans. The different metabolic responses between winners and losers in Stenopus highlight the profound, asymmetric physiological costs of social conflict at the molecular level. Furthermore, their unique reliance on dopamine and acetylcholine reveals diverse evolutionary trajectories in the neuroendocrine regulation of aggression, providing new insights into the current paradigms of invertebrate social behavior.

Sea slugs are vital benthic predators and indicators of marine ecosystem health. Hong Kong hosts a rich but incompletely documented sea slug fauna, with previous inventories recording 257 species as of 2022. This study reports the outcomes of a citizenscience program launched in late 2023 that engaged 142 contributors and generated 1,930 validated observations spanning from 2013 to 2025. These records documented 274 species from 108 genera and 47 families, adding 121 new records to the local inventory and expanding the known fauna to 378 species (47% increase). The newly recorded species span seven orders and one superorder, and are dominated by Doridida (54 species; 44.6%) and Nudibranchia (41 species; 33.9%). While 71.1% of the new records were limited to single observations, requiring further monitoring to confirm their residency, the repeated detection of other taxa reveals that several previously overlooked species are actually relatively common. Spatio-temporal trends in species richness closely track observer activity, with records concentrated during peak diving seasons and at popular offshore diving sites, such as East Ninepin and East Dam. Although these patterns reflect inherent sampling biases, the substantial species richness at these locations identifies them as candidate biodiversity hotspots warranting long-term investigation. Our findings demonstrate the efficacy of citizen science for rapid inventory expansion. To transit from baseline studies into a structured monitoring framework, we recommend integrating professional surveys to address spatial coverage gaps and enhance taxonomic resolution. Furthermore, prioritizing the protection of these identified hotspots will be essential for safeguarding Hong Kong’s marine biodiversity.

Three new species of freshwater atyid shrimp, namely Caridina chui sp. nov., C. bauhinia sp. nov. and C. argilla sp. nov., are described from Hong Kong, southern China using an integrative taxonomic approach. All three species belong to the C. serrata species group and differ from the other members in characteristics of the rostrum, pereiopods, and male first and second pleopods. Phylogenetic analyses based on mitochondrial COI and 16S rRNA sequences show that the C. serrata species group is polyphyletic and comprises at least four lineages. The validity of the C. serrata species group is discussed and more detailed diagnosis of the species group is proposed. The C. serrata species group sensu stricto is restricted to the nominal lineage containing C. serrata and closely related species, distinguished from other species of the C. serrata species group mainly by having relatively stout carpus of first pereiopod, stout appendix masculina of male second pleopod, and large number of spiniform setae on the uropodal diaeresis.

Background Crustaceans exhibit great diversity in anatomical, physiological, and behavioral adaptations, all of which contribute to their evolutionary success, and neuropeptides are important regulators mediating these adaptations. While expansion of certain neuropeptide families in specific animal groups and organs typically reflects their adaptive roles, our understanding of their diversity and evolution in crustaceans has remained incomplete and underrepresented, partly because previous studies typically focused on a limited number of taxa and organs, with large-scale comparisons across multiple groups remaining scarce. Utilizing custom-made crustacean profile hidden Markov models constructed for 66 neuropeptide families, the present study examined their diversity and evolutionary patterns across four crustacean classes (Branchiopoda, Thecostraca, Hexanauplia, and Malacostraca), between non-agonistic and agonistic malacostracans, and among their three sensory and neuroendocrine organs through transcriptome mining. Results Our results revealed that the expansions of neuropeptide families in certain crustacean classes generally corresponded to their evolutionary adaptations. Specifically, those related to osmoregulation and detoxification were expanded in Thecostraca and Hexanauplia; those involved in the development of sessility were specifically expanded in Thecostraca; and those important for visual and nervous systems were expanded in Malacostraca. Furthermore, seven neuropeptide families, six of which play roles in sex-related functions, were potentially crucial in regulating agonism in malacostracans. Additionally, the significantly higher number of members in certain neuropeptide families within specific sensory and neuroendocrine organs likely reflected their greater functional importance to those organs in Malacostraca. Conclusions In conclusion, this study represents the first large-scale comparative analysis of the number of members in different neuropeptide families across multiple crustacean taxa and organs, providing a foundation for future research on neuropeptidergic regulation of physiologies and behaviors in crustaceans, with significant implications for their ecology and evolution.

Octocorals are an important component contributing to habitat forming that enhances the three-dimensional structural complexity and facilitates benthic–pelagic coupling for marine benthic ecosystems. Studies on octocorals in marginal environments could provide insight to researchers and conservationists in the face of the deteriorating seascape under the Anthropocene. The octocoral communities in Hony -30g Kong persist under marginal living conditions created by freshwater influx from the western Pearl River Estuary, transitioning from the turbid, hyposaline conditions of the western waters to the relatively oligotrophic oceanic waters of the East. This study employed genome re-sequencing to investigate the population structure and genetic diversity of three widely distributed octocoral species, Dendronephthya gigantea, Dendronephthya spinifera, and Echinomuricea spinifera, along the water gradient of Hong Kong to investigate the effect of a marginal turbid environment on the population structure of octocorals. Two octocoral genera exhibited differences in vertical distribution in response to sedimentation and turbidity, in which the soft coral Dendronephthya in western waters were restricted to shallower depths while the gorgonian Echinomuricea could occupy deeper waters in the western region despite heavy siltation. A low level of genetic differentiation was reported in D. gigantea in the turbid western region while no significant population differentiation was observed for D. spinifera and E. spinifera between sampling regions. Moreover, low genetic diversity in terms of nucleotide diversity and heterozygosity with highly negatively skewed Tajima’s D was reported for all three studied octocoral species in Hong Kong. The lack of population differentiation and low genetic diversity could reflect a small founder population selected by turbid marginal conditions settled in Hong Kong and expand the population with the aid of asexual reproduction. Despite their low genetic diversity, which suggests low evolutionary potential and intrinsic vulnerability toward diseases and disturbances, octocoral populations in Hong Kong could be selected under a marginal environment and exhibit resilience to suboptimal conditions. This study underscores the necessity of prioritizing marginal octocoral communities in conservation frameworks, as they represent critical units for understanding species persistence under environmental stress and may serve as essential gene sources for regional biodiversity in an increasingly unstable global climate.

Epidemics are often initiated by emerging and re-emerging infectious diseases caused by viruses of animal origin. It is thus important to identify the reservoirs of potentially zoonotic viruses and understand the dynamics of their host shifts. The flu viruses belong to the virus family Orthomyxoviridae, which also contains Isavirus, Quaranjavirus, and Thogotovirus. Many members of this virus family are known to be pathogenic to humans. For initial surveillance of animal-originated or zoonotic Orthomyxoviridae, unclassified viruses were screened by the use of high-throughput transcriptomes as a data source because of their wide species and lineage coverage. We identified 96 novel or unclassified Orthomyxoviridae members with the discovery of three new lineages of the virus, possibly new genera, one sister to Influenza + Thogotovirus, one to Influenza + Thogotovirus + Quaranjavirus, and another one to all orthomyxoviruses except Isavirus. Throughout the evolution of Orthomyxoviridae, there might be multiple host-shifting incidences, shifting between six different animal host phyla. The most common host shifts seemed to be between Arthropoda and Chordata; however, further evidence would be needed to fully support this statement. Nonetheless, Orthomyxoviridae viruses can infect a wide range of animal phyla, while some members hold a higher risk of shifting back to Chordates and humans that warrants surveillance.

Background Agonistic behaviors are crucial and common among animals due to their importance in securing an individual’s fitness, and neuroendocrine regulators are known to mediate the behaviors. Stenopus, a genus of shrimp-like decapod crustaceans characterized by a pair of enlarged pereiopods, exhibits prominent agonistic behaviors when encountering conspecifics of the same sex owing to its monogamous social structure. These shrimps are potentially excellent non-insect model organisms for investigating the neuroendocrine regulation of agonistic behaviors in arthropods, but the underpinning molecular basis has never been studied. Using S. hispidus and S. cyanoscelis as representatives, the present study is the first to systematically examine the genetics of agonistic behaviors of Stenopus. Three organs, including (1) antennae + antennules, (2) central nervous system, and (3) eyestalks, were RNA-sequenced to find out the differentially expressed genes (DEGs) and pathways conserved in winners and losers of Stenopus after fighting interactions. Results Our results demonstrated that Stenopus agonistic interactions likely involved the simultaneous modulation and interplay of multiple signaling cascades, organismal systems, and metabolic pathways. DEGs in both winners and losers typically enriched for gene ontologies involved in neuroendocrine signaling, and sensory and behavioral processes. Regarding enriched pathways, while those related to glycan biosynthesis and metabolism were enriched in winners, cholesterol metabolism and one-carbon pool by folate were enriched in losers. These different sets of pathways suggested that while fighting interactions in Stenopus were injurious to both combatants, the damage in losers appeared to be more traumatic. Four neuroendocrine regulators, including dopamine, acetylcholine, octopamine, and glutamate were identified as the major ones in modulating agonistic behaviors and fighting interactions in both Stenopus species, with the first two believed to play relatively more important roles. A comparison of the neuroendocrine regulators involved in mediating aggression among pan-crustaceans showed that Stenopus shrimps appeared to stand out by its seemingly major reliance on dopamine and acetylcholine, as opposed to the primarily serotonin-based regulation of aggression in most examined pan-crustaceans. Conclusions This study presents a valuable opportunity for studying behavioral genetics and aggression regulation in crustaceans aside from traditional crayfish and lobster models.

Ongoing revision of the classification of the spider crab (Majoidea) families has seen numerous changes in recent years. Polyphyly in the diverse family, Inachidae, has seen numerous genera moved to other families, particularly to Oregoniidae and Macrocheiridae. The genus Eurypodius Gu & eacute;rin, 1828 from southern South America has had a fluid taxonomic history (including being the type genus of its own family) but for the past 100 years, has been considered an inachid. Reconsideration of Eurypodius on the basis of molecular data as well as adult and larval morphology show that it does not belong in Inachidae. Instead, genetic evidence places Eurypodius in a clade together with representatives of the families Epialtidae, Mithracidae, and Majidae, phylogenetically distant from other inachids. As such, the family Eurypodiidae MacLeay, 1838, is removed from the synonymy with Inachidae and here treated as a separate family. Eurypodiidae is rediagnosed and the type species of Eurypodius, E. latreillii Gu & eacute;rin, 1828, described and figured.

For much of terrestrial biodiversity, the evolutionary pathways of adaptation from marine ancestors are poorly understood and have usually been viewed as a binary trait. True crabs, the decapod crustacean infraorder Brachyura, comprise over 7600 species representing a striking diversity of morphology and ecology, including repeated adaptation to non-marine habitats. Here, we reconstruct the evolutionary history of Brachyura using new and published sequences of 10 genes for 344 tips spanning 88 of 109 brachyuran families. Using 36 newly vetted fossil calibrations, we infer that brachyurans most likely diverged in the Triassic, with family-level splits in the late Cretaceous and early Paleogene. By contrast, the root age is underestimated with automated sampling of 328 fossil occurrences explicitly incorporated into the tree prior, suggesting such models are a poor fit under heterogeneous fossil preservation. We apply recently defined trait-by-environment associations to classify a gradient of transitions from marine to terrestrial lifestyles. We estimate that crabs left the marine environment at least 7 and up to 17 times convergently, and returned to the sea from non-marine environments at least twice. Although the most highly terrestrial- and many freshwater-adapted crabs are concentrated in Thoracotremata, Bayesian threshold models of ancestral state reconstruction fail to identify shifts to higher terrestrial grades due to the degree of underlying change required. Lineages throughout our tree inhabit intertidal and marginal marine environments, corroborating the inference that the early stages of terrestrial adaptation have a lower threshold to evolve. Our framework and extensive new fossil and natural history datasets will enable future comparisons of non-marine adaptation at the morphological and molecular level. Crabs provide an important window into the early processes of adaptation to novel environments, and different degrees of evolutionary constraint that might help predict these pathways. [Brachyura; convergent evolution; crustaceans; divergence times; fossil calibration; molecular phylogeny; terrestrialization; threshold model.].

Agonistic behaviors are crucial and ubiquitous among animals for the competition of limited resources. Although the study of aggression has been a popular topic, plenty of studies focused on model organisms, and typically on crayfish and lobsters for crustaceans. Variations of the agonistic behaviors and the underpinning eliciting cues of other crustaceans therefore have not been fully explored. In the present study, we targeted Stenopus, a genus of shrimp-like crustaceans that displays prominent agonistic behaviors when encountering conspecifics of the same sex owing to their monogamous social structure. Using S. hispidus (Olivier, 1811) and S. cyanoscelis (Goy, 1984) as representatives, we characterized their agonistic behaviors and fighting pattern, conducted experiments to investigate the contribution of visual, olfactory and tactile cues to inducing aggression, and examined the effects of antennal and antennular ablation on their agonistic interactions. A total of seven agonistic behaviors were documented, where antennal entwining and tactile contact is the major driver and seemingly important cue, respectively, in inducing agonistic behaviors in Stenopus. Although ablation of antennae and antennules did not inhibit fighting, behavioral changes, such as the prolonged agonistic interactions and the delayed establishment of dominance were observed, suggesting a reduction of aggressiveness. A comparison of agonistic behaviors with other crustaceans showed that certain features appeared to be unique or distinct in Stenopus, including the potential functional overlap of antennae and antennules, a higher aggressiveness of the fighting behaviors, and the exhibition of crouching behavior by submissive individuals. The present study provides a crucial background understanding for subsequent research on Stenopus and paves the way for its establishment as another crustacean model for studying aggression.

IntroductionThe phylogenetic relationships within Pancrustacea (including Crustacea and Hexapoda) remain elusive despite analyses using various molecular data sets and analytical approaches over the past decade. The relationship between the major lineages of Allotriocarida, which includes Hexapoda, the most species-rich animal taxon, is particularly recalcitrant.MethodsTo investigate and resolve the root of phylogenetic ambiguity in Pancrustacea, we re-evaluated the evolutionary relationships of major pancrustacean clades using a phylogenetically informed orthology approach and assessed the effect of systematic errors, with a major focus on long branch attraction (LBA) and incomplete lineage sorting (ILS). A data set comprising 1086 orthologs from 106 species representing all major extant classes of pancrustaceans was assembled and used in gene tree and species tree construction after various filtering processes.Results and discussionRegardless of the filtering criteria and phylogenetic analyses, the resulting trees consistently supported (1) a sister relationship of Remipedia and Hexapoda (hence rejecting the monophyly of Xenocarida, i.e. Remipedia + Cephalocarida), and (2) refuted the monophyly of Multicrustacea, as Copepoda is either sister to or nested within Allotriocarida. Examination of gene trees reveals that the grouping of Multicrustacea and Xenocarida in previous phylogenetic studies may represent LBA artefacts. Phylogenetic signal analyses suggest a low resolution and an incidence of strong conflicting signals at the deep splits. Further analyses indicate a partial contribution of incomplete lineage sorting (ILS) to the contradictory signal in the allotriocaridan phylogeny, leading to limited support for any potential relationships between Branchiopoda, Cephalocarida and Copepoda. This study suggests the need for further examination of other potential sources of signal discordance, such as introgression and gene tree estimation error to fully understand the evolutionary history of Pancrustacea.

A new species of freshwater atyid shrimp, Caridina ngankeeae sp. nov., is described from Hong Kong, southern China. The dorsally armed rostrum, the large number of spiniform setae on the uropodal diaeresis, the relatively long stylocerite and the distally deeply excavated pereiopod 1 carpus indicate a close relationship with the C. serrata Stimpson, 1860 species group and allied species primarily distributed in southern China and northern to central Vietnam. The new species can be distinguished from congeners by characteristics of the rostrum, pereiopods, and male first and second pleopods.

The acquisition of microbial symbionts enables animals to rapidly adapt to and exploit novel ecological niches, thus significantly enhancing the evolutionary fitness and success of their hosts. However, the dynamics of host-microbe interactions and their evolutionary implications remain largely underexplored in marine invertebrates. Crabs of the family Sesarmidae (Crustacea: Brachyura) are dominant inhabitants of mangrove forests and are considered keystone species there. Their rapid diversification, particularly after adopting a plant-feeding lifestyle, is believed to have been facilitated by symbiotic gut microbes, enabling successful colonization of intertidal and terrestrial environments. To investigate the patterns and mechanisms shaping the microbial communities and the role of microbes in the evolution of Sesarmidae, we characterized and compared the gut microbiome compositions across 43 crab species from Sesarmidae and other mangrove-associated families using 16S metabarcoding. We found that the gut microbiome assemblages in crabs are primarily determined by host identity, with a secondary influence from environmental factors such as microhabitat and sampling location, and to a lesser extent influenced by biological factors such as sex and gut region. While patterns of phylosymbiosis (i.e. when microbial community relationships recapitulate the phylogeny of their hosts) were consistently observed in all beta-diversity metrics analysed, the strength of phylosymbiosis varied across crab families. This suggests that the bacterial assemblages in each family were differentially shaped by different degrees of host filtering and/or other evolutionary processes. Notably, Sesarmidae displayed signals of cophylogeny with its core gut bacterial genera, which likely play crucial functional roles in their hosts by providing lignocellulolytic enzymes, essential amino acids, and fatty acids supplementation. Our results support the hypothesis of microbial contribution to herbivory and terrestrialization in mangrove crabs, highlighting the tight association and codiversification of the crab holobiont.

Background Sesarmid crabs dominate mangrove habitats as the major primary consumers, which facilitates the trophic link and nutrient recycling in the ecosystem. Therefore, the adaptations and mechanisms of sesarmid crabs to herbivory are not only crucial to terrestrialization and its evolutionary success, but also to the healthy functioning of mangrove ecosystems. Although endogenous cellulase expressions were reported in crabs, it remains unknown if endogenous enzymes alone can complete the whole lignocellulolytic pathway, or if they also depend on the contribution from the intestinal microbiome. We attempt to investigate the role of gut symbiotic microbes of mangrove-feeding sesarmid crabs in plant digestion using a comparative metagenomic approach. Results Metagenomics analyses on 43 crab gut samples from 23 species of mangrove crabs with different dietary preferences revealed a wide coverage of 127 CAZy families and nine KOs targeting lignocellulose and their derivatives in all species analyzed, including predominantly carnivorous species, suggesting the crab gut microbiomes have lignocellulolytic capacity regardless of dietary preference. Microbial cellulase, hemicellulase and pectinase genes in herbivorous and detritivorous crabs were differentially more abundant when compared to omnivorous and carnivorous crabs, indicating the importance of gut symbionts in lignocellulose degradation and the enrichment of lignocellulolytic microbes in response to diet with higher lignocellulose content. Herbivorous and detritivorous crabs showed highly similar CAZyme composition despite dissimilarities in taxonomic profiles observed in both groups, suggesting a stronger selection force on gut microbiota by functional capacity than by taxonomy. The gut microbiota in herbivorous sesarmid crabs were also enriched with nitrogen reduction and fixation genes, implying possible roles of gut microbiota in supplementing nitrogen that is deficient in plant diet. Conclusions Endosymbiotic microbes play an important role in lignocellulose degradation in most crab species. Their abundance is strongly correlated with dietary preference, and they are highly enriched in herbivorous sesarmids, thus enhancing their capacity in digesting mangrove leaves. Dietary preference is a stronger driver in determining the microbial CAZyme composition and taxonomic profile in the crab microbiome, resulting in functional redundancy of endosymbiotic microbes. Our results showed that crabs implement a mixed mode of digestion utilizing both endogenous and microbial enzymes in lignocellulose degradation, as observed in most of the more advanced herbivorous invertebrates.

A comprehensive molecular analysis of the deep-sea blind lobsters of the family Polychelidae, often referred to as "living fossils", is conducted based on all six modern genera and 27 of the 38 extant species. Using six genetic markers from both mitochondrial and nuclear genomes, the molecular phylogenetic results differ considerably from previous morphological analyses and reveal the genera Polycheles and Pentacheles to be para- or polyphyletic. As the splitting of Polycheles has strong support from both molecular and morphological data, two new genera, Dianecheles and Neopolycheles, are erected for those species excluded from the clade containing the type species of Polycheles. The pattern of polyphyly of Pentacheles, however, is not robustly resolved, so it is retained as a single genus. Fossil evidence suggests that fossil polychelids inhabited deep-sea environments as early as the Early to Middle Jurassic, demonstrating the enduring adaptation of extant polychelid species to the deep-sea. Time-calibrated phylogeny suggested that modern polychelids probably had an Atlantic origin during the Jurassic period. Since their emergence, this ancient lobster group has continued to diversify, particularly in the West Pacific, and has colonized the abyssal zone, with the deepest genus, Willemoesia, representing the more 'derived' members among extant polychelids. Differences in eye reduction among extant polychelid genera highlight the necessity for ongoing investigations to ascertain the relative degree of functionality of their eyes, if they indeed retain any function.

The systematic position of three aberrant pinnotheroid genera: Tetrias Rathbun, 1898, Parapinnixa Holmes, 1895 and Sakaina Serène, 1964, are reappraised. A new family, Tetriasidae fam. nov. is established for Tetrias, and Parapinnixidae Števčić, 2005, is recognised for Parapinnixa and Sakaina. Tetriasids differ from all other pinnotheroids in that the articles of the palp of the third maxilliped (carpus, propodus and dactylus) are large and the dactylus is inserted at the distolateral corner of the propodus, whereas parapinnixids are distinctive in having the buccal cavern relatively small, subtriangular with the third maxilliped ischiomerus small, mesioproximally produced to form a triangular projection and is completely fused. The two families are phylogenetically distinct from the family Pinnotheridae based on the multi-locus gene tree, and are basal or near basal lineages in the Pinnotheroidea.

The pea crabs, superfamily Pinnotheroidea, are exceptional among brachyuran crabs in their diverse symbiotic associations involving both inquilinism and protective symbiosis. While this group presents a rare opportunity for evolutionary comparative study of host switching and morphological evolution in marine macroinvertebrates, previous phylogenetic studies have been focused on systematics. Here, we reconstructed the most extensive phylogeny of Pinnotheroidea based on two mitochondrial and six nuclear markers, with the aim of elucidating the host switching pathways and the correlation between symbiotic lifestyles and selected morphological adaptations. Ancestral state reconstruction of host association revealed a monophyletic origin of symbiosis in the form of inquilinism. Subsequent shifts in microhabitat preference for burrows or worm tubes, and the move to protective symbiosis, primarily in the switch to mollusc endosymbiosis, contributed to radiation in Pinnotheridae. Further parallel colonisations of echinoderms and tunicates occurred but did not lead to extensive diversification, except in the Clypeasterophilus + Dissodactylus lineage, which experienced a unique switch to echinoderm ectosymbiosis. The evolution of the third maxillipeds, carapace shape and ambulatory pereiopods suggests a rather strong coupling with the symbiotic lifestyle (whether inquilinism or protective symbiosis). Phenotypic diversity of these characters was higher among species engaged in protective symbiosis, with convergence in form (or function) among those sharing the same host affiliation. Species having different host affiliations or symbiotic lifestyles might also exhibit convergence in the form of the three morphological traits, suggesting a common adaptive value of the specialisations. Pinnotherid crabs overall exhibited a lower trait diversity than the also symbiotic palaemonid shrimps with comparable species diversity. This may plausibly be attributed to differences in potential for morphological modification to serve additional functions among the traits analysed in the two groups, the less frequent host switching and the less diverse host affiliations, and thus a less complicated evolutionary history in pinnotherids.

The poorly known pilumnid crab Cryptocoeloma haswelli Rathbun, 1923, is redescribed and figured at length on the basis of fresh specimens from Hong Kong and Taiwan, both of which constitute new local records. The male is redescribed and figured as the only previous account of it had been based on a gynandromorph. The species is also compared in detail with Heteropilumnus fimbriatus (H. Milne Edwards, 1834) with which it has been previously confused.

The Thoracotremata is a large and successful group of “true” crabs (Decapoda, Brachyura, Eubrachyura) with a great diversity of lifestyles and well-known intertidal representatives. The group represents the largest brachyuran radiation into terrestrial and semi-terrestrial environments and comprises multiple lineages of obligate symbiotic species. In consequence, they exhibit very diverse physiological and morphological adaptations. Our understanding of their evolution is, however, largely obscured by their confused classification. Here, we resolve interfamilial relationships of Thoracotremata, using 10 molecular markers and exemplars from all nominal families in order to reconstruct the pathways of lifestyle transition and to prepare a new taxonomy corresponding to phylogenetic relationships. The results confirm the polyphyly of three superfamilies as currently defined (Grapsoidea, Ocypodoidea and Pinnotheroidea). At the family level, Dotillidae, Macrophthalmidae, and Varunidae are not monophyletic. Ancestral state reconstruction analyses and divergent time estimations indicate that the common ancestor of thoracotremes already thrived in intertidal environments in the Late Cretaceous and terrestrialization became a major driver of thoracotreme diversification. Multiple semi-terrestrial and terrestrial lineages originated and radiated in the Early Eocene, coinciding with the global warming event at the Paleocene-Eocene Thermal Maximum (PETM). Secondary invasions into subtidal regions and colonizations of freshwater habitats occurred independently through multiple semi-terrestrial and terrestrial lineages. Obligate symbiosis between thoracotremes and other marine macro-invertebrates evolved at least twice. On the basis of the current molecular phylogenetic hypothesis, it will be necessary in the future to revise and recognize seven monophyletic superfamilies and revisit the morphological character states which define them.