Functional morphology of the seminal receptacle of Mithrax, Mithraculus and Omalacantha spider crabs (Brachyura: Mithracidae).

Most Majoidea crabs display high sperm competition rates due to the development of spermatic strata within the seminal receptacle (SR). To verify the occurrence of sperm competition, SR anatomy and histochemistry analyses were performed in Mithrax hispidus, Omalacantha bicornuta, and Mithraculus forceps. The SR of the three Mithracidae species is classified into two regions, one of mesodermal origin (dorsal), consisting of a stratified epithelium with desquamation cells that produce a holocrine secretion, and the other, an ectodermal region (ventral) comprising a simple cubic epithelium covered by a cuticle. The oviduct opens in a ventral position near the transition region, which exhibits more pronounced folds on the opposite face of the oviduct, which may aid fertilization due to the presence of an external musculature. Sperm masses in a circular format were observed in the SR lumen, reminiscent of coenospermic spermatophores, and no morphological evidence strata of sperm packets were observed in any of the three studied species. The ventral SR followed the most common pattern observed in Majoidea. The secretion produced in the receptacle is composed of glycoproteins, with neutral polysaccharides. Acidic polysaccharides probably play a role against microorganisms from male seminal fluid. Due to the absence sperm packets, we were unable to determine whether the investigated females receive material from more than one male nor (if this does, in fact, occur) whether any preference for the spermatozoa of one male over another takes place during fertilization, that can may indicate the absence of sperm competition in the investigated species.

Much more than hooked: Setal adaptations for camouflage in Macrocoeloma trispinosum (Latreille, 1825) (Crustacea: Decapoda: Brachyura).

Several majoid crabs are known to adhere exogenous materials to their bodies, a behaviour called decoration. Until now, the adhesion of exogenous materials to the body is most attributed to the well-known hooked setae. Here, we analysed the carapace of Macrocoeloma trispinosum (Latreille, 1825) under light and electron microscopy to study the different mechanisms allowing majoid crabs to decorate themselves. Five setal types are described here, of which four for the first time: velvet type I, velvet type II, depressa and cattail seta. These setae are morphologically and histologically detailed, and new hypotheses about the fixation of exogenous material on the carapace are explored. M. trispinosum has a complex setal apparatus for the adhesion of the decoration, with tegumental ducts along the shaft of most setae. These tegumental ducts are connected to glands formed by large cells arranged radially (rosette or acini) at the base of the setae, in the connective tissue, just below the epithelium. We could observe these glands in different stages of maturation, and no valve-like structure was observed, which may indicate a continuous flow of protein secretion that could serve as an adhesive substance found in the apex of most setae. This is the first record indicating a potential chemical adhesion mechanism aiding the masking process in decorator crabs.

Revision of Majella Ortmann, 1893 (Crustacea: Brachyura: Majidae), with Description of Two New Species from the Indian Ocean.

The poorly known majid genus Majella Ortmann, 1893, is revised. The genus was previously known only from one species, M. brevipes Ortmann, 1893, described from Japan and reported from east Africa. Majella brevipes is redescribed and figured in detail from the type and material from the type locality, Sagami Bay in Japan. This species is now restricted to Japan. Specimens from east Africa are herein described as two new species: M. skolopion n. sp. and M. pristis n. sp.; they differ markedly from M. brevipes (now restricted to Japan) in the arrangement of spines on the carapace and pereopods, third maxillipeds, male pleon and gonopods.

Setal descriptions, locations, and abundances: Their impacts on decorating behaviors in the spider crab Libinia (Crustacea: Epialtidae).

The setal abundances and distributions of the spider crab Libinia are examined, using two species. These spider crabs decorate their bodies with environmental materials and camouflage from predators. These decorations attach to hooked setae, but other types setae are prevalent in body regions where decorations occur. Adult crabs undergo a terminal molt and lose the ability to regrow setae. Little is known about setal changes over the lifetime in these crabs and if hooked setae abundance decreases with age, resulting in a loss of camouflage strategy in larger individuals. The objective of this study is to determine the prevalence and abundance of hooked and non-hooked setae in the decorated body regions and if these setal patterns vary by species and body size. Scanning electron microscopy is used along with computer software to describe, count, and measure the setal coverage of hooked and non-hooked setae in body regions. Small individuals have more hooked setae than larger individuals and the two species have different setal abundances of hooked and non-hooked setae. Non-hooked setae cover much surface areas of crabs, attach decorations, and remain on the crabs even when hooked setae are damaged and broken. Setal morphologies and fouling are described among different sized crabs and the two species, with setal types and morphologies being similar but fouling being different. Large individuals likely employ a different camouflage strategy than small individuals due to losing hooked setae but retaining non-hooked setae.

Complete mitochondrial genome of Oregonia gracilis Dana, 1851 (Crustacea: Decapoda: Majoidea).

The complete mitochondrial genome of the majoid crab, Oregonia gracilis, was determined from a specimen collected in Korea. The mitochondrial genome is 15,737 bp long and contains 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, and two ribosomal RNA (rRNA) genes. A maximum-likelihood phylogenetic tree based on the 13 PCGs of the mitochondria showed that O. gracilis is closely related to the genus Chinoecetes. The complete mitochondrial genome of O. gracilis provides valuable information on the mitochondrial evolution of majoid crabs.

Male reproductive strategies in two species of spider crabs, Leurocyclus tuberculosus and Libinia spinosa.

Reproductive strategies vary according to season length, individual reproductive traits, and factors associated with the social context such as density and sex ratio of the individuals. The social context predicts the intensity of sexual selection due to its effect on mate choice and intra-sexual competition. Moreover, it exacerbates the costs derived from sexual conflict and allows for the existence of alternative reproductive tactics to counteract such costs. However, these mechanisms are still underexplored in many animal taxa. Majoidea "spider crabs" show a diversity of life history, and behavioral and morphological adaptations for reproduction, which have evolved as an outcome of competition to maximize reproductive success. We aimed to analyze in this study, male reproductive strategies through mating behavior under different social contexts, density of individuals, and males size differences in two species of "spider crabs", Leurocyclus tuberculosus and Libinia spinosa. The experiments were as follows: one female and one male per aquarium (Experiment 1); a female and three males of similar sizes per aquarium (Experiment 2); and a female and three males of distinct sizes per aquarium (Experiment 3). The experiments showed that mating behavior and duration of mate guarding differ between species and among different social contexts. The presence of males with similar or different sizes showed a relevant different behavior, the female copulated with more than one male when males were similar in size (E2), and copulated only with the largest male when males were different in size (E3). In mate guarding, the shape and the use of the chelipeds had an important role: the male of Leurocyclus tuberculosus grasped the female with the chelipeds and did not fight with other males, but the male of Libinia spinosa surrounded the female with the chelipeds and fought with other males. The longest duration of mate guarding was during precopulatory guarding. Precopulatory guarding was present in all the experiments and its duration did not present significant differences among the experiments. However, in both species, the experiments differ in the duration of postcopulatory guarding until spawning. Gonopod morphology revealed important differences between species and are probably associated with sperm competition. The experiments under distinct social contexts linked to the use and shape of the chelipeds and the morphology of the gonopods, could allow to understand the functional significance of the different reproductive strategies.

Extraordinary majoid crabs: the genus Esopus A. Milne-Edwards, 1875 in the new subfamily Esopinae subfam. nov., and erection of Paulitinae subfam. nov. (Crustacea, Decapoda, Brachyura, Majoidea, Inachoididae Dana, 1851).

A rare small species, Esopus crassus A. Milne-Edwards, 1875, recently collected by KARUBENTHOS Expedition 2015 in Guadeloupe, is re-examined. The genus Esopus A. Milne-Edwards, 1875, currently included in the Epialtidae MacLeay, 1838, must be assigned to the Inachoididae Dana, 1851, a rather basal family within the Majoidea Samouelle, 1819, but deviates from the morphotype that is being traditionally associated to this group. It deserves its own subfamily, Esopinae subfam. nov., besides other inachoidid subfamilies, for which a description is here provided (Collodinae Stimpson, 1871; Dasygyiinae Holmes, 1900; Inachoidinae Dana, 1851; Salaciinae Dana, 1851; Stenorhynchinae Dana, 1851). Another inachoidid subfamily is erected here, Paulitinae subfam. nov., for the genus Paulita Guinot, 2012, monotypic with P. tuberculata (Lemos de Castro, 1949, as Dasygyius tuberculatus). A reliable fossil member is recorded from the lower Miocene onwards.

The complete mitogenome of the Chionoecetes opilio (Crustacea: Decapoda: Oregoniidae) and its unique characteristics.

The complete mitochondrial genome of Chionoecetes opilio is a 16,067 bp long, circular molecule which contains 13 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs), and 2 ribosomal RNA genes (rRNAs). Its gene contents and organization are generally similar to other majoid mitogenomes. However, the mitogenome shows unique characteristics; long terminal amino acids, loss or addition of 3 PCGs, a 1216 bp long putative D-loop region, and peculiar secondary structures of 5 tRNAs. The concatenated amino acid sequences of 13 PCGs were used to analyze the phylogenetic tree, which well supported the monophyly of brachyuran clades of Majoidea, Heterotremata, Thoracotremata, and Eubrachyura.

The complete mitochondrial genome of Chionoecetes japonicus (Crustacea: Decapoda: Majoidea).

The complete mitochondrial genome of Chionoecetes japonicus was sequenced using a specimen collected offshore in the East Sea. The genome includes 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, two ribosomal RNA (rRNA) genes, and a control region (D-loop), with a total length of 16,060 bp. The overall nucleotide composition was 34.91% A, 17.29% C, 10.93% G, and 36.87% T, with 71.78% A + T. In the phylogenetic tree was constructed using maximum-likelihood and Bayesian inference analyses, C. japonicus and C. japonicus pacificus formed a genetic clade that was sister to C. opilio.

Hyastenus verreauxii A. Milne-Edwards, 1872, a synonym of Hyastenus elatus Griffin Tranter, 1986: lectotype designation and reversal of precedence (Crustacea: Brachyura: Majoidea: Epialtidae).

The majoid spider crab, Hyastenus verreauxii A. Milne-Edwards, 1872 (family Epialtidae), was described based on the material from "Nouvelle-Hollande" (= Australia) in the "Collection du Muséum", that is, the Muséum national d'Histoire naturelle, Paris (MNHN). It was noted by A. Milne-Edwards (1872: 250) that the species is morphologically similar to H. diacanthus (De Haan, 1839) but differs in having a less developed hepatic region, as well as longer and straighter pseudorostral spines. Miers (1879: 26) synonymised H. verreauxii under H. diacanthus with no detailed explanation, and Miers (1886: 56) noted that this species "is probably a variety of Hyastenus diacanthus". Haswell (1880: 442; 1882: 20) identified Australian specimens as H. diacanthus and listed H. verreauxii as its junior synonym. Ortmann (1893: 55) followed the consensus but regarded H. verreauxii as a subspecific taxon ("var.") of H. diacanthus. Serène Lohavanijaya (1973: 53) treated H. verreauxii as a synonym of H. diacanthus. However, none of these authors provided a detailed explanation for the decision to synonymise this species nor was the type examined. This species has not been treated by subsequent workers.

A new species of deep-water spider crab of the genus Paramaya De Haan, 1837 from the Bay of Bengal, India (Crustacea, Brachyura, Majidae).

The identity of the majid species of Paramaya De Haan, 1837, in the Indian Ocean is clarified with the collection of fresh specimens from the Bay of Bengal. Previously identified as P. spinigera (De Haan, 1837) which is known only from Japan, Taiwan, and Korea, the material from eastern India is here referred to a new species, P. mullisp. n. The new species can easily be distinguished from all congeners by its relatively shorter pseudorostral and carapace spines, more swollen branchial regions, distinctly granulated male thoracic sternum, and the G1 is not prominently curved with the dorsal projection on the sub distal part short and the tip rounded.

From an Old Eroded Carapace: Rediscovery of the Majid Crab Leptomithrax sinensis Rathbun, 1916 (Crustacea, Brachyura, Majidae) from Taiwan and Japan.

Kingsley J. H. Wong, Peter K. L. Ng, and Ming-Shiou Jeng (2018) The majid crab Leptomithrax sinensis Rathbun, 1916 was previously only known from the holotype, a detached and partially eroded carapace collected during the Albatross Philippine Expedition in 1908 from the northern part of the South China Sea. Recent collections of fresh material from precious coral harvest sites off northeastern Taiwan, and Shikoku, Japan made verifying this poorly known species possible. The species is considered to be valid and is here redescribed, illustrated, and compared with its closest East Asian congener, L. bifidus (Ortmann, 1893).

Ultrastructure of spermatozoa of spider crabs, family Mithracidae (Crustacea, Decapoda, Brachyura): Integrative analyses based on morphological and molecular data.

Recent studies based on morphological and molecular data provide a new perspective concerning taxonomic aspects of the brachyuran family Mithracidae. These studies proposed a series of nominal changes and indicated that the family is actually represented by a different number and representatives of genera than previously thought. Here, we provide a comparative description of the ultrastructure of spermatozoa and spermatophores of some species of Mithracidae in a phylogenetic context. The ultrastructure of the spermatozoa and spermatophore was observed by scanning and transmission electron microscopy. The most informative morphological characters analysed were thickness of the operculum, shape of the perforatorial chamber and shape and thickness of the inner acrosomal zone. As a framework, we used a topology based on a phylogenetic analysis using mitochondrial data obtained here and from previous studies. Our results indicate that closely related species share a series of morphological characteristics of the spermatozoa. A thick operculum, for example, is a feature observed in species of the genera Amphithrax, Teleophrys, and Omalacantha in contrast to the slender operculum observed in Mithraculus and Mithrax. Amphithrax and Teleophrys have a rhomboid perforatorial chamber, while Mithraculus, Mithrax, and Omalacantha show a wider, deltoid morphology. Furthermore, our results are in agreement with recently proposed taxonomic changes including the separation of the genera Mithrax (previously Damithrax), Amphithrax (previously Mithrax) and Mithraculus, and the synonymy of Mithrax caribbaeus with Mithrax hispidus. Overall, the spermiotaxonomy of these species of Mithracidae represent a novel set of data that corroborates the most recent taxonomic revision of the family and can be used in future taxonomic and phylogenetic studies within this family.

Spider crabs of the Western Atlantic with special reference to fossil and some modern Mithracidae.

Spider crabs (Majoidea) are well-known from modern oceans and are also common in the western part of the Atlantic Ocean. When spider crabs appeared in the Western Atlantic in deep time, and when they became diverse, hinges on their fossil record. By reviewing their fossil record, we show that (1) spider crabs first appeared in the Western Atlantic in the Late Cretaceous, (2) they became common since the Miocene, and (3) most species and genera are found in the Caribbean region from the Miocene onwards. Furthermore, taxonomic work on some modern and fossil Mithracidae, a family that might have originated in the Western Atlantic, was conducted. Specifically, Maguimithrax gen. nov. is erected to accommodate the extant species Damithrax spinosissimus, while Damithrax cf. pleuracanthus is recognized for the first time from the fossil record (late Pliocene-early Pleistocene, Florida, USA). Furthermore, two new species are described from the lower Miocene coral-associated limestones of Jamaica (Mithrax arawakum sp. nov. and Nemausa windsorae sp. nov.). Spurred by a recent revision of the subfamily, two known species from the same deposits are refigured and transferred to new genera: Mithrax donovani to Nemausa, and Mithrax unguis to Damithrax. The diverse assemblage of decapods from these coral-associated limestones underlines the importance of reefs for the abundance and diversity of decapods in deep time. Finally, we quantitatively show that these crabs possess allometric growth in that length/width ratios drop as specimens grow, a factor that is not always taken into account while describing and comparing among taxa.