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Larval development of the crab Amphithrax hemphilli (Rathbun, 1892) (Decapoda, Brachyura, Mithracidae) described from laboratory-reared material
Abstract
The complete larval development of Amphithrax hemphilli was described, illustrated, and compared with that of the previously described larvae of the genus Mithrax sensu lato. Specimens of A. hemphilli were sampled from the northeastern Brazilian coast. The larval development of A. hemphilli consisted of two zoeal stages and one megalopa. Amphithrax hemphilli shows morphological features in all stages of larval development that differ from those observed in other species of Amphithrax, Mithrax, and Maguimithrax. In the first larval stage, A. hemphilli was the only species with two aesthetascs on the antennule and the coxal endite of the maxillule with five setae. In the second larval stage, A. hemphilli was distinguished from the other species of the genus Mithrax sensu lato by the presence of five setae on the basial endite of the maxilla and 26 setae in the scaphognathite of the maxilla. Finally, the megalopa stage of A. hemphilli is characterized by the number of aesthetascs on the three-segmented exopod of the antennule, the setation of both the endopod of the third maxilliped, and pleonites 1–6. Based on the results obtained in the present study, we verified that the more advanced the larval stage, the more distinct characteristics are observed among species of the genus Mithrax sensu lato, facilitating the identification of species through later larval stages.
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The complete larval development of the spider crab Maguimithrax spinosissimus (Lamarck, 1818) is re-described and illustrated in detail from laboratory-reared material. The development consisted of the typical pattern reported for the Majoidea, two zoeal stages and one megalopa. The complete larval development from hatching to first crab lasted 5-6 days at temperatures that ranged between 24-28°C. Both zoeal stages of M. spinosissimus exhibited moderate reduction in the number of setae in the maxilla and maxillipeds, from the first to the second zoeal stage, when compared with other closely related species. Maguimithrax spi-nosissimus can be easily distinguished from other species belonging to the closely related genus Mithrax by the (i) setation of the endopod of the maxillule, maxilla and second max-illiped in both zoeal stages; (ii) setation of the scaphognathite of the maxilla in the first zoeal stage; (iii) setation of the basis of maxilliped I in the second zoeal stage and megalopa; (iv) morphology of the antennule and antenna in the second zoeal stage; and (v) setation of the antennule, coxal endite of maxilla, and exopod of second maxilliped in the megalopa. All these characters support the recent generic status of Maguimithrax within the Mithracidae. Additional morphological details, not available previously, are provided. This study will provide support for conservation strategies in this species.
The cuticle plays an important role in many aspects of crustacean biology, since it is the interface to the surrounding world. Thus, the cuticle displays many structural specializations all over the body. The structures considered here are setae, setules, denticles, and spines. Seven types of setae are recognised based on their detailed external morphology: plumose, pappose, composite, serrate, papposerrate, simple, and cuspidate. In support of the categorization of these setae, each seems to correlate with a specific functional outcome such as feeding, grooming, and locomotion. Little can be learned about the sensory functions from the external morphology of setae, but their ultrastructure seems to provide better cues. In particular, mechanoreceptors display structures related to transduction mechanisms, with the scolopale as a good example. Still, too few data are available outside malacostracans to draw general conclusions for all crustaceans, underlining the need for multidisciplinary and broad intertaxon studies.
Mithracid crabs comprise a primarily subtidal reef- and rubble-dwelling group inhabiting both tropical and subtropical seas. Despite their relative ubiquity in many hard-substrate environments, there has been little consensus about their phylogenetic relationships or whether their group rank should be that of subfamily or family. We have used a combined molecular dataset of two nuclear (18S, H3) and three mitochondrial (12S, 16S, COI) genes to build a preliminary molecular phylogeny of Majoidea in order to examine the membership of Mithracidae. We then built a second molecular phylogeny based on three mitochondrial genes to assess the internal composition of the family, and conducted comparative morphological examinations of genera and species that resolved in unexpected positions on the phylogram. Four genera are designated under new or resurrected names on the basis of molecular and morphological characters, while memberships of several other existing genera are modified. Following review of molecular and morphological characters, the genera Coelocerus, Cyclocoeloma, Cyphocarcinus, Leptopisa, Micippa, Picrocerodes, Stenocionops and Tiarinia are provisionally excluded from Mithracidae s.s., while Hemus and Pitho are included in it. A key to genera of Mithracidae is provided.
There is a need for improvement and standardization in larval descriptions for them to become useful not only for identification purposes but also in comparative and phylogenetic studies. Details of the rearing technique and equipment used should be given, the spent female crab and representative larval stages need to be deposited at a national museum and a minimum of five specimens of each stage should be used for meristic and morphometric criteria. Diagnostic features of the carapace, all appendages, abdomen, and other characters that are to be included in a standard larval description are presented with specifications concerning format and sequence of substructures. Typical configurations of larval structures are given and some possible exceptions are described. Guidelines for illustrations including the preparation, size, sequence and arrangement of figures are provided.
The present work describes the complete larval development of Mithraculussculptus (two zoeal stages, the megalopa) and the first crab instar from laboratory cultured material. The larval morphology is compared with other descriptions currently available for the MithraxMithraculus complex display uniform morphological characters, the first zoeal stage of M.sculptus differs from other species in the setal meristics of the carapace and the number of aesthetascs of the antennule. The second zoeal stage differs in the number of aesthetascs of the antennule and the number of setae in the distal margin of the coxal endite of the maxillule. The megalopa of M.sculptus can be distinguished by the presence of 3–4 aesthetascs and a simple seta in the distal segment of the antennule. The morphological differences between the larvae from the genus Mithrax and Mithraculus are insufficient to support the separation of the two genera using adult morphology. Future studies should address in detail setal meristics.
Mithrax hispidus (Herbst, 1790) is a mithracid majoid crab occurring on sand, corals and rocks in waters of the western Atlantic. Larval development consists of two zoeal stages and a megalopa. All larval stages are described in detail based on multiple cultures. Prior to this study, larvae of M. hispidus were considered to be different and grouped separately from most other larvae of Mithrax, primarily based on setation. A detailed morphological examination, based on the same specimens used for the first description, revealed that the inclusion of M. hispidus in a separate group is not valid as zoeae now fully agree with the morphological characteristics defined for the other group of five Mithrax species, including M. pleuracanthus, M. verrucosus, M. caribbaeus, M. coryphe, and M. forceps. This illustrates the importance of precisely recording morphological details such as setation, which may otherwise lead to incorrect interpretations with regard to perceived taxonomic affinities. A comparison of larvae of the Mithrax-Mithraculus species complex does not support separation into two genera. Larval evidence supports the recently suggested adult-based synonymization of M. caribbaeus with M. hispidus.
We re-evaluated the larval support for families within majoids using the Wilcoxon signed-rank test with emphasis on Inachoididae. To accomplish our objectives, we added 10 new taxa, two of which are traditionally assigned to the family of special interest, to a previous larval database for majoids, and re-appraised the larval characters used in earlier studies. Phylogenetic analysis was performed with PAUP* using the heuristic search with 50 replicates or the branch-and-bound algorithm when possible. Multi-state transformation series were considered unordered; initially characters were equally weighted followed by successive weighting, and trees were rooted at the Oregoniidae node. Ten different topological constraints were enforced for families to evaluate tree length under the assumption of monophyly for each taxonomic entity. Our results showed that the tree length of most constrained topologies was not considerably greater than that of unconstrained analysis in which most families nested as paraphyletic taxa. This may indicate that the present larval database does not provide strong support for paraphyly of the taxa in question. For Inachoididae, although the Wilcoxon signed-rank test rejected a significant difference between unconstrained and constrained cladograms, we were unable to provide a single synapomorphy for this clade. Except for the conflicting position of Leurocyclus and Stenorhynchus, the two clades correspond to the traditional taxonomic arrangement. Among inachoidids, the clade (Anasimus (Paradasygyius (Collodes +Pyromaia))) is supported, whereas for inachids, the clade (Inachus (Macropodia +Achaeus)) is one of the most supported clades within majids. As often stated, only additional characters will provide a better test for the monophyly of Inachoididae and other families within Majoidea.
Mithrax hispidus, M. caribbaeus, M. pleuracanthus, and M. tortugae are closely related shallow-water crabs that are difficult to distinguish by morphology alone. This led to recent synonymy of the four species under M. hispidus (Herbst, 1790). The use of three mitochondrial genes (12s, 16s, and COI) nevertheless provides evidence for three distinct species (M. hispidus, M. pleuracanthus, and M. tortugae) and the synonymy of M. caribbaeus with M. hispidus. Morphological features of the merus and carpus of the chelipeds serve as characters to separate the three species.
The crustacean cuticle has numerous projections and some of these projections, the setae, have important mechanical as well as sensory functions. The setae display a wide diversity in their external morphology, which has led to great problems separating setae from other projections in the cuticle and problems in making a consistent classification system. Here, the cuticular projections on the mouthparts of seven species of decapods are examined by scanning and transmission electron microscopy. A new definition is given: a seta is an elongate projection with a more or less circular base and a continuous lumen; the lumen has a semicircular arrangement of sheath cells basally. From the details of the external morphology the mouthpart setae are divided into seven types: pappose, plumose, serrulate, serrate, papposerrate, simple and cuspidate setae, which are suggested to reflect mechanical functions and not evolutionary history. This classification system is compared with earlier systems. © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society, 2004, 142, 233–252.
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.
The postembryonic larval stages of Mithrax tortugae are described, illustrated and compared with the described zoeae of other Mithrax species. Mithrax tortugae showed morphological features in all the stages of larval development that differed from those observed in other species of Mithrax, especially M. hispidus. In the Zoea I stage, M. tortugae and M. pleuracanthus lacked the minute spine on the dorsal spine observed in M. hispidus; M. tortugae exhibited a terminal spine on the inner lobe of the coxal endite of the maxilla, which was not observed in M. hispidus or M. pleuracanthus. Also, M. tortugae exhibited furcae with spines that are not spinulated, whereas in M. hispidus and M. pleuracanthus these spines are spinulated. In the Zoea II stage, M. tortugae showed a terminal spine on the coxal endite of the maxillula, whereas in M. hispidus and M. pleuracanthus this spine is absent. In the Megalopa stage, we also observed differences in the sternal plate setation between M. tortugae and M. hispidus, where M. tortugae had eight simple setae and M. hispidus showed two simple and four plumodenticulate setae. Partial sequences of the 16S rRNA and COI genes of the parental female were analysed, providing additional evidence for species identification. Together, our analyses of larval morphology and the results of the molecular analyses reinforced recognition of the relationships among M. tortugae, M. hispidus and M. pleuracanthus.
Mithracid crabs comprise a primarily subtidal reef- and rubble-dwelling group inhabiting both tropical and subtropical seas. Despite their relative ubiquity in many hard-substrate environments, there has been little consensus about their phylogenetic relationships or whether their group rank should be that of subfamily or family. We have used a combined molecular dataset of two nuclear (18S, H3) and three mitochondrial (12S, 16S, COI) genes to build a preliminary molecular phylogeny of Majoidea in order to examine the membership of Mithracidae. We then built a second molecular phylogeny based on three mitochondrial genes to assess the internal composition of the family, and conducted comparative morphological examinations of genera and species that resolved in unexpected positions on the phylogram. Four genera are designated under new or resurrected names on the basis of molecular and morphological characters, while memberships of several other existing genera are modified. Following review of molecular and morphological characters, the genera Coelocerus, Cyclocoeloma, Cyphocarcinus, Leptopisa, Micippa, Picrocerodes, Stenocionops and Tiarinia are provisionally excluded from Mithracidae s.s., while Hemus and Pitho are included in it. A key to genera of Mithracidae is provided.
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.
The complete larval development of the spider crab Mithrax (Mithrax) pleuracanthus from hatching to first crab stage was obtained by culture in the laboratory. A prezoea, two zoeal stages, and one megalopal stage are described and illustrated. Larvae were reared in 12 different combinations of temperature and salinity. Best survival to first crab stage occurred at 25°C and salinities of 30‰ and 35‰ and at cyclic temperatures of 30-35°C in 35‰ salinity. Development to first crab was 14 days (at 25°C) and 9-10 days (at 30°C, cyclic 30-35°C). Morphological characters of M. pleuracanthus larvae were compared with those of two other members within the genus. The subgenera Mithraculus and Mithrax were originally based on only adult characters, but it is suggested that a reevaluation of these subgeneric diagnoses should be considered to include larval characters.
The Western Atlantic species formely assigned to the (sub)genera Mithrax and Mithraculus are revised. Mithrax and Mithraculus are treated here as two distinct genera based on seven (relative) constant generic characters, keys to the various species are given. All species are discussed with full account of their synonymy and distribution.
Crabs from the Mithrax–Mithraculus species complex are known for their diversity of lifestyles, habitats, and coloration. This group includes small, colourful, symbiotic species and much larger, reef-dwelling crabs targeted by fishermen. The evolutionary relationships between the species within this complex are not well-defined. Previous studies based upon morphological characters have proposed the separation of this complex into two genera (Mithrax and Mithraculus), but cladistic analyses based upon larval characters do not support this division. A molecular phylogeny of the group may help to resolve this long-standing taxonomic question and shed light on the ecological conditions driving the diversity of these crabs. Using a 550-bp alignment of the 16S rRNA mitochondrial DNA segment we examined the phylogenetic relationships between 8 species within the Mithrax–Mithraculus complex native to the Caribbean. The resulting phylogeny indicates that this complex is paraphyletic, as it includes the genus Microphrys. The analyses revealed a well-supported, monophyletic group containing four species of Mithraculus (M. cinctimanus, M. coryphe, M. sculptus and M. forceps) and supported one pair of sister species from the genus Mithrax (M. caribbaeus and M. spinosissimus). No complete segregation of species, according to genera, was evident, however, from tree topologies. Bayesian-factor analyses revealed strong support for the unconstrained tree instead of alternative trees in which monophyly of the two genera was forced. Thus, the present molecular phylogeny does not support the separation of the species within this complex into the genera Mithrax and Mithraculus. A review of the literature demonstrated considerable phenotypic variation within monophyletic clades in this group.
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