Spider crabs of the family Hymenosomatidae (Crustacea; Brachyura) with particular reference to Australian species: systematics and biology
The family Hymenosomatidae is revised as a result of discovering a large range of species in Australia. Three new genera and seven new species are described from the Australasian region. The genera Rhynchoplax Stimpson, Neorhynchoplax Sakai and Cyclohombronia Melrose are relegated to junior [synonymy] of Halicarcinus White, Elamenopsis A. Milne Edwards and Hymenosoma Desmarest, respectively. Trigonoplax H. Milne Edwards, treated by some authors as a subgenus of Elamena H. Milne Edwards, is given generic status on the basis of crab and larval morphology. The family now consists of ten genera and sixty-four species; seven genera, including all multi specific genera, and twenty-two species occur in Australia. An annotated key to all genera and species is provided. Diagnostic features for hymenosomatid larvae are given and larval morphology is considered in resolving several taxonomic problems. Absence of a megalopa larva is a notable family feature. Data on the life-cycles, reproduction, behaviour and ecology of hymenosomatids are reviewed. Fecundity is limited by the small sizes of these crabs and some females show morphological adaptations to increase shallow coastal waters and there has been a number of independent invasions of low fecundity by increasing larval survival. The majority of hymenosomatid species occur in shallow coastal waters and there have been a number of independent invasions of low salinity habitats. Twenty-four species, from six genera, occur in fresh and brackish water. Hymenosomatids reach high densities in some habitats and frequently occur in fish gut contents at various localities: they may play significant roles in some food webs. Most hymenosomatid species occur in the tropical and sub-tropical shelf waters of the Indo-West Pacific region and adjacent inland waters. Because they are inconspicuous, geographical distributions of many species are poorly known; the Indo-Malayan Region, which is central to their distribution, is particularly poorly collected.