Higher classification of the flabelliferan and related Isopoda based on a reappraisal of relationships

Abstract

The history of the systematics of isopod suborders is summarised. Several authors have suggested that the traditional suborder Flabellifera is paraphyletic and includes one or more of the suborders Gnathiidea, Epicaridea and Anthuridea. Two suborders, Cymothoida and Sphaeromatidea, have been proposed as replacement taxa for the Flabellifera, but it has not been possible on the basis of phylogenetic analyses to elucidate significant relationships between the suborders and families. Morphological characters are used to explore relationships between 35 genus-, family- and suborder-level taxa of flabelliferan Isopoda in a cladistic analysis (using Phreatoicidea and Asellota as outgroups) and to derive a new classification. The analysis did not find a synapomorphy for 'Flabellifera' sensu lato, but recognises two diverging clades of 'long-tailed' isopods. Members of the Oniscidea are not part of either clade. Nor are the Tainisopidea, a new suborder erected for members of the family Tainisopidae. The Tainisopidea has many synapomorphies and plesiomorphic features, but does not share characters with either clade. The first clade comprises Phoratopidea (for Phoratopus remex) and sister-taxa Cymothoida and Limnoriidea. Representatives of these suborders have uropodal rami ventral to the pleotelson and articulating from side-to-side inside the branchial space. The new suborder, Phoratopidea, is for one species with unique, broad articles of pereopods 3 and 4 with reduced dactyls. It lacks the synapomorphies of the following two suborders. In members of the suborder Cymothoida, the mandibular molar is either a flat triangular blade, reduced to a conical process, or absent, and the maxillipedal endite is rarely longer than palp article 1 (or is absent), distally tapering and has few setae. The suborder Limnoriidea is diagnosed as lacking the mandibular molar, and the non-tapering, slender (except in Keuphylia) maxillipedal endite reaches to at least the distal margin of palp article 4. Members of the second clade share a vaulted pleotelson enclosing a branchial chamber defined by ventrolateral ridges and uropods lateral to the pleotelson margin that fold down alongside the branchial space. It comprises two suborders. Members of the Sphaeromatidea have pleonite 1 much narrower than pleonite 2 and a reduced (or absent) right lacinia mobilis fused to the spine row. They lack operculiform uropods, which characterise Valvifera. The suborder Anthuridea is reduced to superfamily rank and Epicaridea is reduced to two superfamilies within Cymothoida. Unambiguous relationships between most families are resolved, but Sphaeromatidae is suspected to be paraphyletic, Paravireia is placed as the most plesiomorphic of the Sphaeromatoidea and a new family, Basserolidae, is proposed. The Tainisopidea includes freshwater taxa in a relictual environment. The sole species of Phoratopidea is marine, rare, and its ecology is unknown. The Cymothoida are most diverse in tropical regions. Members of the most plesiomorphic family, the Cirolanidae, are mobile predators or scavengers and the more derived families are ectoparasites on fishes and other crustaceans. Members of the Limnoriidea are mainly tropical and at least one family is herbivorous. The Valvifera and Sphaeromatidea are benthic, with respiratory pleopods in a branchial chamber. They are most diverse in the temperate southern hemisphere, and most are detritivores.