Fig. 1
Remarks. At the time of writing, 36 species have been described in the Family Agathotanaidae, distributed in the four genera Agathotanais Hansen, 1913, Paragathotanais Lang, 1971, Metagathotanais Bird & Holdich, 1988, and Paranarthrura Hansen, 1913. The enigmatic Paranarthrurella Lang, 1971, considered to be very doubtfully placed in the Agathotanaidae by Bird & Holdich (1989), has recently been revised and unambiguously removed from the family, albeit placed with an incertae sedis category (Jóżwiak et al 2009). A recent paper on agathotanaids from Japanese waters by Larsen (2007) usefully summarised taxonomic changes and issues up to that date. Phylogenetic analyses are undoubtedly required to interrogate possible paraphyletic genera such as Paranarthrura but are beyond the scope of this paper. These may be linked to ongoing studies following Bird & Larsen (2009).
A remaining taxonomic issue is the structure of the cheliped-cephalothorax articulation, i.e. whether it is formed from a ‘sclerite’ or 'pseudocoxa' (see Larsen 2007). It is evident that the cheliped in all agathotanaids is attached to the cephalothorax via a sclerite, with the cheliped basis lacking any posterior free process (c.f. Tanaidae, Leptocheliidae and Paratanaidae, etc.). This arrangement is variable, with some taxa having discrete and protuberant sclerites (e.g. Paranarthrura insignis, Bird & Holdich 1989, fig 1 j) and others with fused sclerites, with or without a gap between the cheliped bases (Agathotanais ingolfi, Bird & Holdich 1988, fig 2g and Metagathotanais insulcatus, Bird & Holdich 1988, fig 2 k). Phylogenetic analysis of these and other cheliped-cephalothorax morphologies is ongoing.
Morphometric data and the inferred life-history patterns suggest that, at least for Agathotanais ingolfi, Paragathotanais robustus, P. nanus and Paranarthrura insignis, two male phases exist: a (smaller) primary male developing directly through several (indeterminate) moults from the manca/juvenile and a (larger) secondary male developing from a female—i.e. sequential hermaphroditism.