Neoleptastacus acanthus, (Chappuis, 1954) (Chappuis, 1954) Sak et al., 2008

Sak, Serdar, Karaytuğ, Süphan & Huys, Rony, 2024, Review of Neoleptastacus Nicholls, 1945 (Harpacticoida, Arenopontiidae), including an updated key to species and proposal of Phreatipontia gen. nov., Zootaxa 5525 (1), pp. 1-66 : 6-8

(1) acanthus -group

Diagnosis. Anal somite with paired dorsolateral processes. Anal operculum weakly developed, without medial extension. P1 exp-1 with outer spine; exp-3 with four setae/spines (except N. huysi ). P1 enp-2 with outer spine and inner geniculate seta distally. P2 exp-2 with outer spine of normal length (not extending far beyond distal margin of exp-3). Endopod P2–P3 2-segmented. P2 enp-2 with or without inner seta; with two distal spines. P3 enp-2 with 1–2 distal spines. P4 enp-2 with well developed outer seta.

Subgroups included. acanthus -subgroup, gussoae -subgroup, ornamentus -subgroup.

Remarks. Members of this group can readily be identified by the presence of paired dorsolateral spinous processes on the anal somite. The presence of these backwardly directed extensions, not found in any other species of the Arenopontiidae , is an apomorphic character state supporting the monophyletic status of this lineage.

Various authors ( Itô 1978; Mielke 1982 b, 1987; Wells & Rao 1987) have recognized a gussoae -subgroup of morphologically extremely similar species characterized by the presence of only one spine on P3 enp-2: N. longiremis ( Chappuis, 1955) , N. indicus ( Rao, 1967) , N. gussoae ( Cottarelli, 1973a) and N. sakagamii ( Itô, 1978) . Neoleptastacus accraensis ( Lang, 1965) , which is generally believed to display this character (see below for a reinterpretation), has occasionally been cited as part of this species complex ( Mielke 1982 b, 1987). However, the absence of paired anal processes excludes this species from the acanthus -group. The 1-spine condition on the distal endopodal segment of P3 evolved convergently in the common ancestor of the trisetosus -group as well as in N. australis ( Chappuis, 1953) and N. reductaspina ( Mielke, 1987) ( Table 2 ). Wells & Rao (1987), inspired by the extensive variability observed in their material of N. indicus , proposed to sink N. sakagamii as a junior subjective synonym of the latter, but argued that that the caudal ramus and possibly P5 are sufficiently different to maintain A. gussoae as a distinct species. Neoleptastacus secundus should also be considered a member of the acanthus - group since Krishnaswamy (1957: 97) clearly stated that “...the anal segment bears two spines posteriorly” (hinted at in his illustration of the male habitus). Unfortunately, the P3 endopod was not figured and his statement that it resembles that of P2 (except for the absence of the inner seta on enp-1) has made most authors assume that P3 enp-2 bears two distal elements ( Lang 1965; Bodiou & Colomines 1986; Karanovic 2000). Given its close similarity to N. longiremis (see below), which bears one element on this segment, this character must await confirmation. The species is here treated as a species inquirenda in the gussoae -subgroup. Reassessment of Mielke’s (1982 b, 1987) descriptions of “ Arenopontia? gussoae ” resulted in the recognition of three additional members: N. abbreviatus sp. nov., N. chilensis sp. nov. and N. rectus sp. nov. Finally, Neoleptastacus emendatus sp. nov., proposed here (see below) for Arenopontia (Neoleptastacus) acantha accraensis Lang, 1965 sensu Kunz (1971) , also belongs to this subgroup which contains species from the northern Atlantic, Indian Ocean, Japan and the Pacific seaboard of South America.

The ornamentus -subgroup currently includes two closely related Chilean species, N. ornamentus ( Mielke, 1987) and N. reductaspina ( Mielke, 1987) , that display conspicuous surface ornamentation on the abdominal somites in the form of rectangular plates (areas of integumental reinforcement). Similar surface ornamentation has been reported for N. clasingi ( Mielke, 1985) in the spinicaudatus -group. Both N. ornamentus and N. reductaspina share the unique absence of the inner seta on P2 enp-2 ( Table 2 ), giving further credence to the monophyly of this subgroup.

Neoleptastacus acanthus ( Chappuis, 1954) , N. chaufriassei ( Bodiou & Colomines, 1986) and N. huysi ( Karanovic, 2000) exhibit the plesiomorphic 2-spine condition on P3 enp-2 and are united in the acanthus -subgroup. No synapomorphy has as yet been identified for this subgroup which includes species from the Mediterranean and the sub-antarctic Crozet Islands in the southern Indian Ocean.