Atlantocuma

Relationship between Atlantocuma and Pseudopicrocuma

Two genera described in the present study are currently placed in the family Nannastacidae, based on the combination of characters (1) no pleopods in both sexes, and (2) uropod endopod uni-articulate, that are shared by all nannastacid genera and by no bodotriid genera, although some doubt on these criteria remains (see below). The new genus Pseudopicrocuma, which is similar to Picrocuma, is also rather similar to Atlantocuma, except for (1) well developed exopods present on maxilliped 3 and pereopods 1–3 in males and females (maxilliped 3 and pereopods 1–4 in males, on maxilliped 3 and pereopod 1 in female in Atlantocuma), (2) uropod peduncle shorter than rami (much longer in Atlantocuma, arrangement of setae is similar to each other), and (3) male antenna 2 clasping form, peduncle distal 2 articles fused (flagellum normal form, distal 2 articles of peduncle fused in Atlantocuma, (4) ocular lobe large (normal size in Atlantocuma). In addition, ovigerous females of these 2 genera were characterized by a similar trend in degeneration of mouthparts and associated morphological change (1) complete decalcification of mandibles, (2) diminishing or loss of setae on mandibles, maxilla 1, maxilla 2, inner margin of maxillipeds 1 and 2 (terminal seta on dactylus of maxilliped 2 and 3 also reduced in P. japonicum), (3) carapace elevated posteriorly, and (4) maxilliped 3 with row of stiff plumose setae on inner margin of basis. No food residue was observed in all ovigerous females of both species, suggesting they completely stop feeding. Although cumacean life cycles have not been investigated sufficiently, degeneration of mouthparts has only been observed previously in the monotypic genus Claudicuma (Nannastacidae, Roccatagliata 1981), suggesting a possible close relationship to Atlantocuma and Pseudopicrocuma with quite different arrangement of exopods on pereopods. This causes a problem in cumacean taxonomy which largely depends on sexual characters.

Generally, cumaceans show marked sexual dimorphism, including arrangement of their pereopods and pleopods: males typically have higher numbers of well-developed exopods than females (usually on maxilliped 3 and pereopods 1–4) as more advanced swimming apparatus, along with pleopods which are present in males. Along with presence or absence of a telson, these characters are important for taxonomy of cumaceans.

Exceptionally, some cumacean genera are characterized by the close similarity between males and females possibly due to suppression of sexual dimorphism, especially the same arrangement of well-developed exopods in both sexes; Pseudocumatidae (with 2 pairs of pleopods), Kerguelenica (no pleopods in both sexes, Ledoyer 1977; Gerken & McCarthy 2008) and Fontainella (with two pairs of small pleopods in males, Bäcescu & Muradian 1978; López-González et al. 1996); Nannastacidae (with no pleopods in both sexes), Almyracuma (Jones and Burbanck 1959, Petrescu and Heard 2004), Thalycrocuma (Corbera et al. 2008), Picrocuma (Hale 1936, 1945; Muhlenhardt- Siegel 2003); Bodotriidae (usually having five pairs of pleopods in males), Spilocuma (with no pleopods, Watling 1977; Omholt & Heard 1979), Mancocuma (with two pairs of small pleopods in males, Zimmer 1943, 1980), and Speleocuma (with two pairs of pleopods in males, Corbera 2002). Pseudopicrocuma, a new genus, is an additional case. Similarity of this genus to Atlantocuma, which is characterized by normal sexual dimorphism, suggests that these genera should be treated as exceptional cases in cumacean phylogenetic analysis based only on external morphology.

The adult male antenna 2 is rather different between Atlantocuma and Pseudopicrocuma. Male antenna 2 of Japanese Atlantocuma, with the flagellum of 10 (plus 1 minute) articles, is almost normal, but possibly represents a primitive form of a clasping antenna, based on (1) flagellum shorter than peduncle, (2) the appendage occasionally bent between peduncle and flagellum (Fig. 8 A), and (3) flagellum article 1 with steep taper at basal region (Fig. 4 F). In Pseudopicrocuma, this appendage (Fig. 13 E) is obviously clasping, representing a more derived form; (1) the appendage geniculate between peduncle and basal article of flagellum; (2) flagellum of only 4 (1 specimen) or 5 (4 specimens) large articles with triangular process on inner margin, except for distal 2 minute ones. Fusion of the peduncle articles 4 and 5 would make this appendage work as an advanced clasping arm. The flagellum of Pseudopicrocuma consisting of about half the number of articles, compared with Atlantocum a, may be a result of fusion of the articles of the flagellum. In Nippoleucon hinumensis (Leuconidae), incomplete fusion of the articles of the male antenna 2 flagellum occurs at proximal region (Gamô 1967).