Platycuma bamberconfabulor Gerken & Watling, 2015, sp. nov.

Platycuma bamberconfabulor sp. nov.

(FigS 1 & 2)

Material examined. Holotype - subadult male (NIWA 80709) Ross Sea, Antarctica, RV Tangaroa cruise TAN 0 802, Station 88, 76.6097°S, 176.7560°E, 360 m, 17 February 2008.

Diagnosis. Carapace laterally expanded and somewhat dorsoventrally flattened, anterior margin excavate in dorsal view, dorsal margin sinuous in lateral view, uropod peduncles less than twice length of pleonite 6.

Etymology. The species is named bamberconfabulor for Roger N. Bamber in combination with confabulor, from the Latin meaning debate, argue, or discuss in honor of Roger’s fondness for discussion of crustacean terminology.

Description. Subadult male holotype. Body (Figs 1 A, 1B) white. Carapace nearly circular when viewed from above, slightly wider posteriorly, anterior margin excavate; overlapping pereonites 1 and 2; highest about midway along length, with shallow paired posterior swellings extending ventrally to lateral lamellar margin; pseudorostral lobes extending in front of obsolete eyelobe; antennal notch broad, anteroventral corner without teeth; lateral lamellar margin extends from anteroventral corner to posterior margin. Pleon about half body length; pereonites 4 and 5 and pleonites 1 through 4 with paired dorsolateral keels; pleonite 4 and 5 subequal.

Antennule (Fig. 1 C) not geniculate; basal article of peduncle slightly shorter than articles 2 and 3 combined, with terminal seta; accessory flagellum minute, of two articles; main flagellum of three articles, distal two articles with one aesthetasc each.

Antenna not illustrated, immature, without annulations or setae, but extending past posterior border of carapace, suggesting the individual is one molt from adult, and would be expected to have a medium to long antennal flagellum as an adult.

Mouth parts: Mandibles and maxillae not illustrated, not dissected in order to preserve integrity of carapace. Maxilliped 1 (Fig. 1 D) basis wide, dorsomedial ridge armed with four plumose setae, endite simple, with single terminal acuminate seta; ischium not visible; merus shorter than carpus; carpus with few simple setae on ventral surface; propodus half length of carpus, with cluster of pappose setae and one plumose seta distally. Maxilliped 2 (Fig. 1 E) basis equal in length to remainder of appendage, with one short and one elongate plumose seta distally; merus, carpus, and propodus subequal in length, with simple setae along lateral margins; propodus with 3 claw-like setae on inner margin, several plumose setae distally.

Maxilliped 3 (Fig. 1 F) basis subequal in length to remainder of appendage, with many short, simple setae along ventral and dorsal margins, one long plumose seta distally; merus and carpus with plumose setae on ventral margins and distally; propodus longer than merus, with single simple seta terminally; dactyl with cluster of simple, claw-like setae terminally.

Pereopods (Figs 2 A–E). Pereopods 1, 3–5 typical of the genus, pereopods 1–4 with exopods. Pereopod 2 carpus slightly shorter than dactylus. All pereopods with few simple or plumose setae.

Uropods (Fig. 2 F). Uropod peduncle 1.7 times length of pleonite 6, scarcely longer than endopod, with serrated inner margin; endopod longer than exopod, with few simple setae on inner margin.

Distribution. Ross Sea, Antarctica, from the holotype only.

Remarks. The new species described here has a subequal pleonite 4 and 5, while in the other species in the genus pleonite 4 is shorter than pleonite 5. The new species is most similar to Platycuma holti in having a nearly uniformly wide carapace with the anterior margin excavate in dorsal view in this case with a little more curvature than in P. h ol t i Also, in P. holti, the carpus of maxilliped 1, carpus of pereopod 1 and the uropod peduncles are all longer than in P. bamberconfabulor sp. nov.

Species in the genus Platycuma can be divided into two groups, those with a laterally expanded and somewhat dorsally flattened carapace (P. ho l t i, P. triangularis, and P. japonicum) and those with a carapace typical of the family (P. sandersi, P. hessleri, P. candida, and P. lineata). All species, however, have the coiled gut that is diagnostic of the genus. There do not appear to be any additional morphological features suggesting the genus should be divided on the basis of carapace shape.

The new species is a subadult male, and the state of development of the antenna suggests that the adult male will have an antenna that is longer than the antennae described for the two species (P. holti and P. hessleri) in which the adult male is known.

A note on the gut of Platycuma. Species of Platycuma are the only known cumaceans with a coiled gut. Calman (1905) describes in detail the structure for P. holti: “The anterior portion of the gut, corresponding probably to the chitin-lined fore-gut, runs nearly straight backwards, with a slight inclination to the left. Behind the middle of the carapace it turns sharply to the right, then curving upwards and forwards it forms a right-handed spiral of two and a half turns, not quite in a plane, the outermost whorl lying a little to the left and passing straight backwards into the abdomen. The straight anterior portion is a little wider than the rest, and there is a slight constriction at the beginning of the spiral, beyond which the diameter remains fairly uniform. The whole of this portion of the gut is distended with a very fine-grained mud, in which the only recognizable particles of organic origin are numerous coccoliths and fragments of the shells of Peridinia. No trace of hepatic caeca could be discovered. While it is possible and even probable that some vestiges of them may have escaped detection, it is certain that they must be exceedingly reduced” (p. 33). No additional details are available from other species in the genus, but drawings of the gut for other species in the genus show some coiling, but not to the extent of forming a spiral as documented by Calman.

The question thus is, why this peculiar structure of the gut? Platycuma holti is clearly a deposit-feeding animal, ingesting large amounts of fine-grained sediment and it is widely known that deep-sea sediments do not have high organic matter content. Thus, to extract the greatest amount of useful organic content from such sediments, one of two strategies needs to be adopted. The more typical one used by most crustaceans, is to process the sediment in the foregut and then to filter the contents, sending only the slurry of organic molecules and liquid into the hepatic caeca (Watling 2013). Since, for some reason, Platycuma has lost or reduced its hepatic caeca, another strategy for processing low organic content sediment was needed: to greatly elongate the gut to prolong the action of digestive enzymes, allowing more time for digestion to occur (Jumars 2000). A clue that this might be the case comes from Calman’s description of the gut. He notes that there is a slight constriction at the beginning of the spiral; this is probably the transition from foregut to midgut, the latter being excessively long. In other animal groups, the strategy of elongating the midgut in deposit feeders is common, for example, in urchins and polychaetes (Jumars 2000). Of course, in the case of Platycuma, it is impossible to know if the gut elongation developed due to a gradual shortening of the hepatic caeca, or whether it was the other way round.