Arcturina psittacus, Bamber, 2008

Bamber, Roger N., 2008, New species of anthurid and arcturid isopod (Crustacea: Peracarida: Isopoda) from Hong Kong, Journal of Natural History 42 (9 - 12), pp. 855-876 : 870-875

Arcturina psittacus sp. nov.

( Figures 10–12 )

Material

One female, holotype (registration no. NHM.2003.354), four females, eight juveniles (NHM.2003.359–368), two subadult males (NHM.2003.355–356), two males (NHM.2003.357–358), paratypes, station Z, off the mouth of Conic Island Cave, Hong Kong, 22 ° 219540 N 114 ° 239220 E; sediment 1% gravel, 96% sand, 3% silt/clay, depth 13.7 m. Coll. N.J. Evans and P.F. Clark, 25 October 2002 .

Description of adult female

Body ( Figures 10E, G ) 2.5 to 2.7 mm long (rostrum to tip of telson), exoskeleton smooth and strongly calcified, with irregular brown pigment. Cephalon with welldeveloped anterolateral lobes and small, rounded rostrum ( Figure 10A ); eyes present, pigmented. Pereonite 1 fused to cephalon. Pereonites 5 to 7 distinct, without dorsal ridges. Pleonites 1 and 2 distinct dorsally and with raised dorsal ridges; pleotelson pentagonal.

Antennule peduncle naked; flagellum of single articles bearing two distal aesthetascs. Antenna ( Figures 10G , 11B ) peduncle third article twice as long as second and half length of subequal fourth and fifth articles; flagellum of three articles, distal two with ventral row of short spines resembling saw-teeth.

Mandible ( Figure 11C ) with three spines in spine row and fine, comb-like lacinia mobilis; pars molaris peripherally serrate; inner surface with blunt apophysis. Lower lip with slender, setose lobes ( Figure 11D ). First maxilla outer endite ( Figure 11E ) with seven elongate distal spines and setose outer margin.

First pereopod ( Figure 12A ) merus and carpus dorsally lobate, propodus with lateral rows of spines, single distal plumose spine; dactylus and claw robust.

Pereopod 2 ( Figure 12B ) articles simply setose; basis twice as long as wide, of similar width throughout; ischium with ventrodistal lobe bearing two setae; merus and carpus 1.3 times as long as wide with ventrodistal setal groups; propodus reduced with pair of slender, finely plumose distal setae. Pereopod 3 ( Figure 12C ) similar to pereopod 2, but basis expanding distally, ischium without ventrodistal lobe, merus with dorsal setae, carpus 1.6 times as long as wide with midventral setae. Pereopod 4 ( Figure 12D ) cylindrical, simple; distal dactylar plumose setae shorter than those of pereopods 2 and 3. Pereopods 1 to 4 with oostegites, that of pereopod 4 largest by far ( Figure 12E ).

Pereopods 5 to 7 (e.g. Figure 12F ) similar to and progressively smaller than each other; small dorsal lobes on ischium and merus.

First pleopod exopod operculiform, with three sigmoid spines (‘‘retinaculae’’) on inner margin of basis ending in fine teeth. Uropod ( Figure 12G ) with regularly setose outer margin; distal seta of inner ramus finely plumose.

Distinctions of other life stages

Juveniles ( Figure 10B ): first pereonite distinct from cephalon; pereonites 1 to 4 parallel, continuous with rest of body and with dorsal ridges; pereonites 5 to 7 and pleonites similar to adult female. Pigmentation sparse or absent.

Subadult males ( Figure 10C ) with parallel-sided body, as large as adult female; pereonites 1 to 3 fused to cephalon (single suture apparent dorsally); pleonites fused to pleotelson. Parallel dorsal ridges present on pereonite 4 and anterior pleon. Pigmentation as adult female, variable. Antennule ( Figure 11A ) flagellar article more elongate than in female, with numerous aesthetascs. Pleopods as adult female.

Adult male ( Figures 10D, F ) without pigmentation, pereonites 1 to 4 fused with cephalon, and bearing parallel dorsal ridges; pleon completely fused without dorsal ridges. Antennule as subadult male ( Figure 11A ). Pleopod 1 rami subequal in length, longer than protopod, exopod with indentation just proximal of midlength bearing three plumose setae and tuft of fine setae. Pleopod 2 ( Figure 11F ) rami subequal in length, longer than protopod, endopod bearing slender, distally-trifid appendix masculina (‘‘stylet’’) on outer margin longer than rami.

Etymology

From the Latin for a parrot (noun in apposition), the companion of the Expedition’s pirate (see under Cyathura peirates ).

Remarks

The present species shows the characteristic lozenge-shaped morphology of the cephalon and anterior four pereonites of the mature female, as well as the gross morphology of the mouthparts, pereopods, uropods and male pleopods all consistent with the described species of Arcturina .

The most recent and comprehensive treatment of the genus was that of Kensley (1975), who distinguished between four species of Arcturina , all from northwestern or southern Africa, but did not include A. cylindralis Pillai, 1963 , a species described from a subadult male from the southern tip of India ( Pillai 1963).

Pillai (loc. cit.) mistakenly distinguished Arcturina cylindralis on its cylindrical fourth pereonite, not realizing that it was not a female. It can be distinguished from the African species and the present species by the large inner ‘‘expansions’’ (lobes) on the carpus and propodus of pereopod 7.

All of the five described species have a widened carpus and propodus on pereopods 2 and 3, the propodus on pereopod 2 at least being as wide as long. Arcturina psittacus sp. nov. does not have this feature. In addition, the fusion of the pereonites and pleonites in the present species is distinct, notably in the complete fusion of the cephalon and pereonites 1 to 4 in the adult male. The present material has allowed the description of developmental stages for the first time, showing the retention of the juvenile body segmentation into the adult female, but its progressive reduction in the male.

Figures

Figure 10. Arcturina psittacus sp. nov.: A, anterior of cephalon; B, juvenile, dorsal; C, subadult male, dorsal; D, adult male, dorsal; E, adult female, holotype, dorsal; F, adult male, lateral; G, adult female, holotype, lateral. Scale line51 mm for B to G, 0.25 mm for A.

Figure 11. Arcturina psittacus sp. nov.: A, antennule of 2.8 mm subadult male; B, distal articles of antenna; C, left mandible, with detail of lacinia mobilis; D, lower lip; E, outer ramus of first maxilla; F, second pleopod of mature male. Scale line50.1 mm for A, C to F; 0.15 mm for B.

Figure 12. Arcturina psittacus sp. nov.: A to D, pereopods 1 to 4; E, fourth oostegite and pereopod; F, pereopod 7; G, uropod, with enlargement of distal end (rami). Scale line50.2 mm for A to D, F; 1.3 mm for E; 0.6 mm for G.