Scyracepon biglobosus, An & Zheng & Liang & Paulay, 2020

An, Jianmei, Zheng, Wanrui, Liang, Jielong & Paulay, Gustav, 2020, Three new species of Scyracepon Tattersall, 1905 (Isopoda: Bopyridae) from Pacific islands, with comments on the rarity of bopyrids parasitizing brachyurans, Zootaxa 4851 (1), pp. 151-162 : 155-158

Scyracepon biglobosus n. sp.

Figs 3 , 4

Material examined. Holotype female, paratype male, UF Arthropoda 42214, infesting right branchial chamber of Schizophrys aspera (H. Milne-Edwards, 1834) (Majidae) , UF Arthropoda 10459, Kiribati, Line Islands, Kiritimati Atoll, off ocean side of Cook Islet in middle of passage, outer reef slope, under rocks, 10–12 meters, 1.9656°N, 157.4862°W, 6 August 2005, coll. G. Paulay.

Description. Holotype female length 7.0 mm, maximum width 5.10 mm, head length 1.56 mm, head width 2.05 mm. Body broad, bulging, segments distinct, slightly dextrally distorted ( Fig. 3A, B ).

Head deeply bilobate, with two lateral hemispheres separated by sunken middle, with well-developed frontal lamina ( Fig. 3A ), without eyes. Antennula of three articles; antenna of four articles, terminal two articles setose ( Fig. 3C ). Barbula with two slender, pointed, falcate projections on each side, external projection larger than inner one, without projections medially ( Fig. 3D ). Maxilliped with stout, triangular, curved palp, inner margin fringed with setae, plectron long, thin, pointed ( Fig. 3E ).

Pereon broadest across third pereomere ( Fig. 3A ). First four pereomeres with suboval dorsolateral bosses, those on right side larger than those on left. Pereomeres 2–4 with tergal projections only on right side ( Fig. 3A ). Last two pereomeres with well-developed, backward-directed, middorsal projections ( Fig. 3A ). Brood pouch ventrally vaulted, completely closed ( Fig. 3B ). First oostegite with 3 or 4 projections on internal ridge, anterior margin smooth and curved, posterior margin straight, posterolateral point blunt, substantially extended laterally ( Fig. 3F, G ). Pereopods increase in size posteriorly. Ischia of pereopods 1–3 with small tubercles ( Fig. 3H ), of pereopod 7 with large tuberculate extension ( Fig. 3I ). All pereopods with blunt and short dactyli ( Fig. 3H, I ).

Pleon of six distinct pleomeres, first five bearing lateral plates and biramous pleopods, decreasing in size posteriorly ( Fig. 3A ). All lateral plates and pleopods covered with tubercles, both with digitate margins. Endopodites of pleopods triangular and much smaller than exopodites ( Fig. 3B ). Uniramous uropods resemble lateral plates of pleomere 5 ( Fig. 3A ).

Paratype male length 2.62 mm, maximum width, 1.02 mm, head length 0.19 mm, head width 0.40 mm, pleonal length 0.83 mm ( Fig. 3J ; 4 ).

Head broadly suboval, posterior margin long and relatively straight; with black eyes near posterolateral margin ( Fig. 3J ). Antennula of 3 articles; antenna of 5 articles; terminal articles of both setose ( Fig. 4B ).

Pereon much wider than head, fourth widest, anterior three with quadrate margin, others with somewhat sharper margins; lateral extensions curved ventrally ( Fig. 4A ). Pereopods subequal, except dactyli of first three slightly larger than those of others ( Fig. 4A, C ). Meri, carpi and propodi of pereopods with scale plates and setae ( Fig. 4D ). Propodi of pereopods with articular socket comprised of 4 or 5 scale plates to accommodate retracted dactyli ( Fig. 4D ). All pereomeres with midventral projections ( Fig. 4A ); small and round on 1 and 2, larger and rounded on 3 and 4, broad, almost bilobate on 5 and 6, low and slightly bilobate on 7 ( Fig. 4A, E ), all with scales.

Pleon with six distinct pleomeres demarcated by lateral indentations, first three free, clearly demarcated both dorsally and ventrally, last three fused ( Figs 3J , 4A, F ). Five pairs of tuberculate pleopods ( Fig. 4A, F ). Uropods short, tuberculate; with anal cone between them ( Fig. 4G ).

Etymology. The specific name, biglobosus refers to the strikingly bilobate head of the female.

Remarks. The present species is assigned to Scyracepon because of the fused male pleonal segments. The female of the new species is most similar to S. distincta and S. thalamitae , as both of these also have middorsal projections on only the last two pereomeres. Scyracepon biglobosus is distinguished from both by its conspicuously bilobate head (slightly bilobate in both other species). It is also distinguishable from S. distincta by the large (vs very small) middorsal projections on the last pereomeres, and the straight and glabrous posterior margin of oostegite 1 (curved and fringed with setae in S. distincta ). The new species further differs from S. thalamitae in that its pleopods are only ornamented by digitate margins ( S. thalamitae has pleopods with deeply digitate, divided margins and a minutely tuberculate and setose surface). The male of the new species is distinguished in having three free pleomeres ( S. distincta has only two free pleomeres, while S. thalamitae has all pleomeres medially fused). The spider crab Schizophrys aspera is also host of the bopyrid Grapsicepon magnum Shiino, 1936 in Japan ( Shiino, 1936). This species is readily distinguished from S. biglobosus n. sp. by its completely separate pleonal segments in males.

Figures

FIGURE 3. Scyracepon biglobosus n. sp. Holotype female (A–I). A. Dorsal view; B. Ventral view (lp = lateral plates; en = endopodite; ex = exopodite); C. Left antennae; D. Right side of barbula; E. Left maxilliped, external view; F. Left oostegite 1, external view; G. Left oostegite 1, internal view; H. Left pereopod 1; I. Left pereopod 7; Paratype male (J); J. Dorsal view. Scale-bars: A, B = 1mm; C, H, I = 0.13 mm; D, E, J = 0.30 mm; F, G = 0.76 mm.

FIGURE 4. Scyracepon biglobosus n. sp. Scanning Electron Micrograph of paratype male. A. Ventral view of the male; B. Right antennae; C. Right pereopods 1–3; D. Propodus, carpus and merus of pereopod 3; E. Midventral projections of pereomere 6; F. Ventral view of pleon; G. Pleotelson and anal cone. Scale-bars: A = 0.50 mm; B, C, E = 50 um; D = 20 um; F = 80 um; G = 40 um.