Gnathia scabra, Ota, Yuzo, 2012

Ota, Yuzo, 2012, Gnathiidae from Kumejima Island in the Ryukyu Archipelago, southwestern Japan, with description of three new species (Crustacea: Isopoda) *, Zootaxa 3367, pp. 79-94 : 84-87

Gnathia scabra n. sp.

New Japanese name: Ibo-eguri-umikuwagata ( Figs. 4 , 5 )

Material examined. Holotype. Male, 2.40 mm in total length (RUMF-ZC-1429), dead coral rubble or rock rubble, Stn. Trawl 45 (beam trawl), 26°19.907ʹN, 126°43.191ʹE – 26°20.056ʹN, 126°42.622ʹE, 67.5–76 m depth, off Kumejima Island, the Ryukyu Archipelago, southwestern Japan, 16 November 2009.

Description. Male ( Figs. 4 , 5 ). Body 2.40 mm (n = 1).

Cephalothorax ( Fig. 4 A–C). Cephaolothorax, covered with setae and tubercles, almost square. Dorsal sulcus deeply concave. Posterior margin slightly concave. Mediofrontal process broad with remarkably concave apex. Superior frontolateral process with 3 setae. Marginal carina slightly visible below antennae in dorsal view. Eyes with 42 ocelli in 7 horizontal rows. Paraocular ornamentation indistinct, composed of several tiny tubercles. Supraocular lobe not acute.

Pereon ( Fig. 4 A). Pereonite I approximately half length of pereonite II, not fused; lateral parts not visible dorsally. Pereonite II and III subequal in length and width; few setae and tubercles on lateral margins. Pereonite IV with anterior constriction; anterolateral lobe absent. Pereonite V with areae laterales. Pereonite VI as long as combined length of pereonite IV and V. Pereonite VII not extending posterolateral margin of pereonite VI, overlapping pleonite I.

Pleon ( Fig. 4 A). All pleonites sparsely covered with setae. Pleonites II–V subequal in length and width; epimera prominent.

Pleotelson ( Fig. 4 D). Width approximately 1.09 of length. 4 pairs of setae on lateral margin, central part, and apex. Lateral margins slightly concave over proximal third and slightly convex at middle.

Mandible ( Fig. 4 B, C). Mandible approximately 0.6 times as long as cephalothorax; apex curved dorsomesially. Mandibular seta present on mid-dorsal surface near incisor. Dentate blade occupying approximately onethird of mandible length. Erisma prominent.

Antennula ( Fig. 4 E). 3 basal podomeres and 5 flagellar articles. Basal podomeres I, II, and III bearing 2, 3, and 1 flagellar articles on distal margins, respectively. Flagellar articles I and V bearing 2, 3, 1, 1, and 1 penicillate setae on distal margins, respectively. Articles VI–VIII each with 1 aesthetasc; article VIII bearing 2 terminal setae. Antenna ( Fig. 4 F). 4 basal podomeres and 7 flagellar articles. Basal podomeres III and IV bearing 2 and 3 penicillate setae on distal margins, respectively. Flagellar articles I–VII with few setae on distal margins; article VII bearing 3 terminal setae.

Maxilliped ( Fig. 5 A). Endite reaching distal margin of palp article 1. Palp articles I–IV bearing 3, 7, 4, and 1 plumose setae on external margins, respectively; article IV bearing 7 simple terminal setae.

Pylopod ( Fig. 5 B). 3-articulate; 3 setae near distal margin of article I and II, respectively. Article I elliptical with 3 areolae bearing 25 plumose setae on internal margin; 1 seta near external margin and distal margin, respectively. Article II elliptical, fringed with fine setae. Article III semicircular.

Pereopod II ( Fig. 5 C). Basis oblong, outer margin with 3 penicillate setae and 4 processes. Ischium approximately two-third length of basis, becoming wider distally; fine setae bearing inner and outer margins and 2 long setae near distal margin. Merus approximately two-third length of ischium, bearing 4 setae on distal corner. Inner margins of carpus and propodus covered with pectinate scales. Carpus slightly shorter than merus, bearing 1 spine and 2 processes on inner margin. Propodus rectangular and 1.3 times as long as carpus; inner-middle and inner-distal margins with 2 spines and distal corner with 1 penicillate seta.

Pleopod II ( Fig. 5 D, E). Protopod distomedial corner with coupling hook and 1 seta. Both rami elliptic but exopod slightly shorter than endopod; 9 and 7 plumose setae on endopod and exopod, respectively. Appendix masculina reaching half-length of endopod. Both rami of pleopods IV and V shorter than those of pleopods I–III. Endopods each with 7 or 8 simple setae. Exopods each with 8–9 plumose setae.

Uropod ( Fig. 4 D). Both rami subequal in length; endopod extending beyond apex of pleotelson. Exopod bearing 7 setae and 3 plumose setae laterally. Endopod bearing 6 setae and 3 plumose setae laterally. Dorsal surface of endopod bearing 5 penicillate setae.

Penes ( Fig. 5 F). Penes composed of 2 contiguous papillae and not prominent.

Etymology. The scientific name scabra is derived from the Latin meaning “rugged”, referring to its many tubercles on the head.

Remarks. Gnathia scabra n. sp. is most similar to G. lignophila Müller, 1993 , in following characters: many tubercles present on chephalothorax and pereonite I–III, and the apex of mediofrontal process is remarkably concave ( Müller 1993). This new species, however, has appendix masculina on pleopod II (it lacks in G. lignophila ), longer mediofrontal process than frontolateral process (mediofrontal border of G. lignophila is always shorter than frontolateral process).

Figures

FIGURE 4. Gnathia scabra n. sp., holotype, adult male (RUMF-ZC- 1429, total length, 2.40 mm): A, body, dorsal view; B, eye and frontal border, dorsal view; C, cephalothorax and pereonite I, lateral view; D, pleotelson and uropods, dorsal view; E, left antennula, medial view; F, left antenna, medial view.

FIGURE 5. Gnathia scabra n. sp., holotype, adult male (RUMF-ZC- 1429, total length, 2.40 mm): A, left maxilliped, ventral view; B, left pylopod, ventral view; C, left pereopod II, medial view; D, right pleopod II, ventral view; E, right pleopod III, ventral view, plumose setae were omitted; F, penes, ventral view.