Heterosignum unicornis ( Kensley, 1976 ) Shimomura & Mawatari, 2002

Shimomura, M & Mawatari, SF, 2002, A revision of Heterosignum (Asellota, Isopoda, Paramunnidae) with descriptions of three new species, Journal of Natural History 36 (17), pp. 2103-2126 : 2122-2124

Heterosignum unicornis ( Kensley, 1976) comb. nov.

(gure 12)

Coulmanni a unicornis Kensley, 1976: 310–313 , gures 23, 24.

Material examined

Amsterdam Island , southern Indian Ocean, 30 m, 17 January 1971: holotype , one male, 2.0 mm (MNHN-Is. 1010); allotype, one ovigerous female, 1.4 mm (with 11 eggs) (MNHN-Is. 1011, in the same bottle as holotype).

Description

Male. Body (gure 12A) widest at pereonite 3. Pereonite 1 with long, stout mid-dorsal spine and 14 dorsal setae; pereonites 2–7 with some dorsal setae, without mid-dorsal spines. Pereonites 1 and 5–7 laterally rounded, without lateral spine-like processes; pereonites 2–4 each with a pair of short single spine-like processes. Pereonite 1 long; pereonite 2 slightly longer than pereonite 1; pereonite 3 shorter than pereonite 2; pereonite 4 longest; pereonite 5 about two-thirds length of pereonite 4; pereonites 5–7 decreasing in length. Pereonite 1 slightly narrower than pereonite 2; pereonites 2 and 3 subequal in width; pereonite 4 narrower than pereonite 3; pereonites 5–7 decreasing in width. Coxal plates dorsally invisible on all pereonites. Pleonite 1 small, without setae. Pleotelson consisting of two parts, about 1.3 times as long as wide, with scattered small dorsal and distal setae; anterior part of pleotelson long, cylindrical, about one- fth length of posterior; posterior part of pleotelson bulbous, with ve teeth on right lateral margin.

Antenna 2 (gure 12C) with peduncle of six articles: article 1 not visible dorsally; article 2 longer than broad, without setae; article 3 slightly shorter than article 2, with convex proximal protrusion bearing single seta, and with one lateral and one mesial setae.

Pereopod 1 (gure 12D) robust: basis the longest article, 1.9 times as long as ischium, with two dorsal and one ventral setae; ischium with two dorsal, two ventral and one mesial setae; merus trapezoidal, with one simple and one robust setae dorsally and one simple seta ventrally; carpus robust, widest, with four simple and two robust sensory setae ventrally, one simple seta dorsodistally; propodus ovate, with two robust sensory and two simple setae ventrally and three simple setae dorsally; dactylus narrowest, slightly shorter than propodus, with ve distal setae, one robust long unguis, and one acute accessory claw.

Female. Similar to male in morphology of pereonal appendages except for pereopod 1. Body (gure 12B) broader than in male, about 1.7 times as long as maximum width (except lateral spine-like process), widest at pereonite 3. Pereonite 1 narrow and short, without lateral spine-like processes and dorsal spines; pereonites 2–4 each with a pair of short single lateral spine-like processes; pereonites 5–7 without lateral spine-like process. Pereonites 2–4 increasing in length; pereonite 5 about half as long as pereonite 4; pereonite 6 slightly shorter than pereonite 5; pereonite 7 slightly longer than pereonite 6. Pereonites 1–3 increasing in width; pereonites 4–7 decreasing in width. Pereopod 1 (gure 12E) slender: basis the longest article, twice as long as ischium, with one dorsal seta; ischium with one ventrodistal, one dorsodistal and one lateral setae; merus trapezoidal, with one robust dorsal seta, four ventral and one distolateral setae; carpus ovate, ventrally with two robust sensory and one simple setae and seven short acute teeth, dorsally with two setae; propodus ovate, ventrally with two robust sensory and two simple setae, dorsally with three setae; dactylus narrowest, as long as propodus, with ve distal setae, one slender long unguis and one acute accessory claw.

Remarks

Coulmannia unicornis Kensley, 1976 should be assigned to Heterosignum because it shares the following characters with species of Heterosignum : (1) the lateral spine-like processes on pereonites are single; (2) article 3 of antenna 2 is elongate; (3) dorsal surface of body bears some setae; (4) pereonites 5–7 diVer in shape from pereonites 1–4; (5) a waist is present between pereonites 4 and 5; (6) pleotelson consists of two parts, narrow, cylindrical anterior and bulbous posterior; (7) pleotelson has some teeth on its lateral margin.

After careful examinations of the holotype and allotype, we found a marked sexual dimorphism: the rst four articles of pereopod 1 are distinctly broader in the male than in the female. There are no congeners so far known to exhibit such sexual dimorphism in pereopod 1.

Heterosignum unicornis diVers from all congeners in lacking lateral spine-like processes on pereonites 1 and 5–7 and in only the male having a stout mid-dorsal spine on pereonite 1.