Idiotropiscis larsonae

Idiotropiscis larsonae (Dawson, 1984)

Figures 11–13, Tables 2–5

Acentronura (Idiotropiscis) larsonae Dawson, 1984: 157–158, figs. 1, 2 (Alpha Island, Monte Bello Islands, Western Australia).

Diagnosis.— See generic diagnosis. Idiotropiscis larsonae differs from its congeners by the following combination of characters: bony dorsomedial crest on the supraoccipital, strongly elevated, tripartite, the three large segments arcuate in lateral view, spanning the complete frontal from anterior margins of the interorbital to the supraoccipital, compressed laterally, protruding anteriorly; posterolateral margins of post-temporal bones expanded slightly anteriorly taking the form of flat protuberances, not prominent; rim of orbit projecting slightly dorsolaterally and ventrolaterally; small dorsal snout spine present medially on mesethmoid bone; body compressed laterally.

Description.— Morphometric and meristic characters listed in Tables 2–4. Superior trunk and tail ridges discontinuous below dorsal-fin base, lateral tail ridge present, inferior trunk ridge ends at anal ring, lateral trunk ridge confluent with inferior tail ridge. Head angled ventrally 25° from the principal body axis, the anterodorsal profile slightly conoid in lateral aspect; bony dorsomedial crest on the supraoccipital, strongly elevated, tripartite, the three large segments arcuate in lateral view, spanning the complete frontal from anterior margins of the interorbital to the supraoccipital, compressed laterally, protruding anteriorly; posterolateral margins of post-temporal bones expanded slightly anteriorly taking the form of flat protuberances, not prominent and difficult to discern (Fig. 14B); anterior nuchal plate absent; posterior nuchal plate present anterior to cleithrum with dorsomedial crest-like ridge along its dorsum, large gap present between the supraoccipital and posterior nuchal plate; small blunt spine midway between orbit and lobed protuberance on operculum; cleithral ring distinct, discontinuous mid-dorsally; bilateral gill slits situated laterally on head between supraoccipital and cleithrum; median dorsal snout spine present on mesethmoid bone; interorbital narrow, depressed; rim of orbit projecting moderately dorsolaterally and ventrally; opercular ridge low, entire, angled dorsally toward gill opening; swelling of the gular region ventroposteriorly of eye absent; pectoral-fin base without distinct ridges, low, strongly elevated ventrolateral bulge supporting the pectoral-fin base absent; trunk compressed laterally, span of dorsum of pre-dorsal trunk rings less by half of dorsum of anterior post-dorsal tail rings, span of trunk at lateral and inferior ridges somewhat greater than span of dorsum of trunk; subdorsal rings four (spans three trunk rings and one tail rings vs. two trunk rings and two tail rings in paratype); principal body ridges distinct; superior tail ridge with slightly enlarged spines on 4 th, 12 th, and 16 th trunk rings. Caudal fin absent.

Idiotropiscis larsonae exhibits strong sexual dimorphism associated with the presence of a brood pouch in the male (Figs. 11C, 12B). The brood pouch is formed along the ventral midline of the tail and is present below the anteriormost nine tail rings. X-ray image of the holotype revealed the brood pouch is enclosed by nine arcuate bony extensions and reduced in size posteriorly. The female paratype exhibits well-developed ossification of the skeleton, including the strong ossification of the inferior and ventral trunk area as detected by µCT scanning (Fig. 13B).

Distribution.— Idiotropiscis larsonae is known only from the tropical waters of the Monte Bello Islands, north Western Australia (Dawson, 1985).

Material examined.— Idiotropiscis larsonae: NTM S. 10805-001, adult male, holotype, Monte Bello Islands, Western Australia, Australia, 20°36 ' S, 115°37 ' E, depth 3–9 m, 22 April 1983; GCRL 21518, female, paratype, 33.0 mm SL, Monte Bello Islands, Western Australia, Australia, 20°36 ' S, 115°37 ' E, depth 3–9 m, 22 April 1983.