Alderiopsis nigra (Baba, 1937)

Location: Karatsu, Saga, Japan
Date: 2025/02/15
Length: 3mm
Depth: 5.0m
Water temperature: 11.0℃

Description

A tiny sacoglossan, about 3 mm long, with a flattened oval (limaciform) body. The ground colour is black, with white tips on the paired head-horns, on the tips of the dorsal cerata and on the tail; the broad sole is paler. The antero-lateral corners of the head are drawn out into short, flat horn-like processes; despite their appearance these are not true rhinophores, since the rhinophorial nerves do not extend into them. Ten to eleven fusiform cerata are arranged in a single row along each dorso-lateral margin, each containing an unbranched, faintly olive-coloured liver-diverticulum. The anus opens just behind the pericardium, slightly to the right of the mid-dorsal line. The radula has a single sacoglossan series of simple, blade-like teeth without marginal denticles, and a penial stylet is present.

Distribution

Type locality: Tomioka, Amakusa (Kumamoto, Japan), on Zostera marina. Subsequent specimens have been recorded from Tannowa (Sennan, Osaka Bay) and from Mukaishima in the Inland Sea of Seto (Baba, 1968), indicating a wider distribution along Japanese eelgrass beds.

Etymology

From Latin niger (feminine nigra), "black", referring to the dark ground colour of the body.

Remarks

Originally described by Baba 1937 from a single specimen taken on Zostera marina at Tomioka, Amakusa, the species was tentatively placed in Alderia Allman, 1846. After re-examining many additional specimens from Osaka Bay and the Inland Sea of Seto, Baba 1968 showed that the head-horns are not true rhinophores, that the anus lies at the right anterior corner of the pericardium, that the liver diverticula are unbranched, and that the egg masses form a flat coiled band. On these grounds he erected the new genus Alderiopsis with this species as its type. The genus name combines Alderia with the Greek suffix -opsis ("having the appearance of"), in reference to its overall similarity to Alderia. The species lives on the leaves of eelgrass (Zostera marina), on which it is presumed to feed.