Pleurobranchus varians Pease, 1860

Description

A side-gilled sea slug reaching about 70 mm in length, with the foot not projecting beyond the mantle. The background colour ranges from orange to dark red, and the dorsum is densely covered with relatively large, flattened, polygonal tubercles that in some individuals become smaller towards the mantle edge. Opaque white pigment is present on the tubercles of some specimens, and the species shows two sharply distinct colour forms — one with extensive white pigment and one with none at all. The rhinophores bear numerous fine longitudinal striations, and the oral veil and rhinophores are the same colour as the mantle.

Distribution

Type locality: the Hawaiian Islands ("Sandwich Islands"). Recorded from the Hawaiian Islands and Vanuatu.

Etymology

The specific epithet varians is Latin for "varying, variable". The species is markedly variable in colour, showing two distinct forms separated by the presence or absence of white pigment.

Remarks

This species was long confused with the similar, co-occurring Indo-Pacific Pleurobranchus albiguttatus, and the name P. varians fell into disuse until molecular phylogenetic analyses showed the two to be distinct and reinstated it as a valid species. Externally, P. albiguttatus has smaller, conical tubercles and a continuously variable amount of white pigment, whereas P. varians has larger, flattened tubercles and a sharp distinction between only two colour forms.

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References

• Pleurobranchus varians n. sp., Pease W.H. (1860). Descriptions of new species of Mollusca from the Sandwich Islands. Part I. Proceedings of the Zoological Society of London. 28: 18-36.