Proscoloplos Day 1954

Genus Proscoloplos Day, 1954

Type-species: Proscoloplos cygnochaetus Day, 1954 by monotypy.

Diagnosis. Body short, with separation between thorax and abdomen indistinct and with body segments of similar shape and size throughout, with no change in appearance between anterior and posterior segments. Prostomium broadly rounded or elongate, with paired rounded nuchal organs present; eyespots present or absent. Peristomium with two asetigerous rings. Noto- and neuropodia poorly developed, consisting of low tori from which setae emerge; with simple, tapering post-setal lamellae; posterior parapodia not shifted dorsally. Setae including crenulated capillary noto- and neurosetae and distinct swan hooks present from anterior neuropodia; furcate setae entirely absent. Branchiae present from anterior parapodia continuing posteriorly. Pygidium with four cirri.

Remarks. Pettibonella Solís-Weiss & Fauchald, 1989 and Proscoloplos are the only two orbiniid genera having swan hooks. Pettibonella was differentiated from Proscoloplos by Solís-Weiss & Fauchald (1989) in that their type-species, P. multiuncinata has eyes instead of lacking them, has elongate instead of short branchiae, has thoracic neuropodial uncini and capillaries, and has swan hooks of two kinds instead of one in posterior setigers. Within the Orbiniidae the presence or absence of eyespots is widespread among the genera including the closely related genus Orbiniella, where eyespots are typically found in shallow-water species and absent in deep-water species (see above). Differential shape and length of branchiae is a useful species-level character among orbiniids genera but is not used to define any other genus in the family. Proscoloplos cygnochaetus is now believed to be the only species of that genus and there are swan hooks of larger and smaller sizes within a single neuropodium (see below). The presence of thoracic neuropodial uncini is therefore the most obvious character that differentiates Pettibonella from Proscoloplos and is similar to the manner in which Leitoscoloplos is separated from Scoloplos.