Chaetocirratulus Blake 2018, new genus

Chaetocirratulus new genus

Type species: Heterocirrus andersenensis Augener, 1932. Original designation.

Diagnosis. Prostomium broadly rounded anteriorly or wedge-shaped; eyespots absent; with a pair of small nuchal slits or depressions at posterior edge. Peristomium with a single pair of grooved dorsal tentacles arising from posterior margin or interface with setiger 1. First pair of branchiae arising from posterior margin of peristomium, an achaetous segment, or setiger 1. Body typically thick and fusiform over many segments, rarely with middle or posterior body segments beaded or moniliform; individual segments short, numerous. Setae include capillaries on most setigers and thick, pointed acicular spines in neuropodia and a few in notopodia or spines in neuropodia only; spines few, often small and inconspicuous, not forming cinctures. Individual spines straight to weakly sigmoid. Pygidium with a simple ventral lobe.

Etymology. The name Chaetocirratulus is derived from the Greek chaeto for hair or bristle and the Latin cirrus for curl or tendril and is a combination of the generic names Chaetozone and Cirratulus emphasizing the close similarity of these bitentaculate cirratulids with their multitentaculate relatives.

Remarks. Chaetocirratulus n. gen. is established to accommodate several bitentaculate cirratulids having few acicular spines that do not produce the distinct posterior armature formed of cinctures of numerous sigmoid spines typical of species of Chaetozone. As will be seen, in some Antarctic species, the spines are so inconspicuous that some species were originally described as lacking them and as such, have been misidentified in recent studies when only the characteristics from the original descriptions or those restated by Hartman (1966) were cited. In addition, species of Chaetocirratulus n. gen. typically have a prostomium that is enlarged, broad, or wedge-shaped and an enlarged thickened body, where segments are short and often swollen, producing a fusiform shape that is not divided into obvious anterior and posterior regions. Petersen (1991) recognized that some cirratulids with these characteristics were more closely aligned with multitentaculate cirratulids and referred at least one of these, Chaetozone gayheadia Hartman, 1965, to the multitentaculate genus Cirratulus Lamarck, 1818; she also emended the definition of Cirratulus to include species with two or more dorsal tentacles.

In the present study, the types of four Antarctic species, Heterocirrus andersenensis, Tharyx fusiformis, Tharyx epitocus, and Chaetozone pinguis have been examined and are all referred to the new genus Chaetocirratulus n. gen. Heterocirrus andersenensis and Tharyx fusiformis are considered the same species, referred to here as Chaetocirratulus andersenensis n. comb. Both have short, inconspicuous straight spines in posterior segments. The same situation exists for Tharyx epitocus, which after examination of both syntypes has been found to have posterior spines and is also referred to Chaetocirratulus n. gen. Chaetozone pinguis was originally described with neuropodial spines from setiger 1 and agrees with the definition of Chaetocirratulus n. gen. In addition to these three species, three new species of Chaetocirratulus n. gen have been discovered from deep water off South America and the Ross Sea. The following six species are here assigned to Chaetocirratulus n. gen:

1 Chaetocirratulus abyssalis n. sp. Ross Sea Antarctica, slope depths.

2 Chaetocirratulus andersenensis (Augener, 1932) n. comb. Antarctica, shelf depths.

[Tharyx fusiformis Monro, 1939 new syn. Antarctica.]

3 Chaetocirratulus epitocus (Monro, 1930) n. comb. Antarctica, shelf depths.

4 Chaetocirratulus furvus n. sp. Off Peru, slope depths.

5 Chaetocirratulus neogracilis n. sp. Peru-Chile Trench, abyssal.

6 Chaetocirratulus pinguis (Hartman, 1978) n. comb. Antarctica, shelf depths.

Three other species, Chaetozone benthaliana McIntosh, 1885, C. gayheadia, and C. gracilis (Moore, 1923), all from deep water off North America are also believed to belong to Chaetocirratulus n. gen. and are noted in the Discussion Section (below); these species will be treated in a separate publication.