Chaetocirratulus Blake 2018

Genus Chaetocirratulus Blake, 2018

Type species: Heterocirrus andersenensis Augener, 1932. Original designation by Blake 2018.

Diagnosis. (after Blake 2018) Prostomium broadly rounded anteriorly or wedge-shaped; eyespots absent; with a pair of small nuchal organs as 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 a simple ventral lobe.

Remarks. Species of Chaetocirratulus include a heterogeneous group of cirratulids that share characteristics of both the multitentaculate genus Cirratulus Lamarck, 1818 and the bitentaculate genus Chaetozone. The genus was established by Blake (2018) to accommodate a group of Chaetozone species that had large, often fusiform bodies with only a few acicular spines that were not arranged in spreading fascicles or in the more typical cinctures. Petersen (1991) had earlier recognized that these species did not agree with Chaetozone and referred them to the multitentaculate genus Cirratulus and redefined the genus to include species having two tentacular cirri in addition to multiple pairs of tentacles.

Two species previously described from the western North Atlantic Ocean considered to possibly belong to Chaetocirratulus are Chaetozone benthaliana McIntosh, 1885 from off Nova Scotia in 2275 m and Chaetozone gayheadia Hartman, 1961 from off New England in 300 m.

Comments on Chaetozone benthaliana . Chaetozone benthaliana is an enigma because the holotype (NHM ZK 1885.12.1.278) is represented only by a large posterior end collected as part of the Challenger Expedition from deep-water off Nova Scotia. This posterior fragment has been cut into two pieces, the largest of which has 80 setigers and is about 40 mm long and up to 7 mm wide. Most setae are broken off, but a few straight acicular spines are present in posterior setigers. Blake (2018: 121), without documentation, referred this taxon to Chaetocirratulus. However, given the lack of any anterior morphology, it is not possible to characterize this species to any genus of Cirratulidae based on the type specimen. I suggest, therefore, that C. benthaliana McIntosh be considered a species inquirendum and removed from the list of valid species of Chaetozone.

Kirkegaard (1983) referred a specimen from deep-water (1900 m) from the northeastern Atlantic southwest of Ireland to C. benthaliana and provided an illustrated description, but few details were provided and no explanation as to why he thought his specimen was the same species as the posterior end of C. benthaliana reported by McIntosh (1885) from off Canada. As part of a series of unpublished notes and sketches prepared by the late Dr. Mary E. Petersen and now in my possession are details of her observations on the specimen reported by Kirkegaard (1983). These notes were present in documents recovered from Dr. Petersen’s laboratory following her death in 2014. Dr. Petersen noted that the specimen was widest in the middle of the body, narrowing at both anterior and posterior ends and having ca. 100 setigers, 36 mm long and 5 mm wide. She noted that the prostomium was broadly triangular and wedge-shaped, but had a mid-dorsal notch or depression. The nature of the peristomium was not noted, but her sketches indicate that two peristomial rings were present, the anteriormost being large with a pair of lateral creases, not crossing the dorsal or ventral surfaces. The dorsal tentacles were missing but their scars were present in a notch on the anterior border of setiger 1; most branchiae were missing, but the scars of the first pair of branchiae were visible on setiger 1 dorsal to the notosetae. Parapodia were well developed with ridges apparent along most of the body. Most of the setae were broken off, but smooth capillaries and a few acicular spines, when present, were straight, not curved; their first occurrence could not be determined, however one or two spines were present in both noto- and neuropodia from setiger 91. The notes and detailed sketches prepared by Dr. Petersen of the NE Atlantic specimen suggest a species that differs from any specimens of Chaetocirratulus observed along the U.S. Atlantic coast in the present study and likely represents an undescribed species.

Unlike C. benthaliana, where only a posterior fragment is available, numerous specimens of C. gayheadia were available for study including postlarvae, juveniles, and mature adults. Detailed observations on these specimens are presented below. In addition, representatives of three additional species, new to science, were encountered. In the present study, the following species have been identified off the U.S. Atlantic coast and are treated in this study.

2. Chaetocirratulus hessleri n. sp.

3. Chaetocirratulus sandersi n. sp.

4. Chaetocirratulus tomaculus n. sp.