Chaetocirratulus hessleri Blake 2022, new species

Chaetocirratulus hessleri new species

Figures 8–9

urn:lsid:zoobank.org:act: 823AE4D8-8C7C-42A8-A423-A4FCA8EDC548

Chaetozone sp. B: Maciolek et al. 1987a: D-2 (in part).

Material examined. Off New Jersey and Delaware, U.S. Mid-Atlantic ACSAR Program, coll. R. Petrecca, Chief Scientist. Sta. 3: Cruise Mid-2, Rep. 3, 03 Aug 1984, 38°36ʹ.84ʹN, 72°51.46ʹW, 2056 m, holotype (USNM 1660936). — Off New England, U.S. North Atlantic ACSAR Program, coll. G.W. Hampson, Chief Scientist. Sta. 3: Cruise NA-3, Rep. 1, 03 Jul 1985, 41°01.37ʹN, 66°21.21ʹW, 1337 m, paratype (USNM 1660937).

Description. A large species, holotype complete, with 90 setigers, 40 mm long and 3 mm wide across anterior setigers. Body elongate, not inflated along body but gradually narrowing in far posterior segments. Parapodia located laterally along body with intersegmental grooves not apparent in anterior setigers and only weakly so in posterior segments; dorsal surface elevated above parapodia, rounded and relatively smooth (Fig. 8A); venter similarly flattened, also lacking distinct intersegmental grooves in first half of body; distinct intersegmental grooves only apparent laterally between anterior parapodia. Dorsal and ventral grooves absent but with a narrow white line along venter, probably representing ventral nerve cord extending from posterior margin of peristomium posteriorly along most of body. Color in alcohol tan in first half of body, grey in posterior half. Distinct areas of dark, dusky pigment apparent on ventral surface of peristomium posterior to mouth and first two setigers, extending higher on sides; same pigment present on posterior border of first 4–5 parapodia (Fig. 9A, C–D).

Pre-setiger region about as wide as long. Prostomium broadly triangular, tapering to rounded apex (Fig. 8A); eyespots absent; nuchal organs at posterior lateral margins, mostly hidden by peristomium. Peristomium with three rings (Figs. 8A, 9A, C); first ring surrounding prostomium dorsally and forming posterior lip of mouth ventrally (Fig. 9A, D); second and third rings incomplete dorsally with a weakly developed dorsal crest (Fig. 8A). A short transverse notch present dorsally between first and second peristomial rings (Fig. 8A). Dorsal tentacles arise from posterior margin of third peristomial ring, with first pair of branchiae lateral to tentacles (Figs. 8A, 9C). Subsequent segmental branchiae dorsal to notosetae on posterior margin of individual parapodia. Branchiae present to about setiger 40.

Parapodia of anterior segments swollen, bearing distinct noto- and neuropodia from which setae arise (Fig. 9C). Parapodia less conspicuous in middle and posterior segments. Setae include long, smooth capillaries and acicular spines. Capillaries numbering 19–21 per noto- and neuropodia in anterior setigers, reduced to 6–7 in middle segments and 2–3 in posterior setigers. Some capillaries with thicker shafts transitioning to acicular spines at about setiger 55–57 in both noto- and neuropodia (Fig. 8C). Individual spines with straight, not sigmoid, shafts and with bluntly rounded tips (Fig. 8C–D). Spines 1–2 per notopodium and 2–3 per neuropodium, accompanied by 2–3 capillaries.

Pygidium with smoothly rounded ventral lobe and 3–4 short dorsal lobes surrounding anal opening (Figs. 8B, 9B).

Variability. The paratype (USNM 1660937) is damaged, mostly complete, measuring 9.0 mm long and 1.2 mm wide with about 45 setigers, but at least five posterior segments missing. Last peristomial ring and first three setigers with black pigment spots on venter as in holotype. Most anterior and middle setae broken, but thick acicular spine observed in neuropodia of setiger 10. Three peristomial rings observed; dorsal crest present, but not as well developed as on holotype.

Methyl green staining. Methyl green imparts a distinctive pattern to the pre-setiger region and anterior parapodia (Fig. 9E–F). The entire prostomium and peristomium retain stain except for mid-dorsal and posterior-lateral clear areas. The anterior parapodia concentrate stain, which extends ventrally as a band onto the venter. The branchiae typically have numerous stained spots along their length; dorsal tentacles do not stain.

Remarks. Of the four species of Chaetocirratulus encountered in this study, three, including C. hessleri n. sp., have three peristomial rings. However, C. hessleri n. sp. is the only one to have a dorsal crest on the peristomium. In addition, C. hessleri n. sp. is the only one with conspicuous dark pigment on the ventral surface of the peristomium and first two setigers where it continues up and onto the lateral sides; the same pigment is also found on the posterior border of setigers 1–5. A comparison of all four species of Chaetocirratulus treated in this study is found in Table 2.

Etymology. This species is named for the late Dr. Robert E. Hessler, prominent deep-sea ecologist whose pioneering studies with Dr. Howard Sanders on the North Atlantic deep-sea benthos introduced the world to the undiscovered richness and diversity of deep-sea benthos.

Distribution. U.S. Atlantic continental slope, off New Jersey and Delaware, 2056 m; off New England, 1337 m.