Chaetozone reticulata Blake 2018, new species

Chaetozone reticulata new species

Figures 50, 51A

Tharyx sp. Hartman 1978: 167 (in part).

Material examined. Antarctica, Weddell Sea, USCG Glacier, Sta. 69-22, 13 Mar 1969, 73°28.4′S, 30°26.9′W, 3111 m, holotype (USNM 46817).

Description. A moderate-sized species, with body elongate, narrow in anterior segments (Fig. 50A), then widened in middle segments, narrowing posteriorly (Fig. 50E). Holotype a mature male with numerous sperm packets in coelom; complete but twisted with some damage, 8.7 mm long, 0.45 mm wide across anteriormost setigers, 0.65 mm wide across mid-body setigers, 0.22 mm wide across far posterior setigers, with 43 setigerous segments. Dorsal and ventral grooves and ridges absent, but with narrow mid-ventral white line apparent upon staining with MG extending posteriorly from mouth along most of body. Anterior segments narrow, about 5–6 times wider than long; posterior segments longer, about 1.5 times as wide as long, but never with oval or moniliform shape (Fig. 50E). Color in alcohol, tan, with prominent dark brown to black pigment along body; midbody segments with pigment arranged in distinct reticulated pattern (Fig. 50A, D).

Pre-setiger region elongate, about as long as first five setigerous segments (Figs. 50 A–C, 51A). Prostomium short, triangular in shape, narrowing to rounded tip (Fig. 51A); eyespots absent; nuchal organs low mounds on posterior lateral margins (Fig. 51A). Peristomium elongate, with smooth surface, longer than wide with no obvious grooves or annular rings, merging seamlessly with setiger 1 (Fig. 51A); dorsal crest absent; ventrally forming upper and lower lips of mouth (Fig. 50C). Dorsal tentacles arising mid-dorsally about two-thirds along peristomium (Figs. 50B, 51A). First pair of branchiae arising immediately posterior to dorsal tentacles; second pair of branchiae arising on setiger 1, dorsal and posterior to origin of notosetae (Figs. 50B, 51A); subsequent branchiae in similar location on following segments. Branchiae or scars observed on most segments through middle body.

Parapodia of anterior and middle segments reduced to low mounds and ridges from which setae arise; posterior segments with more prominent ridges from which acicular spines arise, but not elevated into raised membranes. Setae of anterior three-fourths of body all capillaries with 4–5 setae per noto-and neuropodia; notopodia with 1–3 extra-long, natatory-like setae variable in number but present along entire body (Figs. 50B, D–E, 51A). Acicular spines first present in neuropodia from setiger 31 and notopodia from setiger 37. Neuropodial spines numbering 1–3 until last few setigers, then increasing to 5–6 in posteriormost setigers; notopodial spines numbering in 1–2 until last few setigers, then increasing to 4–5 in posteriormost setigers. Last 2–3 segments with acicular spines forming partial cinctures with up to 11 spines on a side. Capillaries irregularly distributed with spines, not alternating. Individual acicular spines initially short, sigmoid in shape, narrowing to rounded tip; last few setigers with spines longer, less curved (Fig. 50G); internal core of individual spines relatively clear.

Body terminating in simple pygidium with rounded ventral lobe (Fig. 50F).

Methyl Green stain. Body staining uniformly with no pattern, destaining entirely. As noted above mid-ventral unstained line extends from posterior border of mouth to near posterior end.

Etymology. The epithet is from the Latin, reticulatus for netlike and refers to the reticulated pigment pattern that characterizes this species.

Remarks. The general shape of the pre-setiger region, seamless merger with setiger 1, and arrangement of the dorsal tentacles and first two pair of branchiae of Chaetozone reticulata n. sp. is similar to that of C. australosetosa n. sp. and C. biannulata n. sp. also reported in this study (see above). However, the two latter species have a prominent annular groove that divides the peristomium into two parts; in C. reticulata n. sp. there is no such groove and the entire peristomium is a single smooth unit. Globally, the shallow-water North American species, C. hystricosa from off New England has a similar peristomial structure but differs morphologically from C. reticulata n. sp., an abyssal species, in having an elongate prostomium instead of a short one, more spines in the posterior cinctures, and is not pigmented. Additional comments on these species are in the Remarks for C. australosetosa n. sp. above.

Globally, there are few species of Chaetozone with a similar distribution of body pigment as found in C. reticulata n. sp. Chaetozone pigmentata Blake, 1915 from the North American Arctic shelf depths does have similar pigment but an entirely different pre-setiger region and arrangement of the peristomium, achaetous segment, and distribution of the dorsal tentacles and anterior branchiae (Blake 2015).

Distribution. Weddell Sea, Antarctica, 3111 m.