Chaetozone castanea Blake 2018, new species

Chaetozone castanea new species

Figures 43–44

Material examined. Peru: Off Lima, SEPBOP, R / V Anton Bruun Cr. 17, Sta. 657-K, 24 Jun 1966, 12°10ʹS, 77°27ʹW, rock dredge, 150 m, holotype (USNM 1490739) and 90 paratypes (USNM 1490740); Sta. 657-F, 24 Jun 1966, 12°03S, 77°18ʹW, rock dredge, 105 m (1, USNM 1490741); North of Lima, SEPBOP, R / V Anton Bruun Cr. 16, Sta. 654-A, 10 Jun 1966, 09°29ʹS, 78°54ʹW to 09°32ʹS, 78°47ʹW, trawl, 140 m, (2, USNM 1490742); Cr. 18B, Sta. 751, 04 Sep 1966, 10°02ʹS, 78°38ʹW, 160 m (5, USNM 1490743); South of Lima, SEPBOP, R / V Anton Bruun Cr. 18A, Sta. 735, 24 Aug 1966, 16°20ʹS, 73°27ʹW, trawl, 110 m (80, USNM 1490744).— Chile: Bahia Mejillones, SEPBOP, R / V Anton Bruun Cr. 18A, Sta. 721, 19 Aug 1966, 23°02ʹS, 70°27ʹW, trawl, 65–98 m (135, USNM 1490745).

Description. A small species, holotype complete, 7.0 mm long, 0.5 mm wide for 50 setigerous segments; largest paratypes 7.6–7.8 mm long, 0.5 mm wide with 45–50 setigers. Body elongate, widest in middle segments, narrowing posteriorly; anterior segments about five times as wide as long; middle body segments about twice as wide as long (Fig. 44A). Dorsum rounded, without groove or ridge; venter somewhat flattened, without groove or ridge. Far posterior setigers narrowing, somewhat moniliform (Figs. 43B, 44A, C). Dorsal blood vessel and heart body prominent in larger specimens (Fig. 44B). Color in alcohol distinctly light to dark brown from numerous subdermal pigmented cells throughout body.

Prostomium triangular, narrowing to a rounded anterior margin (Figs. 43A, 44 A–B); eyes absent; nuchal organs elevated lobes, lateral on posterior margin of prostomium. Peristomium with anterior section merged with prostomium, followed by three nearly equal annular rings (Fig. 43A), peristomial grooves best developed laterally; dorsum relatively smooth, with a low raised mound forming weak dorsal ridge. Dorsal tentacles arising from middle of last peristomial ring (Fig. 43A); first pair of branchiae lateral and slightly posterior to tentacles on anterior margin of setiger 1 (Fig. 43A); second pair of branchiae on posterior edge of setiger 1, dorsal to notosetae; subsequent setigers with branchiae in similar location (Fig. 43A); branchiae continuing into middle body segments, absent in posterior half of body.

Setiger 1 of approximately same size as following segments (Fig. 43A); podial lobes reduced to low mounds throughout, best developed in far posterior setigers (Figs. 43B, 44C). Noto- and neurosetae from setiger 1 all capillaries; notosetae 4–5 per fascicle, neurosetae 5–6 per fascicle; each capillary long, thin, with no fibrils apparent along edge of shaft; long natatory-like notosetae absent. Capillaries in both noto- and neuropodia transitioning to acicular spines from about setiger 39–45, earlier in shorter specimens; notoacicular spines longer than neuroaciculars, numbering 2–3 per fascicle and accompanied by an equal number of capillaries; neuroacicular spines shorter, 5–6 per fascicle (Fig. 44E), companion capillaries rare. Individual spines with narrow shaft, tapering to blunted tip (Fig. 43 C–D). Posterior spines organized into partial cinctures; with only 7–9 thin, narrow spines on a side (Fig. 44D); accompanying capillaries long, irregularly spaced, not noticeably alternating with spines.

Pygidium a simple achaetous segment with terminal anus and ventral disk-like lobe (Figs. 43B, 44C).

Methyl Green stain. Pre-setigerous region and far posterior segments not retaining stain; rest of body staining uniformly with no pattern.

Etymology. The epithet is from the Latin, castaneus for brown or the color of chestnuts in reference to the overall brown pigment on this species.

Remarks. Chaetozone castanea n. sp. is readily identified by its overall brown color due to numerous subdermal pigmented glands. The last peristomial ring before setiger 1 may be an achaetous segment because it bears the dorsal tentacles; the first pair of branchiae occurs at its junction with setiger 1 suggesting that a setiger has been lost. Here the first pair of branchiae is interpreted as being on the anterior border of setiger 1, which means that two branchiae occur on that segment. Given the reduced number of posterior spines and its distinct brownpigmented body, C. castanea n. sp. differs from all other described species of Chaetozone. Chaetozone castanea n. sp. has acicular spines that are elongate, thin, and taper to a narrow, blunted tip instead of being short, sigmoid, thick, and taper to a curved, blunted tip. In this respect, T. castaneus n. sp. is most similar to the Antarctic species, C. homosetosa (see below). Chaetozone homosetosa, however, is a much larger species, up to 24 mm long vs. 7 mm for T. castaneus n. sp. and lacks pigment on the body.

Biology. Both males with sperm packets and females with large yolky eggs measuring up to 230 µm in diameter are present in the type collection. Eggs this large suggest either a lecithotrophic or direct development.

Distribution. Offshore Peru, 105–160 m; Chile, 65– 98 m.