Chaetozone diodonta Doner& Blake, 2006

Figure 32–33

Chaetozone sp. A: Maciolek-Blake et al. 1985: B-5 (in part).

Chaetozone sp. B: Maciolek-Blake et al. 1985: 33, B-5.

Chaetozone diodonta Doner & Blake, 2006: 71–72, Fig. 4, 5D; Blake & Levesque 2017: 572–573, 576–577.

Material examined. (883 specimens) Off Massachusetts, Georges Bank, MMS Benthic Infauna Monitoring Program, coll. G.W. Hampson, Chief Scientist. Sta. 8: Cruise M-1, Rep. 2, Jul 1981, 40°27.0′N, 67°37.0′W, 145 m (3, USNM 1661080) ; Rep. 3, (1, USNM 1661081) ; Rep. 5, (5, USNM 1661082) ; Rep. 6 (7, USNM 1661083) ; Cruise M-2, Rep. 1, 11 Nov 1981, 40°27.2′N, 67°37.1′W, 140 m (1, USNM 1661084) ; Rep. 2 (2, USNM 1661085) ; Rep. 3 (1, USNM 1661086) ; Rep. 4 (1, USNM 1661087) ; Rep. 5 (1, USNM 1661088) ; Rep. 6 (3, USNM 1661089) ; Cruise M-3, Rep. 1, 17 Feb 1982, 40°27.2′N, 67°37.1′W, 147 m (1, USNM 1661090) ; Rep. 4, (1, USNM 1661091) ; Rep. 5 (3, USNM 1661092) ; Rep. 6 (3, USNM 1661093) ; Cruise M-4, Rep. 2, 13 May 1982, 40°27.2′N, 67°37.2′W, 140 m (2, USNM 01661094) ; Rep. 4 (1, USNM 1661095) ; Rep. 5 (10, USNM 1661096) ; Rep. 6 (5, USNM 1661097) ; Cruise M-5, Rep. 1, 23 Jul 1982, 40°27.1′N, 67°37.1′W, 140 m (1, USNM 1661098) ; Rep. 3 (6, USNM 11661099) ; Rep. 6 (6, USNM 1661100) ; Cruise M-6, Rep. 1, 22 Nov 1982, 40°27.1′N, 67°37.2′W, 145 m (2, USNM 1661101) ; Rep. 2 (2, USNM 1661102) ; Rep. 3 (20, USNM 1661103) ; Rep. 4 (4, USNM 1661104) ; Rep. 5 (7, USNM 1661105) ; Rep. 6 (7, USNM 1661106) ; Cruise M-7, Rep. 1, 09 Nov 1982, 40°27.1′N, 67°37.2′W, 142 m (2, USNM1661107) ; Rep. 2 (3, USNM 1661108) ; Rep. 4 (3, USNM 1661109) ; Rep. 5, (6, USNM 1661110) ; Rep. 6 (10, USNM 1661111) ; Cruise M-8, Rep. 2, 16 May 1983, 40°27.1′N, 67°37.43′W, 146 m (6, USNM 1661112) ; Cruise M-9, Rep. 1, 15 Jul 1983, 40°27.1′N, 67°37.43′W, 152 m (11, USNM 1661113) ; Rep. 2 (10, USNM 1661114) ; Rep. 4 (14, USNM 1661115) ; Rep. 5 (5, USNM 1661116) ; Cruise M-10, Rep 1, 16 Nov 1983, 40°27.1′N, 67°37.4′W, 152 m (3, USNM 1661117) ; Rep. 2 (2, USNM 1661118) ; Rep. 3 (3, USNM 1661119) ; Rep. 5 (4, USNM 1661120) ; Cruise M-11, 03 Feb 1984, Rep. 2, 40°27.2′N, 67°37.1′W, 152 m (3, USNM 1661121) ; Rep. 3 (12, USNM 1661122) ; Rep. 4 (10, USNM 1661123) ; Cruise M-12, 05 Jun 1984, Rep. 1, 40°27.2′N, 67°37.1′W, 152 m (7, USNM 1661124) ; Rep. 3 (10, USNM 1661125) ; Rep. 5 (5, USNM 1661126) . Sta. 16: Cruise M-1, Jul 1981, Rep. 1, 40°34.2′N, 67°12.3′W, 140 m (1, USNM 1661127) ; Rep. 4 (2, USNM 01661128) ; Cruise M-2, 18 Nov 1981, Rep. 3, 40°34.2′N, 67°12.3′W, 140 m (1, USNM 1661129) ; Cruise M-3, 17 Feb 1982, Rep. 3, 40°34.3′N, 67°12.5′W, 141 m (5, USNM 1661130) ; Rep. 5 (1, USNM 1661131) ; Cruise M-4, 12 May 1982, Rep. 1, 40°34.3′N, 67°12.4′W, 140 m (3, USNM 1661132) ; Rep. 3 (4, USNM 1661133) ; Rep. 5 (1, USNM 1661134) ; Cruise M-6, 21 Nov 1982, Rep. 1, 40°34.3′N, 67°12.4′W, 138 m (5, USNM 1661135) ; Rep. 2 (1, USNM 1661136) ; Rep. 3 (13, USNM 1661137) ; Rep. 4 (6, USNM 1661138) ; Rep. 5 (4, USNM1661139) ; Rep. 6 (4, USNM 1661140) ; Cruise M-7, 09 Feb 1983, Rep. 1, 40°34.3′N, 67°12.4′W, 143 m (3, USNM 1661141) ; Rep. 2 (9, USNM 1661142) ; Rep. 3 (5, USNM 1661143) ; Rep. 4 (6, USNM 1661144) ; Rep. 5 (18, USNM 1661145) ; Rep. 6 (7, USNM 1661146) ; Cruise M-8, 16 May 1983, Rep. 1, 40°34.3′N, 67°12.4′W, 139 m (9, USNM 1661147) ; Rep. 6 (4, USNM 1661148) ; Cruise M-9, 15 Jul 1983, Rep. 3, 40°34.2′N, 67°12.3′W, 142 m (2, USNM 1661149) ; Rep. 4 (18, USNM 1661150) ; Rep. 5 (10, USNM 1661151) ; Cruise M-10, 15 Nov 1983, Rep. 1, 40°34.2′N, 67°12.3′W, 142 m (14, USNM 01661152) ; Rep. 2 (2, USNM 1661153) ; Rep. 3 (23, USNM 1661154) ; Rep. 4 (13, USNM 1661155) ; Rep. 5 (11, USNM 1661156) ; Rep. 6 (14, USNM 1661157) ; Cruise M-11, 03 Feb 1984, Rep. 1, 40°34.2′N, 67°12.3′W, 142 m (7, USNM 1661158) ; Rep. 2 (7, USNM 1661159) ; Rep. 3 (9, USNM 1661160) ; Rep. 4 (17, USNM 1661161) ; Rep. 5 (8, USNM 1661162) ; Rep. 6 (11, USNM 1661163) ; Cruise M-12, 05 Jun 1984, Rep. 2, 40°34.2′N, 67°12.3′W, 142 m (18, USNM 1661164) ; Rep. 3 (5, USNM 1661165) ; Rep. 4 (8, USNM 1661166) ; Rep. 5 (2, USNM 1661167) ; Rep. 6 (4, USNM 1661168) . Sta. 17: Cruise M-1, Rep. 4, Jul 1981, 40°35.0′N, 67°11.7′W, 145 m (1, USNM 1661169) ; Cruise M-2, Rep. 1, 18 Nov. 1981, 40°35.0′N, 67°11.7′W, 145 m (5, USNM 1661170) ; Cruise M-3, Rep. 1, 18 Feb 1982, 40°34.90′N, 67°11.1′W, 145 m (1, USNM 1661171) ; Rep. 4 (3, USNM 1661172) ; Rep. 6 (3, USNM 1661173) ; Cruise M-4, Rep. 4, 12 May 1982, 40°35.0′N, 67°11.3′W, 130 m (3, USNM 1661174) ; Cruise M-5, Rep. 1, 23 Jul 1983, 40°35.0′N, 67°11.2′W, 135 m (3, USNM 1661175) ; Rep. 5 (3, USNM 01661176) ; Cruise M-6, Rep. 2, Nov 1982, 40°35.0′N, 67°11.2′W, 140 m (5, USNM 1661177) ; Rep. 3 (3, USNM 1661178) ; Rep. 4 (5, USNM 1661179) ; Rep. 5 (5, USNM 1661180) ; Cruise M-7, Rep. 1, 08 Feb 1983, 40°35.0′N, 67°11.3′W, 144 m (3, USNM 1661181) ; Rep. 2 (3, USNM 1661182) ; Rep. 3 (2, USNM 1661183) ; Rep. 5 (1, USNM 1661184) ; Rep. 6 (5, USNM 1661185) ; Cruise M-8, Rep. 6, 16 May 1983, 40°35.0′N, 67°11.3′W, 141 m, (5, USNM 1661186) , SEM stubs (4, USNM 1661187); Sta. 18: Cruise M-1, Rep. 4, Jul 1981, 40°33.5′N, 67°11.3′W, 145 m (3, USNM 1661188) ; Rep. 5 (5, USNM 1661189) ; Rep. 6 (4, USNM 1661190) ; Cruise M-2, Rep. 2, 18 Nov 1981, 40°33.5′N, 67°13.7.6′W, 145 m (3, USNM 1661191) ; Rep. 3 (5, USNM 1661192) ; Rep. 4 (3, USNM 1661193) ; Cruise M-3, Rep. 2, 17 Feb 1982, 40°33.5′N, 67°13.4′W, 147 m (3, USNM 1661194) ; Rep. 3 (2, USNM 1661195) ; Rep. 4 (3, USNM 1661196) ; Rep. 5 (9, USNM 1661197) ; Rep. 6 (7, USNM 1661198) ; Cruise M-4, Rep. 1, 13 May 1982, 40°33.5′N, 67°13.6′W, 150 m (9, USNM 1661199) ; Rep. 2 (7, USNM 1661200) ; Rep. 3 (9, USNM 1661201) ; Rep. 5 (4, USNM 1661202) ; Rep. 6 (1, USNM 1661203) ; Cruise M-5, Rep. 4, 23 Jul 1982, 40°33.5′N, 67°13.5′W, 140 m (4, USNM 1661204) ; Rep. 6 (11, USNM 1661205) ; Cruise M-6, Rep. 1, 21 Nov 1982, 40°33.5′N, 67°13.6′W, 144 m (2, USNM 1661206) ; Rep. 2 (8, USNM 1661207) ; Rep. 3 (22, USNM 1661208) ; Rep. 5 (9, USNM 1661209) ; Rep. 6 (9, USNM 1661210) ; Cruise M-7, Rep. 1, 09 Feb 1983, 40°33.5′N, 67°13.5′W, 147 m (30, USNM 1661211) ; Rep. 2 (18 USNM 1661212) ; Rep. 3 (15, USNM 1661213) ; Rep. 4 (5, USNM 1661214) ; Cruise M-8, Rep. 1, 16 May 1983, 40°33.5′N, 67°13.5′W, 141 m (8, USNM 1661215) ; Rep. 2 (12, USNM 1661216) ; Rep. 3 (13, USNM 1661217) ; Rep. 4 (25, USNM 1661218) .

Description. A moderately sized species, 6.0–9.5 mm long, 0.6–0.8 mm wide for up to 100–120 setigers. Specimen from USNM 1661105 complete, 9.5 mm long with 115 setigers. Body thickest in anterior half, then gradually tapering posteriorly to narrow pre-pygidial segments (Fig. 32A). Body rounded dorsally with narrow groove anteriorly (Fig. 32D); flattened ventrally with prominent mid-ventral groove along entire length (Fig. 32A–B). Anterior setigers short, crowded, about ten times as wide as long (Fig. 32D); segments of posterior segments not as short, but still 5–6 times wider than long. Posterior segments with partial cinctures with wide dorsal and ventral gaps (Fig 32F). Color in alcohol light tan, lacking distinctive body pigment.

Pre-setiger region relatively thick, short, as long as first 8–10 crowded anterior setigers. Prostomium thick, elongate, conical, tapering to pointed tip (Fig. 32A–B, D), sometimes curved ventrally with tip not clearly visible when viewed dorsally (Fig. 32C); eyespots absent; nuchal organs oval, elongate curved grooves (Fig. 32D inset). Peristomium as wide as long or wider; with two lateral grooves producing three distinct rings (Fig. 32B–D), but these only partially visible dorsally; with distinct dorsal crest (Fig. 32B–D), best developed in anterior half. Dorsal tentacles arising from posterior margin of peristomium; first branchiae lateral to tentacles and anterior to setiger 1(Fig. 32B–D). Subsequent pairs of branchiae positioned dorsal to and slightly posterior to notosetae (Fig. 32D).

Noto- and neuropodia reduced to low tori bearing setal fascicles. Parapodia of anterior setigers forming distinct protruding lateral shoulders (Fig. 32A–B), these becoming reduced and entirely absent in middle and posterior segments. Notosetae of anterior setigers simple, smooth capillaries, 7–8 per fascicle with one or two transitioning to long, acicular spines from about setiger 65–70 with addition of one, rarely two, bidentate hooks at setiger 80–90 in inferior-most position; up to 5–6 spines per notopodium alternating with capillaries (Fig. 33A–D). Neurosetae of first setigers simple capillaries, 5–6 per fascicle with transition to short acicular spines by about setiger 15–20; posterior setigers with 5–7 spines per fascicle alternating with capillaries; one or two bidentate spines typically present in inferior-most position in fascicle (Fig. 32D). Posterior setigers with partial cinctures with elevated membranes bearing up 10–13 spines on a side. Setae of last 5–10 setigers reduced to a single capillary and spine in both podia. Individual acicular spines thick, weakly curving to pointed tip; bidentate hooks with long, pointed main tooth curving laterally to shaft, surmounted by short pointed, apical tooth (Fig. 33E).

Pygidium with 5–6 elongate lobes lateral and dorsal to anal opening, large semi-circular flattened disk ventral to anal opening (Fig. 32E–G).

Variability. There is considerable variability in the kinds and numbers of acicular spines along the body. In some specimens the notopodial bidentate hooks do not appear until the last 5–10 setigers. In other specimens, unidentate neuropodial spines are present initially with a few transitioning to bidentate hooks more posteriorly. However, the tips of the spines are curved and details including the small apical tooth may be difficult to observe at certain angles in light microscopy.

Methyl green staining. No apparent pattern.

Remarks. Locally, Chaetozone diodonta is most similar to C. donerae n. sp., in having partial cinctures of posterior spines with wide dorsal and ventral gaps between the noto- and neuropodia. The presence of bidentate hooks among acicular spines in C. diodonta is a distinct difference from C. donerae n. sp., which has only unidentate spines. In addition, C. diodonta has three distinct peristomial rings instead of two.

The only other North American species of Chaetozone having both uni- and bidentate spines in posterior cinctures is Chaetozone lunula Blake, 1996 from the continental shelf off northern California in depths of 77– 190 m. The two species are also similar in having reduced cinctures with up to 13 spines on a side and a prominent ventral gap between the neuropodia. However, unlike C. diodonta, which has bidentate hooks along much of the body and in both noto- and neuropodia, C. lunula has only one or two bidentate hooks limited to the ventral-most position in the neuropodia. In addition, C. diodonta differs from C. lunula in having three peristomial rings instead of two and a large rounded pygidial disk instead of a short lobe bearing a terminal anal cirrus.

Biology. On Georges Bank, Chaetozone diodonta populations are mainly concentrated at four stations (Sta. 8, 16, 17, and 18) along the 140–150 m isobath. Station 8 is at the head of Lydonia Canyon. These four stations have sediments that consist of more than 95% gravel and sand-size particles with minimal silt and clay (Maciolek et al. 1985). Owing to the high percentage of coarse particles, the corresponding percent carbon is low (0.12% at Station 8 and <0.05% at Stations 16, 17, and 18). Chaetozone diodonta, as Chaetozone sp. B, was the tenth most abundant benthic invertebrate species at Station 8, ninth at Station 16, and third at Station 18 (Maciolek et al. 1985). In the present study, the 883 specimens reported were distributed at Sta. 8 (225), Sta. 16 (315), Sta. 17 (75), and Sta. 18 (268). The holotype (USNM 1076549) is from Sta. 17 collected on 19 Nov 1982, at 40°35.0′N, 67°11.2′W, and 140 m (Doner & Blake 2006). Paratypes reported in the 2006 paper were from Sta. 8 (3), 16 (8), and 17 (6). The four paratypes reported from Sta. 2 should be re-examined because the majority of specimens of the similar appearing Chaetozone donerae n. sp. were from that location at 60–70 m (see below).

Distribution. Off New England, 140– 152 m.