Chaetocirratulus neogracilis, Blake, 2018

Blake, James A., 2018, Bitentaculate Cirratulidae (Annelida, Polychaeta) collected chiefly during cruises of the R / V Anton Bruun, USNS Eltanin, USCG Glacier, R / V Hero, RVIB Nathaniel B. Palmer, and R / V Polarstern from the Southern Ocean, Antarctica, and off Western South America, Zootaxa 4537 (1), pp. 1-130 : 66-68

Chaetocirratulus neogracilis new species

Figure 35

Material examined. Off Peru, Peru-Chile Trench, SEPBOP, R / V Anton Bruun, Cr. 17, Sta. 663-C, 13°44ʹS, 77°33ʹW, Menzies trawl, 4100 m, holotype ( USNM 1490737 ) and 6 paratypes ( USNM 1490738 ) .

Description. All specimens incomplete, seven anterior ends and one posterior end. Three longest anterior fragments with 24, 26, and 31 setigers (holotype); these 6, 5.5, and 8 mm long, respectively, and 0.6–0.8 mm wide. Posterior fragment with 13 setigers, 3 mm long and 0.5 mm wide. All fragments elongate, narrow, and cylindrical in cross-section; ventral groove absent. One specimen with weak mid-ventral ridge. Color in alcohol: flesh colored with no pigment. Two anterior fragments and posterior fragment gravid with large yolky eggs 140–150 µm in diameter with large germinal vesicle.

Prostomium broadly triangular, wedge-shaped, tapering to narrow anterior end ( Fig. 35A ); eyespots absent; nuchal organs forming prominent lobes on posterior lateral border ( Fig. 35A ). Peristomium divided into a broad, smooth anterior section that merges with a wider and shorter posterior section, likely in part an achaetous segment that bears a pair of dorsal tentacles anteriorly and first pair of branchiae on posterior border ( Fig. 35A ); second pair of branchiae on setiger 1 dorsal to notosetae ( Fig. 35A ). All dorsal tentacles and branchiae missing or limited to stumps or scars.

Parapodia reduced to low lobes from which setae emerge. Noto- and neuropodial setal fascicles close together. Some posterior parapodia with small papilla between noto- and neurosetae, likely a sense organ ( Fig. 35C ); present on posterior segments of anterior fragments and most segments of posterior fragment. Anterior setae all capillaries, 3–4 broad and brass-colored, a few others thin, more elongate, possibly natatory. Neuropodial acicular spines first present from setigers 11–14; one spine per neuropodium at first, then increasing to 3–4 spines over following 5–6 segments. Notopodial spines not present in available anterior ends, but both noto- and neuropodial acicular spines present on 13-segment posterior fragment. Form and arrangement of neuropodial spines identical in anterior fragments and single posterior fragment. Notopodial spines usually a single large spine with 1–2 capillaries; neuroacicular spines 2–3 with two capillaries ( Fig. 35C, G ). Individual spines golden in color, gently curved, tapering to pointed tip ( Fig. 35 D–F). Pygidium with a rounded lobe ventral to anal opening ( Fig. 35B ).

Methyl Green stain. Specimens staining uniformly with no pattern, destaining rapidly.

Etymology. The species name neogracilis is derived from the Latin, neo for new and gracile for slender or thin and indicating the close similarity with Chaetozone gracilis of Moore (1923).

Remarks. Chaetocirratulus neogracilis n. sp. from 4100 m off Peru is similar to Chaetozone gracilis from 4016 m off southern California ( Moore, 1923), which is known only from the holotype, redescribed by Blake (1996). Initially, these new specimens were thought to be C. gracilis because they come from similar abyssal depths, both have a broad conical prostomium, and a peristomium divided into a smooth anterior section and posterior section that bears both the dorsal tentacles and first pair of branchiae.

Chaetozone gracilis , however, is a much larger species, the holotype being 29 mm long with 110 setigers. In contrast, the largest anterior fragment of C. neogracilis n. sp. is only 8 mm long and has 31 setigers. A posterior fragment in the same collection has only 13 setigers and only adds 3 mm to the length. However, the size differential would not account for observed differences in the number and distribution of acicular spines along the body.

In Chaetozone gracilis , the notosetae are all capillaries except for the last, pre-pygidial segments (ca. setiger 108–110), where two spines accompany two capillaries; in C. neogracilis n. sp., the last 13 setigers of the one posterior fragment each have one large conspicuous notoacicular spine and two capillaries. The posterior neurosetae of C. gracilis include 4–6 acicular spines and an equal number of alternating capillaries. In contrast, the posterior neuroacicular spines of C. neogracilis n. sp. include 2–3 heavy spines and two capillaries. Both the noto- and neuroacicular spines of posterior segments in C. neogracilis n. sp. are large and provide the species with a conspicuous posterior armature. In C. gracilis , the posterior spines are not overly large or conspicuous. Chaetozone gracilis , however, should also be referred to Chaetocirratulus n. gen. and will be dealt with in a subsequent paper (Blake unpublished).

Distribution. Off Peru in abyssal depths, 4100 m.

Figures

FIGURE 35. Chaetocirratulus neogracilis n. sp. A, anterior end, dorsal view; B, posterior end, dorsal view; C, parapodium from posterior segment; D–F, neuropodial acicular spines; G, posterior end, lateral view. A, holotype (USNM 1490737); B–G, paratypes (USNM 1490738).