Tharyx tierralobos Blake 2018, new species

Tharyx tierralobos new species

Figures 60–61

Material examined. Off northern Peru, Isla Lobos de Tierra, SEPBOP, R / V Anton Bruun, Cr. 16, Sta. 635-A, coll. 05 Jun 1966, 06°27ʹS, 80°56ʹW, trawl, 160 m, holotype (USNM 1490757) and 3 paratypes (USNM 1490758).

Description. A small species, holotype complete, 7.1 mm long, 0.5 mm wide across anterior segments, with approximately 115 setigerous segments; one complete paratype smaller, 3.4 mm long, 0.3 mm wide across anterior segments, with 110 setigerous segments. All segments short, crowded throughout, not becoming longer in middle body (Figs. 60A, 61 A–B). Body generally dorsoventrally flattened, especially in posterior half; anterior segments with dorsum weakly elevated and rounded over lateral parapodial shoulders (Fig. 60A); venter flattened with midventral groove extending along entire body (Fig. 61B); venter of far posterior segments with broad shallow groove into which neurosetae project (Fig. 61C). Color in alcohol: light tan, no body pigment present on holotype; one paratype with dusky pigment on peristomium and at interface with setiger 1.

Prostomium triangular, tapering to narrow tip (Figs. 60A, 61A); eyespots absent; nuchal organs narrow slits on posterior lateral margins. Peristomium with three weakly developed rings with grooves not crossing dorsum (Fig. 60A); with prominent dorsal crest extending onto setiger 1 (Fig. 60A). Dorsal tentacles on posterior margin of peristomium, lateral to posterior extension of dorsal crest (Fig. 60A). First pair of branchiae on setiger 1, dorsal to notosetae (Fig. 60A); subsequent branchiae in similar position; branchiae present on all segments of first one-third of body, irregularly present on middle and posterior segments.

Noto- and neuropodia moderately separated from one another, most noticeably in middle and posterior segments; anterior notopodia forming distinct dorsal shoulders in anterior segments (Fig. 60A), less distinct shoulders posteriorly; neuropodia less well developed. In middle and posterior segments, setae arising from low ridge. Anterior setae all capillaries, numbering about 5–6 per fascicle. Neuropodial hooks first present from setiger 28, numbering 1–2 at first with 2–3 thin capillaries, increasing to 3–4 hooks in middle and posterior segments with 2–3 thin capillaries; neuropodial hooks short, geniculate, tapering to rounded weakly expanded tip, appearing hollow internally (Fig. 60 B–C, inset). Notosetae all capillaries through middle segments replaced in posterior segments by 3–4 long geniculate setae that taper to narrow pointed tip (Fig. 60D); these accompanied by 3–4 long, thin capillaries.

Pygidium with about 6–8 glandular papillae surrounding anal opening.

Methyl Green stain. Stain retained on dorsum and venter of prostomium and peristomium producing distinct pattern, best seen on holotype (Fig. 61A). No stain retained on individual segments.

Etymology. The name tierralobos is derived from the name of the island “Lobos de Tierra” near the collecting site off northern Peru.

Remarks. Tharyx tierralobos n. sp. differs from most of its congeners by having a relatively thick body with numerous short segments throughout and with the tips of the neuropodial geniculate hooks expanded, but with a rounded tip instead of the sub-bidentate or irregular knobby tip characteristic of most Tharyx species. Based on the body shape and details of the noto- and neuropodial hooks, T. tierralobos n. sp. is most similar to T. robustus Blake & Göransson, 2015, from northern Europe. The two species differ in that T. robustus has the first pair of branchiae on the peristomium posterior to the base of the dorsal tentacles whereas T. tierralobos n. sp. has the first branchiae arising directly on setiger 1. Additional differences are that (1) T. robustus has characteristic pigment spots on the posterior lateral margins of the prostomium that are absent in T. tierralobos n. sp.; (2) both species have relatively flattened bodies, but T. tierralobos n. sp. has a prominent ventral groove along most of the body that is not welldeveloped in T. robustus, which instead has a distinct mid-ventral ridge; and (3) the notopodial acicular hooks of T. robustus consist of both short and long curved setae, whereas the notopodial hooks of T. tierralobos n. sp. are limited to long, geniculate hooks that taper to a point.

It is noted that the noto- and neuropodia of T. tierralobos are moderately separated from one another, especially in middle and posterior segments, a characteristic of species of Caulleriella. However, by lacking any evidence of bidentate hooks, this species has been assigned to Tharyx, but is clearly an outlier and deserves further study.

Distribution. Known only from off Peru, near Isla Lobos de Tierra, 160 m.