Tharyx killariensis Southern 1914

Tharyx killariensis (Southern, 1914)

Figure 1

Chaetozone killariensis Southern, 1914: 113 –115, Plate XII, figs. 27 A–D; McIntosh 1923: 491 –492, Plate CXXXVI, Fig. 13; Hartmann-Schröder 1971: 355.

Heterocirrus killariensis: Fauvel 1927: 97 –98, Fig. 86 d–h.

Caulleriella killariensis: Hartman 1959: 401.

Tharyx killariensis: Blake 1991:19; Hartmann-Schröder 1996: 399 –400, Fig. 192.

Material examined. IRELAND, Killary Harbor, Station W. 86, 1914, dredged in rich black mud at 7 fathoms (12.8 m), lectotype (NMINH 1914.348). Note: the label suggests two specimens (syntypes) in vial, but these are fragmented parts of the same specimen, here designated as the lectotype.

Additional material. Same station, collected in 1909 from Killary Harbor. Two specimens in vial labeled syntypes are fragments. No head or posterior ends available (NMINH 1909.151).

Description. Lectotype composed of three large fragments and two smaller fragments clearly all from the same specimen and with evidence of prior dissection; collectively about 10 mm long with 77 setigerous segments; anterior fragment 5 mm long with 47 setigers. Largest specimen recorded by Southern (1914) 11 mm long with 84 setigerous segments. Body not pigmented, light tan in alcohol. Body with dorsum of anterior and middle segments elevated above parapodia, forming distinct ridge along body (Fig. 1 A–B); first 30 segments narrower and more crowded than middle segments, middle segments not moniliform; far posterior segments again becoming narrow. Venter somewhat flattened throughout body, with narrow, weakly developed mid-ventral ridge formed of raised pads located in middle of each segmental annulus. Pygidium with dorsal anus and flattened ventral lobe (Fig. 1 C).

Prostomium triangular, pointed on anterior margin; eyes absent (Fig. 1 A–B); nuchal organ a narrow slit on posterior margin. Peristomium about as wide as long, annulations not visible dorsally, but with 2–3 weak divisions ventrally (Fig. 1 A). Paired dorsal tentacles arising from posterior margin of peristomium, first pair of branchiae arising on peristomium posterior and slightly lateral to tentacles, but in line with subsequent segmental branchiae (Fig. 1 A–B); second pair of branchiae on setiger 1, subsequent branchiae present on at least another 20–25 setigers (Fig. 1 B).

Parapodia low mounds from which setal fascicles arise; noto- and neuropodial setal fascicles positioned close to one another throughout. Setae all capillaries for first 50 or so setigers, with 5–7 capillaries in noto- and neuropodia. Southern (1914) reported spines (crotchets) from setiger 56 in neuropodia and setiger 61 in notopodia; lectotype similar with neuropodial spines present from setiger 51; notopodial spines first appearing with a single elongate spine among capillaries on setiger 56; posterior setigers with up to two long, narrow blunted spines and 2– 3 capillaries in notopodia and 3–4 short curved spines and 2–3 long capillaries in neuropodia (Fig. 1 D). Notoacicular spines long, thin, with narrow rounded tip bearing only a single knob (Fig. 1 E); Neuroacicular spines curved, somewhat geniculate with blunt tip bearing two weakly developed knobs; shortest spines ventralmost (Fig. 1 F) grading dorsally into longer and thinner capillaries.

Methyl green Stain. No stain retained.

Remarks. The general features originally described by Southern (1914) for Chaetozone killariensis are confirmed in the present description, including the nature of body segmentation, prostomium/peristomium, position of the dorsal tentacles, branchiae, and nature of the pygidium. However, Southern’s depiction of the spines or crotchets as having two small pointed teeth is not confirmed. The lectotype clearly shows that the tips of the noto- and neuropodial spines are blunted with, at best, two low knobs, not teeth. In this respect, this species belongs in the genus Tharyx as defined by Blake (1991). Further, the setal fascicles of noto- and neuropodia are positioned close to one another instead of being widely separated as in most reported species of Caulleriella (see Blake 1996).

Tharyx killariensis is similar to most of the known species of Tharyx in having spines in both the noto- and neuropodia of posterior setigers. However, the notoacicular spines of T. killariensis are few in number, narrow, and blunted, with heavier recurved spines limited to the neuropodia; the spines are also accompanied by capillaries. By having different types of spines in the noto- and neuropodia, T. killariensis most closely resembles T. alaskensis Blake, 2015 from the Alaskan Arctic. Further, the paired knobs on the tips of the spines are poorly developed in both T. killariensis and T. alaskensis compared with other species (Blake 1991, 1996, 2015). The two species differ in that capillaries accompany the spines in the posterior noto- and neuropodia of T. killariensis, whereas they are absent in T. alaskensis. Further, T. alaskensis has both the first and second pairs of branchiae located on setiger 1, whereas T. killariensis and other Tharyx species have the first pair of branchiae either on the posterior margin of the peristomium or on an achaetous segment anterior to setiger 1.

Most published accounts of Tharyx killariensis (e.g. Fauvel 1927; Hartmann-Schröder 1971, 1996) have largely repeated the descriptions and figures of Southern (1914) and as such the actual nature of any specimens subsequently collected and reported as this species in the intervening years from different localities cannot be confirmed. McIntosh (1923) appears to have examined some of Southern’s material, but he did not change the description of the spines; his only illustration was of a notopodial capillary. The brief description by Eliason (1962) from the Skagerrak appears to be a different species because distinct pigment is present on the dorsolateral margins of the peristomium, neuropodial hooks are first present in the anterior one-third of the body instead of the posterior one-third, and notopodial hooks are entirely absent.

Biology. Since the original report from Killary Harbor, Ireland, in shallow subtidal muds, Tharyx killariensis has been reported from SW England, the west coast of France, the southern North Sea, and the Wadden Sea as part of various marine benthic community studies. In some recent accounts the species is among the dominant species in low intertidal and shallow subtidal benthic assemblages (Reiss & Kröncke 2001; Nehmer & Kröncke 2003; Callier et al. 2009; Schückel et al. 2013).

Distribution. Northern Europe: Ireland, SW England, West coast of France, North Sea and Wadden Sea, low intertidal to shallow subtidal.