Tharyx alaskensis Blake, 2015, new species

Tharyx alaskensis new species

Figures 19‒20

Chaetozone setosa: Pettibone 1954: 287 ‒288, Fig. 33d, in part. Not Malmgren 1867. Chaetozone cf. gracilis: Busdosh 1984. Not Moore 1923.

Material examined. Alaskan Arctic, Beaufort Sea, Prudhoe Bay, coll. M. Busdosh, 0 6 Aug 1983, 70°24.1ʹN, 148°32.3.0ʹW, 2.5 m, push cores, Waterflood 6 project, R/V Annika Marie, Sta. 50-1, holotype (LACM-AHF Poly 6554), 10 paratypes (LACM AHF Poly 6555). Off Point Barrow, coll. G.E. McGinitie, 0 9 Aug 1949, 7 miles offshore, 128 m, stones and gravel, 1 specimen (USNM 1263247); coll. 0 6 September 1949, 7.5 miles offshore, 66 m, removed from growth on rocks, 2 specimens (USNM 1263248); 14 Oct 1949, 4 miles offshore, 53 m, gravel and small stones 1 specimen (USNM 22810).

Description. Holotype 11 mm long, 0.7 mm wide for ca. 75 setigerous segments; paratypes of similar size. Body light tan in alcohol, without pigment. Body with dorsum of anterior and middle segments slightly elevated above parapodia; venter somewhat flattened throughout body, with weak ventral groove in posterior 30 segments. First 12‒15 segments narrower, more crowded than middle segments, no moniliform segments except in some juveniles; far posterior expanded region again with narrow, crowded segments (Fig. 20 C).

Prostomium triangular, narrow, pointed on anterior margin; eyes absent (Figs. 19 A, 20A); nuchal organs small, crescent shaped notch at posterior lateral margin of prostomium, not pigmented. Peristomium longer than wide, with 2‒3 distinct annulations visible dorsally, depending on preservation (Fig. 20 A), with 2–3 weak divisions ventrally; dorsal midline with a long ridge, extending to end of peristomium (Figs. 19 A, 20A), better developed on some specimens. Paired dorsal tentacles arising from posterior margin of peristomium, first pair of branchiae arising lateral and posterior to tentacles on setiger 1 (Fig. 19 A); second pair of branchiae on posterior margin of setiger 1, subsequent branchiae on following setigers.

Parapodia low mounds from which setal fascicles arise; noto- and neuropodial setal fascicles positioned close to one another throughout. Notosetae all capillaries for first 55 setigers with 5–7 capillaries in noto- and neuropodia; notosetae of far posterior setigers becoming spinous, straight, with blunt tips (Fig. 19 D–E); natatory setae present in mature individuals; neurosetae transitioning from capillaries in anterior and middle setigers to short curved spines at about mid body, setiger 34 in holotype. Neuropodial spines curved, somewhat geniculate with tip blunt; shortest spines ventralmost (Fig. 19 D, F).

Pygidium with dorsal anus and flattened ventral lobe (Figs. 19 B‒C, 20B).

Methyl Green staining pattern. No distinct staining pattern apparent.

Remarks. Tharyx alaskensis was one of the dominant polychaetes in surveys offshore Prudhoe Bay in shallow water as part of benthic monitoring associated with oil and gas development in the early 1980s (Busdosh 1984). At the time, the species was tentatively referred to Chaetozone cf. gracilis Moore, probably because posterior neuropodial spines appeared to be limited to the neuropodia. In actuality, there are spinous notosetae in far posterior notopodia, but they are stiff, narrow, thinner and with a blunt point instead of short, thick, and curved neuropodial spines. Based on Blake’s (1991) revision of some cirratulid genera, these specimens should be referred to the genus Tharyx because the posterior spines are few, blunt-tipped, and not formed into cinctures. The species is unusual in that the posterior notopodial and neuropodial spines differ from one another: the notopodial spines are straight, stiff, thin, and elongate, whereas the neuropodial spines are curved, somewhat geniculate, short, and thick; both sets of spines are blunt-tipped and not pointed, but are not of the typical knob-tipped form of related species. A few specimens of T. alaskensis n. sp. were encountered among the cirratulids identified as Chaetozone setosa by Pettibone (1954) from off Point Barrow, Alaska, in deeper water and are described as C. pigmentata n. sp. elsewhere in this paper. Pettibone’s Figure 33 D is most definitely T. alaskensis n. sp. because it clearly depicts two pairs of branchiae on setiger 1. The Point Barrow specimens agree very well with the shallower water specimens from Prudhoe Bay.

Tharyx alaskensis n. sp. appears most similar to T. killariensis (Southern, 1914), which was originally described from Ireland as a species of Caulleriella and is likely widespread in northern Europe (Blake & Göransson, in preparation). In T. killariensis, however, the posterior noto- and neuropodial spines are few in number and are accompanied by capillaries, which are not present in the far posterior parapodia of T. alaskensis n. sp. A major difference between T. alaskensis n. sp. and other species is that the first and second pairs of branchiae both occur on setiger 1. Other species either have the first pair of branchiae on the posterior margin of the peristomium or on an achaetous segment anterior to setiger 1 and the second pair on the subsequent setigerous segment.

Biology. Tharyx alaskensis n. sp. (as Chaetozone cf. gracilis) was the single most abundant benthic invertebrate collected during a monitoring survey in the Beaufort Sea, Prudhoe Bay, in August 1983 (Busdosh 1984). A total of 3,458 specimens were identified, accounting for 30.6% of the entire infaunal abundance. Other commonly associated species included the polychaetes Eteone longa (Fabricius, 1780), Capitella capitata (Fabricius, 1780), and Chaetozone ruffi n. sp. (as C. setosa), the cumacean Diastylis sulcata Calman, 1912, and the isopod Saduria entomon (Linnaeus, 1758). Tharyx alaskensis occurred in soft sediments, often with debris. One specimen with natatory setae was gravid, with the body filled with eggs of about 150 µm in diameter. The few specimens available from offshore Point Barrow in deeper water were largely collected from coarse rocky substrates.

Etymology. The name refers to Alaska, where the species occurs and appears to be endemic.

Distribution. Known only from Alaska, Beaufort Sea; shallow subtidal to 128 m offshore.