Tharyx robustus, Blake, James A. & Göransson, Peter, 2015

Blake, James A. & Göransson, Peter, 2015, Redescription of Tharyx killariensis (Southern) from Ireland and description of two new species of Tharyx from the Kattegat, Sweden (Polychaeta, Cirratulidae), Zootaxa 4039 (4), pp. 501-515 : 508-510

publication ID	https://doi.org/ 10.11646/zootaxa.4039.4.1
publication LSID	lsid:zoobank.org:pub:4B12AA52-6A3F-44B9-BBEC-EBABF0E22DF5
DOI	https://doi.org/10.5281/zenodo.5658081
persistent identifier	https://treatment.plazi.org/id/EE2B2061-FFC0-027E-FF2F-0F1EFC03F9FA
treatment provided by	Plazi
scientific name	Tharyx robustus
status	sp. nov.

Treatment

Tharyx robustus View in CoL new species

Figures 3 View FIGURE 3 D, 4

Material examined. SWEDEN, Kattegat, near Varberg, 0 3 Aug 2012, in sediments near eelgrass beds, 57.11501°N; 12.22611°E, 0.3–1.7 m depth, coll. P. Göransson, holotype ( SMNH 8756), six paratypes ( SMNH 8757).

Description. A moderate-sized species, holotype complete, 8 mm long, 1.0 mm wide for 105 setigerous segments; paratypes include complete and incomplete specimens, 4–6 mm long, 0.5 mm wide, with 80–116 setigerous segments. Color in alcohol light tan to opaque white, with a cluster of black pigment on posterior lateral margins of peristomium ( Fig. 4 View FIGURE 4 C); some specimens with brown pigment in lateral segmental grooves in middle of body. Body broad, dorsoventrally flattened throughout with numerous narrow segments ( Figs. 3 View FIGURE 3 D, 4A–B). Anterior segments becoming widest at about setiger 15, continuing posteriorly, narrowing at about setiger 60, then expanding again in posterior region, narrowing in far posterior segments to a simple pygidium consisting of a narrow, conical lobe; anal cirri absent ( Fig. 4 View FIGURE 4 B). Venter flattened, with shallow ventral groove bearing mid-ventral ridge formed of two raised pads where each segment meets at ventral mid-line ( Fig. 3 View FIGURE 3 D).

Prostomium triangular, acutely pointed on anterior margin, broadening posteriorly ( Figs. 3 View FIGURE 3 D, 4A, C); peristomium wider than long with weak dorsolateral groove forming two indistinct annuli ( Fig. 4 View FIGURE 4 C). Nuchal organs inconspicuous, located on posterior margin of prostomium ( Fig. 4 View FIGURE 4 C), not pigmented; eyes absent. Dorsal tentacles present on posterior margin of peristomium, with first pair of branchiae located immediately posterior to tentacles, also on peristomium ( Figs. 4 View FIGURE 4 A, C); second pair of branchiae on posterior margin of setiger 1, medial to notosetal fascicle; in posterior setigers branchiae arise in middle of segment rather than on posterior margin.

Parapodia on both sides of body separated from broad dorsum by a narrow groove extending from peristomium to end of body; individual notopodia slightly elevated above this groove, less obvious in posterior one-third of body ( Fig. 4 View FIGURE 4 A). Broad dorsum of body raised above parapodia, rounded, low, without mid-dorsal groove ( Fig. 4 View FIGURE 4 A).

Noto- and neuropodia with setal fascicles arising close together ( Figs. 4 View FIGURE 4 D, E), anterior parapodia with spreading fascicles of about 7–8 capillary setae in notopodia and 8–10 capillaries in neuropodia; in far posterior notopodia, 2–3 transitional pointed setae ( Fig. 4 View FIGURE 4 F) occur with capillaries from about setiger 85 replaced by stiff elongate spines from about setiger 88–90 ( Fig. 4 View FIGURE 4 H); neuropodial spines first present from setigers 80–90 with 3–7 capillaries and 1–3 short, spines with variable lengths ( Figs. 4 View FIGURE 4 H–J); all spines blunt-tipped, with apex weakly expanded, knob-like ( Figs. 4 View FIGURE 4 I–J). Transitional notopodial spines with angled pointed tip, sometimes with fibrils visible along shaft.

Methyl Green Stain. No distinct stain apparent; most of the body stains darkly, but de-stains rapidly leaving no pattern after differentiation. The prostomium and pygidium never retain stain.

Remarks. In having a robust dorsoventrally flattened body with crowded segments along its entire length, T. robustus n. sp. most closely resembles T. retusisetus ( Hutchings & Murray, 1984) from New South Wales, Australia. The latter species however, has posterior spines with sub-bidentate knob-like tips that are typical of many species of Tharyx ( Blake 1991, 1996). In contrast, the posterior spines of T. robustus n. sp. are variable in length and have only a single knob on the end of the spine and are only weakly expanded. None of these spines resemble the sub-bidentate spines found in several other species. The kind of transitional notopodial spines found in T. robustus has not been reported in other species, but may have been overlooked. The robust body and narrow crowded segments of T. robustus n. sp. are atypical for species of Tharyx and are more reminiscent of species of Aphelochaeta . However, another feature that allies this species with Tharyx is the position of the dorsal tentacles and first pair of branchiae. In nearly all previously described species of Tharyx , the tentacles occur on the peristomium with the first pair of branchiae occurring directly posterior to them, also on the peristomium ( Blake 1991, 1996, 2015). This is exactly how these two structures are positioned in T. robustus n. sp. In addition, lateral peristomial pigment spots are present on T. robustus n. sp. These are known from other species of Tharyx including T. kirkegaardi Blake, 1991 , T. circacutus Blake, 2015 , T. maryae n. sp., and other species as yet undescribed (Blake, pers. obs.). Such pigment has not been reported for species of Aphelochaeta or other bitentaculate cirratulids.

Etymology. The name of this species is derived from the word “robust,” and refers to the nature of the overall body shape which is unlike most other species of Tharyx .

Biology. Same data as for T. maryae (see above).

Distribution. Known only from the west coast of Sweden in low water.