Tharyx Webster & Benedict 1887

Genus Tharyx Webster & Benedict, 1887

Type-species Tharyx acutus Webster & Benedict, 1887, designated by Hartman (1959). Revised by Blake (1991).

Diagnosis. Prostomium conical; peristomium elongate, with pair of grooved dorsal tentacles arising on posterior margin anterior to setiger 1; first pair of branchiae typically arising immediately posterior to dorsal tentacles either on posterior margin of peristomium or rarely on anterior margin of setiger 1; abdominal segments sometimes beadlike. Noto- and neuropodia located close to one another, not widely separated. Setae including simple capillaries in anterior and middle setigers, acicular spines in posterior setigers with irregular notched tips, sometimes appearing weakly bidentate, with pair of stunted or rounded knobs but never with distinct, sharply pointed teeth; spines present either in both posterior noto- and neuropodia or only in neuropodia. Pygidium with terminal anus and small ventral lobe or disk.

Remarks. Blake (1991) determined that the type species of the genus Tharyx, T. acutus Webster & Benedict, 1887, had knob-tipped hooks or spines in addition to capillaries. This discovery differed from the traditional concept of Tharyx, which had been based upon specimens having all capillaries (Hartman 1961). Blake (1991) restricted species of Tharyx to those having blunt or knob-tipped spines and moved species having simple and denticulated capillaries to the genera Aphelochaeta and Monticellina, respectively. The genus Tharyx appears most closely related to Caulleriella in that both genera have species with modified spines that are more or less bidentate. In addition to the nature of the posterior spines, Caulleriella and Tharyx species are easily distinguished by the position of the noto- and neuropodia. In Caulleriella, the setal fascicles are widely separated from one another, so much so, that in cross section of some species, they appear at four corners of the body. In Tharyx, on the other hand, the setal fascicles are close together.

Blake (1991) originally included five species in the genus Tharyx, one of which, T. killariensis is redescribed in the present paper. Four additional species were added by Blake (1996, 2015) and Magalhães & Bailey-Brock (2013). Two new species discovered in shallow-water sediments in Sweden are here described, bringing the total number of species in the genus to eleven. In addition, another eight species of Tharyx are known to the first author from North America and Antarctica. With the number of species thus nearly tripled over the past 24 years, there is now a larger suite of characters available to define the genus than was available in the past. The morphological characters that best define the known species of Tharyx are reviewed.