Thelepus Leuckart, 1849

Jirkov, Igor, 2018, Three new species of Thelepus Leuckart, 1849 from Europe and a re-description of T. cincinnatus (Fabricius, 1780) (Annelida, Terebellidae), ZooKeys 759, pp. 29-56 : 29

Thelepus Leuckart, 1849

Type species.

Amphitrite cincinnata Fabricius, 1780.

Diagnosis.

Branchiae formed of numerous simple filaments arranged in more or less distinct parallel transverse rows arising from S2-S4; notochaetae from S3 (= BS2), uncini from C3 (= S5); lateral lobes absent.

Remarks.

The genus includes 48 species ( Hsueh and Li 2016), distributed from the Arctic to the Antarctic and from the littoral to abyssal zones. The most important taxonomic characters used for species separation based on Day (1955), Hutchings and Glasby (1987), and this study are:

The number of branchial segments.

The number of BS varies from zero to three; most species have three BS. Only six species currently accepted as valid have two BS. Very little variation in the number of BS was observed; only one specimen amongst more than a thousand of all four species had a third branchia, on one side only. Of course, juveniles may have fewer BS, and some of the very small worms in the examined material had only one BS, or branchiae were absent. The final number of BS seems to appear when the size of the worm is approximately 1% of maximum.

The branchial fields from which the filaments arise.

A distinct median gap and lateral extension of the filaments appears to be constant within a species, but in species with numerous filaments both tend to change with size: as the gap becomes narrower, the extension goes further laterally.

The number of branchial filaments.

Some species have very few filaments in total, while others have many (10-40 or more). The number of filaments tends to increase with increasing size of the animal. Once adulthood is achieved, there is little variation in the number of filaments, independent of the size of the worms. According to our data, the maximum size of worms varies between localities for the same species, but the maximum number of filaments is relatively constant within a species. Hutchings and Glasby (1987) suggested that the relative number of branchial filaments between BS2, BS3, and BS4 is more important than the actual number of filaments. However, if there are only a few filaments, variation in their number leads to significant changes in the relative number of branchial filaments and this feature becomes unreliable.

The number of segments with notopodia and notochaetae.

There are two groups of species within Thelepus : (1) notochaetae present only on the anterior half of the body; there are numerous fully-developed segments without notopodia that differ from notopodial segments only by the absence of notopodia, and (2) species with notochaetae present for most of the body, absent only in the segments clustered near the pygidium. This difference seems to be diagnostic.

The number of rows of uncini.

Uncini can be in a single row or form a loop; all of the species investigated have a single row, but T. nucleolata Claparède, 1870, described from the Mediterranean (Gulf of Naples) has uncini forming a loop after S14. The species is poorly known and has not been recorded since the original description. The presence or absence of the loop seems to have high taxonomic value.

The shape of the uncinus.

The most important features seem to be the shape of the prow, the position of the attachment button, and the arrangement of teeth above the main fang-forming crest. The last character is better seen in SEM photographs, whilst the first two are better observed using a compound microscope. Three of the four investigated species with two BS have very similar U1 uncini, but other species inhabiting European waters, T. setosus (Quatrefages, 1866) and T. triserialis (Grube, 1855), have very different uncini (Fig. 1B, C). The shape of the uncini may vary along the body; they usually decrease in size but, in T. parapari sp. n., the shape also changes. Therefore it is best to examine and compare uncini from a specified unciniger, such as U1; comparison of previously described uncini without detail of the segment of origin has limited value.

Presence/absence of eyespots.

Hutchings and Glasby (1987) reported that, in some specimens of T. plagiostoma Schmarda, 1861 and T. robustus (Grube, 1878), eyespots may be absent. The species examined for this paper either have eyespots or not. Eyespots are sub-epithelial and disappear if the epithelium is macerated due to poor fixation.

Comparative size of notopodia.

In some species, the first notopodia are distinctly underdeveloped (for example Fig. 4D), whilst other species have all anterior notopodia of almost equal size. However, this difference may only be apparent in large worms.

Notochaetae.

The notochaetae of the four investigated species look very similar. The shape of the notochaetae is of limited taxonomic value, at least for the species examined here.

Tubes.

The tubes of all the investigated species are constructed using local material (shell fragments, small stones, spicules etc.) without specificity. Tubes are also attached to larger substrata, usually stones, if possible. Some tubes have a branched crown very similar to that reported for Axionice conchilega (Pallas, 1766) by Holthe (1986); this was observed in material examined in this study from the Norwegian Sea.