Thelepus davehalli, Jirkov, Igor, 2018

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 davehalli sp. n. Figs 4, 11

Material

(Table 1): 444 specimens from 27 stations, collected at depths from 94-495 m, 1.73 ° C– 11.3 °C. Holotype st. Sevastopol 2587. Material is deposited at the KGB, three paratypes from Sevastopol st.1453 are deposited at the MNCN 16.01/17772. Material from Aveiro (DBUA0000389.01) and Naples (MNCN 16.01/488) is not included in the type series as it was collected too far away from the type locality, despite seeming to be morphologically identical.

Description

(based on holotype and paratypes). Holotype with 97 segments, 32 segments with notopodia, 95 mm length. Paratypes up to 100 mm long and 5 mm wide; number of segments increased with body size, up to 91.

Several tens of grooved buccal tentacles as long as half body length. Eyespots absent. Branchial filaments numerous, long and tangled (Fig. 4A, C). Due to tangling, it was impossible to count number of branchial filaments in large worms (>5-6 mm width) without removing them one by one. Maximum number of BS1 filaments ca. 20 (13 in holotype), extending laterally to a point level with upper edge of row of U1 uncini. BS2 with ca. ten filaments (nine in holotype). Filaments attached to a transverse elevated stump in 1-2 irregular rows but, due to numerous filaments, there appear to be more rows. Number of filaments increases with body size; small worms (1-2 mm width) with ca. 5 filaments on BS1. Lateral extension of filaments depends upon worm size: in small worms, filaments extend only to a point level with notopodia. Lateral lobes absent. Dorsum with warts or subepithelial honeycomb, forming more or less regular rows (Fig. 4A, C); number of rows increases with size of segments and worms. Segmentation distinct. Nephridial papillae on S5-S7 above neuropodia (Fig. 4C, arrowed), usually poorly visible or not visible; papillae on S4 apparently absent. Ventrum glandular, with “wrinkling” (Fig. 4B) increasing with worm size.

Notopodia from BS2. In small worms, more or less similar, almost cylindrical; in large worms, anterior notopodia transversely flattened, those in first few anterior segments several times smaller than those that are most developed (Fig. 4). Largest specimens in each sample with about 30-40 segments with notopodia, the smallest with fewer, but even specimens ten or more times smaller than largest (by size) with over 30; the next 40-60 segments without notopodia, i.e. about 1/3-1/2 of body length without notopodial segments. Notochaetae with narrow brims (Fig. 11A).

Neuropodia from C3; tori increasing in size to U10, then becoming progressively smaller. Uncini in a single row, uncini of U1 with well-developed prow and crest with one tooth in profile (Fig. 4F, G); posterior uncini (U48) very similar (Fig. 4H).

Pygidium with crenulated margin, without cirri or papillae.

Differential diagnosis.

Only one previously known species, T. pascua (Fauchald, 1977) from the Atlantic coast of Panama, has two pairs of branchiae and no eyespots. It differs from T. davehalli sp. n. in its lower number of branchial filaments: single filament in BS1 and BS2 in T. pascua ; up to 20 filaments in BS1 and up to 10 filaments in T. davehalli . Only one previously known species, T. hamatus Moore, 1905 from Pacific Alaska, has two pairs of branchiae and segments of the posterior half of the body without notopodia. It differs from T. davehalli in the presence of eyespots and a lower number of branchial filaments: five in BS1 and BS2 in T. hamatus ; up to 20 filaments in BS1 and up to 10 filaments in T. davehalli . Thelepus davehalli differs from the other species described in this paper and other known species with two pairs of branchiae in the presence of fully developed segments without notopodia in the posterior 1/3-1/2 of the body.

The last biramous parapodia of Thelepus davehalli is well developed (not reduced), following uniramous parapodia with well-developed neuropodia, contrary to other species described in this study (Fig. 4D, E). Anterior segments lack well-developed notopodia, contrary to those in T. cincinnatus and T. marthae .

Remark.

Thelepus cincinnatus var. andreanae McIntosh, 1922 was described from within the range of T. davehalli . However, McIntosh clearly stated "Dorsal cephalic collar with eye-specks" while this new species has no eyespots.

Etymology.

The species is named after my friend Mr. David Hall, Head of Marine and Freshwater Laboratories, Associate Director APEM Ltd., UK (Fig. 5).