Lumbricillus dubius (Stephenson, 1911)

Klinth, Marten J., Rota, Emilia & Erseus, Christer, 2017, Taxonomy of North European Lumbricillus (Clitellata, Enchytraeidae), ZooKeys 703, pp. 15-96 : 50

Lumbricillus dubius (Stephenson, 1911) Fig. 23

Enchytraeus dubius Stephenson, 1911: pp. 54-58, figs 10-12 & pl. II, figs 12-14;

Lumbricillus dubius ; Nielsen and Christensen 1959: p. 96; Finogenova and Timm 1988: pp. 92-93, fig. 1; Klinth et al. 2017.

Type material.

Typus amissus (Nomenclatura Oligochaetologica). Type locality: Firth of Clyde, Wemyss Bay, United Kingdom ( Stephenson 1911). We did not designate a neotype as we do not have material from the type locality.

Material examined.

SMNH 152726 (CE5221) & SMNH 152727 (CE5223), two mature specimens from Sweden, and ZMBN 107835 (CE22767), ZMBN 107836 (CE23370), ZMBN 107837 (CE23371), ZMBN 107839 (CE24700), ZMBN 107840 (CE24711) & ZMBN 107841 (CE24726), six mature specimens from Norway. For information on specimen collection localities and GenBank accession numbers see Appendix 1.

Description.

White to yellow worms. Length (fixed worms) more than 2.1-6.1 mm (amputated specimens), first 15 segments 1.5-2.5 mm long, width at clitellum 0.32-0.55 mm. More than 20-44 segments. Chaetae straight or slightly sigmoid (Fig. 23A). All observed bundles with two chaetae. Each worm’s longest measured chaetae 50-75 µm long, about 5 µm wide. Clitellum extending over XII– 1/2XIII. Head pore at 0/1. Epidermis with transverse rows of gland cells.

Coelomocytes numerous, 15-30 µm long, spindle-shaped, oval, round, granulated with distinct nucleus. Paired pharyngeal glands present in IV, V and VI. Each pair converges dorsally, connection, if present at all, indistinct (Fig. 23B). Dorsal vessel originating in XIII. Nephridia observed in XVIII–XXI, 50-65 µm long, anteseptale funnel only, postseptale oval, tapering into efferent duct. Brain longer than wide, with posterior incision.

Male genitalia paired. Testes (Fig. 23D) originating in XI, in some specimens extending forwards into X, with testis sacs covering mass of irregularly arranged lobes and detached fragments, fragments spreading in XI–XII. Sperm funnels (Fig. 23D) in XI, 180-390 µm long, 70-145 µm wide, making them 2.5-4 times longer than wide, funnels tapering towards vasa deferentia. Most of vasa irregularly coiled in XI–XII, 5-10 µm wide. Penial bulbs (Fig. 23E), 110-190 µm in diameter, divided into two bulbs each with an extending horn. Ovaries in XII. One to five mature eggs present at a time.

Spermathecae (Fig. 23C) in V, pouch-shaped, without distinct ampulla, gradually widening, entally connecting with oesophagus. Sperm completely occupying lumen of duct and ampulla, regularly arranged with spermatozoan heads facing the wall and tails along the duct, forming denser aggregation throughout the centre of the spermathecae. Spermathecae 95-205 µm long, 40-100 µm wide at widest part of ampulla. Gland cells surrounding ectal pore, divided into few flaps, whole glandular body 70-120 µm in diameter at its widest part. Up to two midventral subneural glands in XIV– XV, 60-80 µm, 60-65 µm long, respectively; glands in XV not observed in all specimens.

Geographical distribution including BOLD data.

Genetically identified from Norway, Russia (White Sea), Sweden and the United Kingdom ( Klinth et al. 2017). BIN-number: BOLD:AAU0151.

Remarks.

The specimens examined match the description of Lumbricillus dubius by Stephenson (1911) well, but they are smaller in body size and have testes that seem to form an irregular mass rather than being made up of branches as observed by Stephenson. The clearly divided penial bulbs and the morphology of the spermathecae bear a close resemblance between our specimens and the original description. Stephenson wrote that no sperm were observed in the spermathecae, but his illustrations depicting sections of the same clearly show the unique distribution of spermatozoa with heads regularly arranged perpendicular to the spermathecal wall. It is possible that he did not recognize them as sperm, simply because of this unusual arrangement.

Lumbricillus dubius has irregularly lobed testis sacs and spermathecae that are at least superficially similar to those of L. arenarius . The chaetae are straight to slightly sigmoid and few in number, which further supports the close relationship with L. arenarius and L. sp. H.

Lumbricillus dubius is genetically closely related to L. sp. H and L. arenarius (Fig. 1).