Enchytraeus albidus

Erséus, Christer, Klinth, Mårten J., Rota, Emilia, De Wit, Pierre, Gustafsson, Daniel R. & Martinsson, Svante, 2019, The popular model annelid Enchytraeus albidus is only one species in a complex of seashore white worms (Clitellata, Enchytraeidae), Organisms Diversity & Evolution (New York, N. Y.) 19 (2), pp. 105-133 : 127-129

E. albidus B clade C^; Erséus and Gustafsson 2009.

Material examined SMNH 172891-172892 ( CE 1684 and CE 1689), two sexually mature and COI-barcoded specimens from algal compost in Galicia, Spain. For more details, including GenBank accession numbers for genetic data, see Table 1 .

Diagnosis A few chaetal bundles with more than three chaetae; sperm funnels at least 1.5–2 times longer than wide; vasa deferentia tripartite, ental and ectal sections thin-walled, middle section thick-walled and lacking ciliation; penial bulbs larger than accessory glands; spermathecae without diverticula.

External characters Color white. Length of first 16–19 segments> 2 mm (fixed, amputated specimens); first 12 segments (anterior end to clitellum) 2.9–3.8 mm long; width at clitellum 0.40–0.62 mm. Chaetae straight or slightly curved. Lateral bundles with three chaetae anterior to clitellum, two chaetae in XII and postclitellar segments. Ventral bundles with three–four chaetae anterior to clitellum, missing in XII, two chaetae in postclitellar segments. Chaetae longest in preclitellar ventral bundles (VIII–XI), 55–75 μm long, about 5 μm wide. Clitellum extending over XII–¾XIII. Head pore not observed. Epidermis with transverse rows of gland cells. Internal characters Coelomocytes numerous, 10–15 μm long, round, oval, or spindle-shaped, granulated and with distinct nucleus. Paired pharyngeal glands present in IV, V, and VI. All pairs seemingly connected dorsally and possessing secondary lobes. Esophageal appendages (peptonephridia) extending from dorsal wall of esophagus in III. Origin of dorsal vessel not observed. Nephridia observed in 6/7–9/10, about 110 μm long, with oval postseptale tapering into posteroventral efferent duct. Brain truncate posteriorly.

Male genitalia paired. Testes in XI, paired, each enclosed in sac containing different stages of spermatogenesis; sacs extending forwards into IX. Sperm funnels at least 215–305 μm long, 135–160 μm wide at the widest point, making them at least 1.5–2 times longer than wide, tapering towards vasa deferentia. Vasa irregularly coiled in XII–XVIII, tripartite, ental and ectal portions thinner, 15–30 μm wide with 2.5–5 μm thick wall, widening to thicker mid portion, 50–55 μm wide with 15 μm thick wall ( Fig. 12a ). The transition between portions gradual, but with an abrupt change in the thickness of the duct wall; ciliation only in parts with thin wall. All parts lacking conspicuous musculature. Ventral surface of XII with invaginations creating two recesses with overhanging lips. Penial bulbs compact, round, 85–90 μm in diameter, not pierced by vasa, surrounded by accessory glands of smaller size ( Fig. 12b ). Ovaries in XII. About one to five mature eggs present at a time.

Spermathecae in V. Ectal pore at lateral line. Ectal duct thin, entally opening into a small, rounded chamber, distinctly set off from round ampulla; ampulla of similar length as entire duct ( Fig. 12c ). Ampulla round, without diverticula, connect- ed to lateral side of esophagus. Sperm in lumen of ampulla. Spermathecae 155–180 μm long, 95–125 μm wide at widest part of ampulla. Gland cells surrounding ectal duct, forming compact mass 60–65 μm in diameter at its widest part; small chamber devoid of gland cells. No obvious midventral subneural glands observed.

Remarks Our association of this species with the Mediterranean L. krumbachi Čejka, 1913 , from a beach in Rovinj ( Croatia), is largely based on the tripartition of the vasa deferentia. The three parts have different widths and wall thicknesses, and cilia are absent in the middle, thickened, tracts, conforming with Čejka’ s observation. Similarly, Lasserre and Erséus (1976, plate 1C) showed a cross section of the thick-walled, but unciliated, part of a vas deferens in their Bermudian form of B E. albidus^ (see also below). We observed that the vasa deferentia change in width over their length also in E. albidus s. str. and E. albellus sp. nov. In E. albidus s. str., they gradually (and slightly) widen from the ental to the middle part and then taper ectally, without any clear differentiation in wall thickness between the parts. In E. albellus , the difference in width between the ental, middle, and ectal parts is not as distinct as in our E. cf. krumbachi , but the middle part clearly has a thicker duct wall than the ental and ectal parts. However, E. albellus has cilia all along the vas’ walls, and spermathecae with diverticula, two features distinguishing it from E. cf. krumbachi .

Nevertheless, there are some morphological discrepancies between our Galician material and Čejka’ s species. First, our form has fewer postclitellar chaetae per bundle; Čejka’ s (1913) form had three chaetae in lateral, four chaetae in ventral bundles. Second, in a horizontal section of segment V (ibid., fig. 9), Čejka did not observe any separate chambers at the inner end of the spermathecal ducts (cf. our Fig. 12 ), although, in the same figure, the spermathecal ampullae were shown as barely wider than the glandular ducts, and his specimen may have been in a stage, where such chambers were still undeveloped.

The combination of the distinctly separated parts of the vasa deferentia, latitude of collection, and the adiverticulate spermathecae prompted us to denote our species as E. cf. krumbachi . And besides, based on the morphological variation within the E. albidus complex revealed here, and the more stringent definition of E. albidus proposed above, the proposed synonymization of E. krumbachi with E. albidus (see Lasserre and Erséus 1976) does not seem to be justified. However, it would be premature to formally attach the name E. krumbachi (without B cf.^) to our Galician specimens, considering that (1) we have no access to topotypes of the Adriatic E. krumbachi , (2) the number of postclitellar chaetae per bundle are fewer in our specimens than in the original description, and (3) additional forms within the E. albidus complex with the distinctly tripartite vasa deferentia are known from seashores of the North Atlantic/Mediterranean area; however, these forms have not been described in sufficient detail to establish whether or not our Galician specimens are conspecific with any of these proposed species.

There are at least three other literature reports of white worms with bi- or tripartite vasa deferentia of the kind described here. In 1874, Verrill described the species H. littoralis from the coast of Massachusetts ( USA), in the Northwestern Atlantic. His brief morphological description (in Verrill and Smith 1874: 329–330) was later amended by Smith (1895), who transferred the species to Enchytraeus and described a long vas deferens with expanding thickness going from quite thin to one fifth the diameter of the entire worm in that region. Due to the pronounced thickening of the vasa deferentia, we do not consider E. littoralis as a junior synonym of E. albidus , as suggested by Michaelsen (1900). Furthermore, we had no access to topotypical material of E. littoralis , and thus, we cannot assess whether these two species are similar in other characters.

As noted above, Lasserre and Erséus (1976) referred to worms found on subtropical Bermuda, i.e., about 1200 km SSE of the type locality of E. littoralis , as E. albidus . The Bermudian worms also had massive outer parts of the vasa deferentia (op.cit.: pl. 1C): the latter were 65–70 μm wide, at least one fifth of the total body diameter, and with 20–25 μm thick walls. These measurements match rather well with the ones we observed for both E. cf. krumbachi (up to 55 μm wide, wall up to ca. 15 μm thick) and E. albellus (up to 65 μm wide, wall up to 20 μm thick). Lasserre and Erséus did not provide a complete morphological description, and we have neither new morphological nor any genetic data for the Bermudian form. Nevertheless, with regard to the similarities in the vasa deferentia, we cannot rule out the possibility that this form was conspecific to one of the two aforementioned taxa. If so, E. cf. krumbachi is more likely to be conspecific with the Bermudian specimens, as its latitudinal distribution is more similar. By contrast, the distribution of E. albellus extends into the High Arctic, which may indicate that this species is better adapted to a colder environment.

The third report is the description of Enchytraeus mediterraneus Michaelsen, 1926 from the coast of Tunisia. In this species, the width of the vas’ thick portion measured up to 85 μm. However, spermathecal ectal glands are absent in E. mediterraneus but present in our Galician worms (see more under B Remarks^ to Enchytraeus sp. 1 ); we therefore do not consider our Galician specimens to be conspecific with E. mediterraneus .

The Galician E. cf. krumbachi was recovered as the sister species of Enchytraeus sp. 1 (see Fig. 2 ) from the Aegean Sea in the present molecular study, suggesting that the two represent a southern lineage (as opposed to the northern group of E. albidus , E. albellus , and E moebii ).

Geographical distribution and habitat Enchytraeus cf. krumbachi has been genetically identified from Spain only (present material). Čejka’ s (1913) original material of E. krumbachi was from a seashore in Croatia, while our Spanish worms come from an algal compost.

Figures

Fig. 12 Enchytraeus cf. krumbachi a vas deferens, showing the transition between the thin- and thick-walled portions; note the lack of cilia inside the thick-walled part. b Male copulatory apparatus. c Spermatheca. ag

Fig. 2 Species tree for the E. albidus species complex (shaded), estimated using the multi-locus coalescent model. The number of included speci- mens for each species is given after the species name. Values above branches are posterior probability (PP) support values, values under terminal branches are mean PP from species delim- itation analyses in BPP, written as: mean analysis A/mean analysis B/ mean analysis C. Scale measures the branch lengths in proportion to total tree length