(FIGS 9A, 14)
Material examined: SMNH198155 (CE34647) and SMNH198156 (CE34648), two semi-mature specimens from Snow Island (South Shetland Islands), Antarctica. For information on collection localities and GenBank accession numbers for COI barcodes, see Table 1 and the Supporting Information (Table S1).
Description: Live colour unknown; and specimens apparently without pigmentation. Length of first 16–19 segments 2.4–2.9 mm (fixed, amputated specimens); first 15 segments 2.2–2.3 mm long; width at XII 0.27 mm. Chaetae slightly sigmoid (Fig. 14A). Upper bundles dorsolateral (closer to lateral line than ventral bundles), with three to four chaetae anterior to clitellum, (three) four in postclitellar segments, at least to XIX. Ventral bundles with four chaetae throughout to XIX. The longest measured chaetae of each worm 55 µm long, ~3 µm wide. Epidermis densely covered with rows of pale gland cells. Clitellum not developed. Head pore at 0/I.
Coelomocytes numerous, 10–15 µm long; round, oval or spindle shaped; granulated, with distinct nucleus. Paired pharyngeal glands (Fig. 14B) present in IV, V and VI; only third pair with ventral lobes and dorsally converging but connection not discernible. Origin of dorsal vessel indiscernible, but with peristomial bifurcation. Nephridia observed in 7/8–8/9, anteseptale with funnel only, postseptale elongated oval with terminal efferent duct. Brain with posterior incision, with clusters of perikarya on prostomial nerves and at ventral ends of circumoesophageal connectives (Fig. 9A). Ventral nerve cord with small subbuccal bulb (Fig. 9A); see Marionina discussion above.
Male genitalia not fully developed. Developing male cells free floating in XI. Developing sperm funnels in XI, 60–65 µm long, 40 µm wide. Thin line of cells extending backwards from sperm funnels to what will become the male opening. Developing spermathecae in V, shape uncertain. No midventral subneural glands observed.
Geographical distribution: Known only from Snow Island (South Shetland Islands).
Remarks: The two small specimens of this species were first believed to be juvenile Lumbricillus, but they were placed genetically within a separate clade together with the two larger, darkly pigmented species, M. aestuum and M. fusca. Upon closer examination, despite their submature condition, we found our specimens to have free-floating developing male cells, more akin to Marionina s.s. than to Lumbricillus. Our Marionina sp. ‘SnowIs.’ isclose to M.georgiana inlacking pigmentation, having a densely glandular epidermis, about four chaetae per bundle, similar pharyngeal glands, presence of perikarya on the prostomial nerves and at ventral ends of circumoesophageal connectives, and in the presence of the subbuccal bulb. However, fully mature specimens from Snow Island and/or DNA from M. georgiana will be needed to establish whether the two are the same or separate species.
CHRISTENSENIDRILUS BLOCKI DÓZSA- FARKAS & CONVEY, 1998
Christensenia blocki Dózsa-Farkas & Convey, 1997: 483–486, figs 1–8.
Christensenidrilus blocki – Dózsa-Farkas & Convey, 1998: 292.
Material examined: SMNH198141 – 198143 (CE35374–35376), three mature specimens from the type locality on Signy Island (South Orkney Islands). For information on collection localities and GenBank accession numbers for COI barcodes, see Table 1 and the Supporting Information (Table S1).
Remarks: Our specimens perfectly resemble those originally described by Dózsa-Farkas & Convey; therefore, we decided not to give complete descriptions and illustrations, but instead a few additional comments about the morphology. Schmelz & Collado (2008), in their comparison between Ch.blocki and M. georgiana, were left with a few questions that we will try to answer. First, we did not observe neuron perikarya in the prostomium of Ch. blocki, similar to those described by Schmelz & Collado (2008) in M. georgiana and observed by us also in M. aestuum and M. sp. ‘Snow Is.’. Second, the dorsal blood vessel bifurcates in both species under the brain in a lumbricilline (peristomial) pattern. Third, the penial bulbs of Ch. blocki, if they can be considered as such, which were described originally as compact and medium sized, were hard to distinguish, composed of a small aggregation of cells (<50 µm in diameter) surrounding the superficial (non-invaginate) male pores, at least apparently similar to the penial bodies of M. georgiana, but lacking any prostate glands. The developing male gametes, although numerous and completely occupying segments X–XII, do not show any sign of being contained in lobed testis sacs. Based on these new elements and the presence of anucleate coelomic corpuscles in place of nucleated coelomocytes in Ch. blocki, we cannot but agree with Rota et al. (2008) and Schmelz & Collado (2008), in that Ch. blocki does not share enough morphological characters with M. georgiana to assume it to be closely related to Marionina s.s., which has also been supported by our molecular data. Unfortunately, we found no support for any specific placement of Christensenidrilus within the family Enchytraeidae as a whole, probably because of the lack of suitable outgroups with which to make comparison.