Marionina fusca Klinth, Rota & Erseus 2022, SP. NOV.

MARIONINA FUSCA KLINTH, ROTA & ERSÉUS SP. NOV.

(FIG. 13)

Z o o b a n k r e g i s t r a t i o n: u r n: l s i d: z o o b a n k. org:act: 720EDF21-78D8-4169-BE8F-EC4860A9B909

Holotype: SMNH Type Coll. 9311 (CE12475), a mature amputated specimen stained in Paracarmine and mounted on a slide. Leg. Paul Brewin & Alison Massey (Shallow Marine Survey Group), 7 December 2010. No COI barcode, accession numbers for other genetic data in Tables 1 and S1. Holotype illustrated in Fig. 13D and 13F.

Type locality: South Georgia, Corral Bay, Cumberland East Bay, intertidal zone, 54.3023 S, 36.3768 W.

Paratypes: SMNH Type Coll. 9312 (CE12476) and SMNH Type Coll. 9313 (CE12478), one mature and one semi-mature specimen from type locality, amputated and mounted on slides. COI barcode (CE12476), GenBank MZ 393959; accession numbers for additional genetic data are given in Table 1 and the Supporting Information (Table S1).

Etymology: From the Latin adjective fuscus, dark or swarthy.

Diagnosis: This species can be separated from other darkly pigmented, southern marine enchytraeid species in the following characters: (1) the presence of penial bodies, connected to one anterior and one posterior prostate gland; and (2) the absence of a thickened ventral epithelial plate in X.

Description: Dark grey worms (at least when preserved), with subepithelial black pigmentation, densest dorsally, decreasing in intensity ventrally. Length of first 19–27 segments 2.9–4.3 mm (fixed, amputated specimens); first 15 segments 2.1–3.0 mm long; width at clitellum 0.45–0.50 mm. Chaetae sigmoid (Fig. 13A). Upper bundles dorsolateral (closer to lateral line than the ventral bundles), with four to eight chaetae anterior to clitellum, three to five in postclitellar segments, at least to XXVII. Ventral bundles with six to 12 chaetae anterior to clitellum, and six to ten chaetae posteriorly, at least to XXVII. Longest measured chaetae of each worm 95–105 µm long, ~5 µm wide. Epidermis covered with rows of pale gland cells, and with a few rows of deeply staining granular gland cells, with the latter rows aggregated anterior and posterior of chaetae. Clitellum with reticulate pattern of gland cells, extending over (1/2XI) XII–XIII, absent ventrally. Head pore at 0/I.

Coelomocytes numerous, 10–15 µm long; round, oval or spindle shaped; granulated, with distinct nucleus. Paired pharyngeal glands (Fig. 13B) present in IV, V and VI, with third pair extending back into VII; dorsal connection indiscernible, dorsal and ventral lobes smallest in IV, ventral lobes absent in VII. Dorsal vessel originating in XII–XIII, with peristomial bifurcation. Nephridia (Fig. 13D) ~120 µm long, observed in 7/8– 9/10. Anteseptale consisting of a distinct funnel and a small part of nephridial body. Postseptale oval, tapering into posteroventral efferent duct. Brain with posterior incision. Ventral nerve cord with small subbuccal bulb; see Marionina discussion above.

Male genitalia paired (Fig. 13E). Testes originating in anterior of XI, with maturing male cells enveloped in testis sacs, cleft lengthwise into irregular lobes, distally breaking up into a mass of free-floating cysts. Sperm funnels in XI, ≥ 170–270 µm long, 65–130 µm wide, that is, about two to three times longer than wide; funnels tapering towards vasa deferentia. Most of vasa irregularly coiled in XII, 15 µm wide. The male pores are surrounded by distinct, small, spherical, penial bodies (35) 60–95 µm in diameter, with cells mostly embedded in the epidermis, encapsulated by musculature. Each penial body bearing two prostate glands, one anterior and one posterior, 95–160 and 95–100 µm long, respectively; each prostate with thread-like ventral connections to the penial bodies. Ovaries in XII. One mature egg observed.

Spermathecae (Fig. 13C, E) in V, spindle or tube shaped; ectal duct difficult to distinguish from ampulla, and exact shape of ental part of ampulla difficult to discern owing to heavy pigmentation. Bundled sperm observed in lumen of ectal duct and entally towards ampulla. Spermathecae 120–180 µm long, 40–65 µm wide at the ectal duct and 30 µm wide at widest part of ampulla. Two separate glands at ectal end of each spermatheca, one dorsal and one ventral to spermathecal pore; glandular bodies 45–60 µm in diameter at widest part. One midventral subneural gland, 105 µm long, observed in XV of one specimen.

Geographical distribution: Known only from South Georgia.

Remarks: We found two darkly pigmented species from South Georgia, both reminiscent of M. aestuum. Although genetically distinct, these two species are morphologically similar. Both have two pairs of large prostate glands (one pair anterior and one posterior to the male pores), sperm funnels between two and three times longer than wide, and spermathecae shaped as long tubes, slightly wider in the ectal duct than the ental (ampullar) part, and with two glands at each of the pores. In the specimen attributed to M. aestuum (see above), we did not observe any midventral subneural glands, whereas M. fusca has one such gland in XV, neither condition of which matches the original description of M. aestuum that reported two glands, one in XIII and one in XIV. The development of said glands might vary with sexual maturity, and slight variation does occur in some enchytraeid species (Schmelz, 2003). There are two principal differences between our two darkly pigmented species. First, M. aestuum lacks distinct penial bodies, whereas M. fusca has small but distinct penial bodies (more like those of M. grisea; Fig. 12C). Second, the average number of chaetae per bundle is larger in M. aestuum, with as many as 15 in a bundle, compared with M. fusca, with a maximum of 12 chaetae in a bundle and usually no more than ten (M. aestuum was originally described as having as many as 17 chaetae per bundle).