Marionina, MICHAELSEN, 1890

Klinth, Mårten J., Rota, Emilia, Martinsson, Svante, Prantoni, Alessandro L. & Erséus, Christer, 2022, New insights into the systematics of Lumbricillus and Marionina (Clitellata: Enchytraeidae) inferred from Southern Hemisphere samples, including three new species, Zoological Journal of the Linnean Society 194 (4), pp. 1103-1133 : 1119-1122

MARIONINA MICHAELSEN, 1890

Type species: Pachydrilus georgianus Michaelsen, 1888 .

In addition to the Lumbricillus specimens described above, we received six other enchytraeid specimens from the recent southern expeditions. Two of them were small, semi-mature and unpigmented individuals from Snow Island, and four were large, mature worms with dark pigmentation from South Georgia. Although the six specimens differed greatly in appearance, the molecular data strongly supported them as a monophyletic group of three species. The two unpigmented specimens belonged to the same, but yet unidentified, species, whereas the four pigmented specimens (constituting two additional species) were all highly reminiscent of M. aestuum , a taxon placed in Lumbricillus by Nielsen & Christensen (1959), but here regarded as a member of Marionina s.s.

As mentioned in the Introduction, Marionina s.l. has grown to become a large genus, with a wide definition allowing for the inclusion of many unrelated taxa. A formal revision of this genus is beyond the scope of our study, but we aim here to provide enough information to allow for such a revision in the near future. The definition of Marionina s.s. can only originate from the type species M. georgiana , which fortunately has been given two extensive redescriptions ( Rota et al., 2008; Schmelz & Collado, 2008). Marionina georgiana is marine and has the following characters: epidermis densely glandular; sigmoid chaetae, with upper bundles dorsolateral (closer to lateral line than the ventral bundles); ventral nerve cord with continuous, scattered occurrence of perikarya (and not concentrated in separate ganglia like that of Achaetinae; Rota et al., 2008); brain with posterior incision; three pairs of pharyngeal glands; gradual transition between oesophagus and intestine; oesophageal appendages absent; dorsal vessel arising in XIII and bifurcating anteriorly at front of peristomium (= lumbricilline pattern); nephridia with anteseptale consisting of funnel only and nucleated coelomocytes. Unfortunately, all these somatic characters also fit most species of Lumbricillus ( Rota et al., 2008; Schmelz & Collado, 2008).

However, the reproductive organs of M. georgiana are not very similar to those of Lumbricillus . The sperm funnels and vasa deferentia are smaller than in most Lumbricillus species, although the general shape is the same. Most Lumbricillus species have testes divided into multiple lobes, each surrounded by a peritoneal sheath, and the maturing male cells are thus enclosed (with the exception of the L. arenarius group) in a fan-like cluster of testis sacs. The male gonads of M. georgiana are not divided into multiple lobes, but our re-examination of the type material has revealed a peritoneal sheath surrounding these organs, except on their distal frayed edge, from which free-floating maturing sperm cysts are released. As the mass of male cells grows and bulges into adjoining segments it becomes enveloped in a seminal vesicle. The spermatheca of M. georgiana is simple, more or less a straight tube from the ectal pore to the oesophagus, with which it is connected. The tube is slightly wider at the ampulla, making it somewhat club shaped. The ectal pore is not surrounded by a rosette of glands, commonly found in Lumbricillus , but instead associated with two pairs of separate glands ( Rota et al., 2008: fig. 6C, D; Schmelz & Collado, 2008: fig. 9).

A most striking feature of M. georgiana , and which might be a potential apomorphy of the genus, is the penial body, a rudimental glandular cushion centrally pierced by the vas deferens and lacking a bursa, that is, an epidermal invagination, such that the pore is on the surface. The male opening is accompanied anteriorly and posteriorly by two large pedunculate glands whose stalks join the body wall close to the male pore. These accessory glands [by us referred to as prostate glands, following Stephenson (1932)] were overlooked by Rota et al. (2008) but illustrated by Schmelz & Collado (2008). An architecture of the copulatory organ such as that just mentioned has never been reported among terrestrial enchytraeids assigned to Marionina before. The ‘extra glandular bulb’ occurring submedially in front of the paired penial bulbs in Marionina vesiculata Nielsen & Christensen, 1959 is not connected to the male pores. Instead, a number of marine enchytraeids from the Southern Hemisphere, some of which were placed in Lumbricillus by Nielsen & Christensen (1959), but which we now transfer back into Marionina s.s., have the simple penial body, devoid of bursa and accompanied by prostate glands described for M. georgiana : Marionina werthi Michaelsen, 1905a from Kerguelen Island (‘vasa deferentia open out through a tiny, onion-shaped bulb that is completely hidden in the body wall. On this bulb, which sometimes protrudes somewhat as a tiny outer papilla, sits a weakly lobed prostate protruding into the body cavity’; Michaelsen, 1905a: 15), Marionina antipodum Benham, 1905 from Antipodes Island [‘the penial apparatus (Plate XIV, fig. 9) is comparatively small and it scarcely exceeds the thickness of the longitudinal muscles of the body wall. However, opening into it is a conspicuous prostate gland’; Benham 1905: 294], Marionina benhami Stephenson, 1932 (= M. werthi sensu Benham, 1922 ) from Macquarie Island (‘there is, so far as I can make out from my sections, no “bulb” in the sense in which the term is used in Lumbricillus , &c. The sperm duct passes nearly vertically into the body wall, between groups of gland cells constituting the prostate gland; it rims down on the mesial side of one of these groups, to perforate the body wall simply; there is neither glandular investment nor muscular covering. The prostate glands, some in front of, and others behind the sperm pore, rise up inside the body-wall to the level of the intestine and are separated from the body cavity by a sheet of obliquely vertical muscles fibres; a few fibres also pass between the groups of gland cells. The glands open through the body-wall independently of the duct’; Benham, 1922: 13–14), Marionina grisea Stephenson, 1932 from the Antarctic Peninsula (‘the vas deferens … forms a loose coil, which pierces through a rudimentary penial body to end on a rather indefinite male papilla. […] The penial body … is pierced by the end of the vas deferens and also by the stalks of two large glands, the “prostates”’; Stephenson, 1932: 244–245) and Marionina aestuum Stephenson, 1932 from South Georgia (‘The vas deferens reaches the surface through a cleft in the muscular mass which represents the penial body, then runs between the stalks of the two “prostates” …, and finally penetrates the junction of the two stalks and the mass of cells which mingles with them where they abut on the surface. The “prostate” glands, in segment xii, are large, and lie, one anterior and the other posterior in position’; Stephenson, 1932: 247–248). We also consider Marionina colpites ( Stephenson, 1932) from South Georgia to be part of this group (‘The penial body is of the enchytraeine rather than of the lumbricilline type. It consists of a number of pearshaped masses of gland cells, about eight such masses being visible in a single longitudinal section, and the total number on each side being perhaps in the neighbourhood of two dozen. These gland masses are closely compacted together, but separated from each other by, and each individually pear-shaped mass more or less enveloped in, muscular strands; there is no common capsule binding the whole together, and the upper (dorsal) ends of the masses are without covering. The glands are composed of cells derived from the surface epithelium, and discharge on the surface around the small aperture of the vas deferens, which comes to the surface after passing between the glandular masses’; Stephenson, 1932: 262). Marionina antipodum and M. colpites were already placed in Marionina by Nielsen & Christensen (1959), but all the species mentioned except M. antipodum were erroneously treated as Lumbricillus by Klinth et al. (2017b), and all but M. colpites were placed in the L. lineatus group.

All these species have high numbers of sigmoid chaetae per bundle, prostate glands (one, two or more) associated with the male pore, and spindle- or tube-shaped spermathecae lacking rosettes of glands around the ectal pore ( Table 2 ), and many of them have been discussed as similar to M. georgiana ( Schmelz & Collado, 2008) . If one were to focus on only the condition of the male apparatus, one might ask whether the intertidal marine Marionina appendiculata Nielsen & Christensen, 1959 of the Northern Hemisphere is also part of this group (‘the penial bulb consists of one or two separate compact structures attached to the body wall immediately in front of and behind the male pore; if only one is present it is the posterior one’; Nielsen & Christensen, 1959: 122, fig. 174; see also Coates & Ellis, 1981: 2137), but in the absence of genetic data we are more inclined to consider it a result of convergent evolution, because this species has a marionine branching of the dorsal vessel and a different morphology of the spermathecae. Prostate-like glands associated with the male pore are also described for the marine genus Randidrilus Coates & Erséus, 1985 , known from the north-western Atlantic, and for the terrestrial/freshwater Holarctic genus Mesenchytraeus Eisen, 1878 . However, in both these taxa the male pore opens into an epidermal invagination surrounded by an eversible glandular bulb.

Unlike M. georgiana , most of the southern species we consider part of Marionina s.s. are pigmented and have testes and masses of developing male cells said to be lobed and somewhat reminiscent of those of Lumbricillus ( Michaelsen, 1905a; Stephenson, 1932). However, the shape of such structures is hard to compare between species, because the descriptions are often poor and do not distinguish between what is the testis and the free-floating maturing sperm cysts, the testis sacs or the seminal vesicles ( Table 2 ). In our two pigmented species, we found the mass of developing male cells closest to the testes and the testes themselves appearing slightly lobed, somewhat similar to the arrangement typical of Lumbricillus . Distally, however, the testes break off into freefloating maturing sperm cysts. This seems to indicate the presence of testis sacs with thin membranes enveloping the early stages of developing male cells but interrupted at the most apical, frayed edge of the gonad; something speculated by Michaelsen (1905a) and Stephenson (1932). Although we did not observe any basal separation into lobes in M. georgiana , a much smaller species than our pigmented ones, we did observe the same membrane enveloping the developing cells but distally breaking up.

*One or two pedunculate glands and two or more sessile spherical glands anterior to the ectal pore ( Rota et al., 2008: fig. 6C, D; Schmelz & Collado, 2008: fig. 9). New species in bold.

Further morphological characters of potential importance, concerning Marionina s.s., are to be found in the epidermal gland cells and the nerve system. Rota et al. (2008) and Schmelz & Collado (2008) illustrated the epidermis of M. georgiana as intensely glandular, that is, covered with densely packed rows of pale cells. We found similar densely glandular epidermis in both the newly collected specimens and the borrowed type material of the species we consider to be Marionina s.s. However, the epidermis of the southern Lumbricillus species included in this study was also densely glandular; hence, this character might be more related to habitat than to genus (see Rota, 2001). Schmelz & Collado (2008) reported clusters of perikarya (‘prostomial ganglia’) on the prostomial nerves of M. georgiana (see their fig. 1) similar to those discovered by Rota et al. (1999: fig. 3b, f) in Achaeta and Henlea . We observed similar clusters on the prostomial nerves of M. aestuum and M. sp. ‘Snow Is.’ ( Fig. 9A ). We also found similar aggregations at the ventral ends of the circumoesophageal connectives, close to where they merge with the ventral nerve cord, in all of M. aestuum , M. sp. ‘Snow Is.’ and M. georgiana . Furthermore, we found a globular mass of deeply staining cells apparently connected with the anterior end of the ventral nerve cord, and also anchored to (discharging in?) the ventral groove of the body wall between I and II ( Fig. 9A–D ). This unprecedented ‘subbuccal bulb’ (seemingly glandular, perhaps similar to the subneural glands found in the proximity of the clitellum in many enchytraeids) was observed by us in M. aestuum ( Fig. 9B, C ), M. colpites , M. fusca , M. grisea , M. sp. ‘Snow Is.’ ( Fig. 9A ) and M. georgiana ( Fig. 9D ). It was not observed in all specimens and was particularly hard to find in M. georgiana , only clearly visible in two of 15 studied specimens, both of which were unstained. It is possible that this organ varies in size with development, in a similar manner to the subneural glands. We did not observe a subbuccal bulb in any of the studied species of Lumbricillus or in Ch. blocki .

Figures

Figure 9. A, Marionina sp. ‘Snow Is.’ (CE34647), anterior body (lateral view). B, Marionina aestuum, sectioned specimen BMNH 1933.2.23.895, cephalic region (lateral view). C, M. aestuum (CE12477), cephalic region (ventral view). D, Marionina georgiana (paralectotype 13), cephalic region (ventral view). Abbreviations are defined under ‘Taxonomy’. Scale bars: 100 μm.