Manayunkia baicalensis ( Nusbaum, 1901 )

Bick, Andreas & Bastrop, Ralf, 2025, Revision of the genus Manayunkia Leidy, 1859, and establishment of two subfamilies within Fabriciidae (Annelida, Sabellida), Zootaxa 5661 (1), pp. 1-58 : 19-21

Manayunkia baicalensis ( Nusbaum, 1901)

( Figure 11 )

Dybowscella baicalensis Nusbaum, 1901: 8–16 , fig. 1–4. 1901 b: 270–273. – Dybowski 1929: p. 455 –461.

Manayunkia baicalensis . – Zenkevitsch 1925: 6–36. – Sitnikova et al. 1997: 20–23, fig. 1, 2, 4, 7, 9, 10. – Pudovkina et al. 2016: 132–136.

Trichosobranchella novobaicalensis minor Dybowski, 1929: 463–474 , 484–485.

? Manayunkia baicalensis hydani Slastnikov, 1940: 76 .

Material examined. Russia, Lake Baikal : Aya Bay, depth 6 m, sandy bottom, coll. 09. July 2001, 4 specimens (complete male, female, juvenile and several fragments) ( USNM 1128559 ) .

Description (based on the descriptions of Nusbaum (1901), Sitnikova et al. (1997) and own observations). Total length of specimens, including radiolar crown, between 5 and 8 mm, usually 5 to 6 mm long; width about 0.6–0.8 mm; length of radiolar crown 0.5–1 mm; ratio between length of radiolar crown and body length, without radiolar crown, between 0.14 and 0.2; body slender, more or less distinctly elongated and tapering posteriorly; margins of thoracic chaetigers clearly visible, but barely visible on abdominal chaetigers ( Fig. 11A, E ).

Radiolar crown with three pairs of radioles and one pair of unbranched vascularized ventral filamentous appendages; surface of radioles and ventral filamentous appendages wrinkled; dorsal radioles unbranched, median and ventral radioles with a large number of branches, usually 2 (or more?) branches arise from a common base, so that the branches appear to be arranged in two rows; in total between about 30 and 62 branches (on average 50), rarely more; ventral filamentous appendages and almost all branches of the radioles end at about the same height ( Fig. 11B, C )

Peristomium distinctly longer than first chaetiger, about as long as chaetiger 3, with anterior and posterior rings; border between anterior and posterior rings indistinct; anterior margin of anterior peristomial ring developed as a low membranous collar ventrally, narrowly separated mid-dorsally; faecal groove dorsally deeply sunken in peristomial region, becoming considerably lower on first and following chaetigers ( Fig. 11B, C ); faecal groove shifts from dorsal to ventral midline at border between thorax and abdomen.

First chaetiger shorter than peristomium; chaetigers 1–6 each successively longer, chaetigers 7 and 8 shorter again; first 4 thoracic chaetigers wider than long, last 3 thoracic chaetigers distinctly longer than wide; margins of chaetigers 6–8 often not clearly visible in females with brood chamber; abdominal chaetigers short, chaetiger 9 longest, chaetiger 10 and 11 decreasing in length; pygidium about same length as chaetiger 11, clearly elongated and pointed; pygidial eyes absent ( Fig. 11A–C, E ).

Notopodia of chaetigers 2–8 superiorly with about 15–20 elongate, narrowly hooded chaetae of about 0.3– 0.4 mm in length, inferiorly with short, narrowly hooded chaetae; neuropodia of chaetigers 2–5 (females) or 2–8 (males) with 6–13 (on average 9) uncini with a main fang and apical with about 4–6 rows of progressively smaller teeth; length of uncini about 0.3–0.4 mm; abdominal neuropodia with 6 to 10 elongate and short narrowly hooded chaetae; abdominal notopodia with 28–40 uncini, about 0.1 mm long; mature females with transitional chaetae on neuropodia of chaetigers 6–8; brood chamber, formed by bulge-like protrusions ventrally on chaetigers 6 and 8, also present in females ( Fig. 11A, D ).

Radiolar crown and thorax until about chaetiger 6 pigmented, then transparent; gut dark coloured; fixed specimens completely without pigment.

Remarks. In addition to the original description of M. baicalensis by Nusbaum (1901), there are further detailed descriptions of this species by Zenkevitsch (1925) and Dybowski (1929). However, these descriptions can only be used with some reservations as Zenkevitsch (1925) has synonymised M. baicalensis with M. godlewskii , the characters of both species are obviously mixed. Dybowski's (1929) description of Trichosobranchella novobaicalensis Dybowski , 19289, probably does not separate the two species mentioned above either. The above-mentioned authors each gave a different number of branches of the radiolar crown. According to Nusbaum (1901), the number varies between 30 and 40 branches, according to Zenkevitsch (1925) between 48 and 72, juvenile individuals with a total of 24 branches, and according to Dybowski (1929) also between 48 and 72. The number of branches of the median and ventral radioles is relatively high, at least in comparison to most of the smaller species. This can possibly be explained by the fact that the branches divide further immediately after branching, so that 2 or even more branches could branch off from a common base. However, we were unable to investigate this with the material available to us.

A small, 2.1 mm long juvenile had significantly fewer thoracic and abdominal chaetae and uncini than adults: 4–6 thoracic and 1–3 abdominal chaetae, and 3–4 thoracic and 8–15 abdominal uncini. A smaller number of chaetae and uncini in juveniles was also found in other species. The pygidium of this small specimen is also elongated but rounded.

Sitnikova et al. (1997) compared M.baicalensis and M.godlewskii and described a third species, M.zenkewitschii , from Lake Baikal. The data for these three species are therefore the most reliable.

Manayunkia baicalensis is characterised by an elongated and pointed pygidium. In the other two species from Lake Baikal, the pygidium is not elongated. In addition, the first 6 chaetigers are clearly pigmented. In M. zenkewitschii all chaetigers are pigmented, in M. godlewskii the pigment is almost completely absent or at most the first two chaetigers have pigment.

A supposed subspecies of M. baicalensis , M. baicalensis hydani Slastnikov, 1940 , was found in the Gyda River, Gyda Peninsula, on the Siberian coast of the Kara Sea ( Slastnikov 1940). However, as there is no description of this subspecies, it must be considered a nomen nudum. In addition, M. baicalensis has only been found in Lake Baikal and in no other lakes in Siberia (see Pudovkina et al. 2016). It can therefore be assumed that this is a different species that has not yet been described.

Manayunkia species from Lake Baikal exhibit also sexual dimorphism, which is evident in the presence of transitional chaetae or transitional uncini of chaetiger 6–8 in females and their absence in males, as well as the morphology of the posterior thoracic chaetigers. Chaetigers 6 to 8 are elongated in males, while in females bulge-like protrusions ventrally at chaetigers 6 and 8 are interpreted as the boundary of a brood chamber in which eggs and embryos develop.

Geographic distribution. Known only from Lake Baikal, Russia.

Biology. The eggs develop in females in chaetiger 4 ( Zenkevitsch 1925). They are oval, and have a length between 0.28 and 0.33 mm, and a width of 0.23 and 0.25 mm. The maximum number of eggs and juveniles of different developmental stages in a maternal tube was 36. We were able to count 25 eggs in the brood chamber of a female collected in July 2001, which was part of the loan from the NMNH (USNM 1128559). The main reproduction period is in July and August ( Sitnikova et al. 1997).

Ecology. Manayunkia baicalensis lives on sandy bottoms, among stones and boulders at depths between 0.8 and 80 m in Lake Baikal.

Figures

FIGURE 11. Manayunkia baikalensis (Nusbaum, 1901), light micrographs of Shirlastain A stained specimens. A. Total view of female, ventral view (left) and male, dorsal view (right). B. Anterior end, ventral view. C. Anterior end, dorsal view. D. Posterior end of female with bulge-like protrusion on chaetigers 6 and 8, ventral view. E. Anterior end with elongated pygidium, dorsal view. Scale bars. A–E 1 mm.