Manayunkia caspica Annenkova, 1929

(Figures 13–14)

Manayunkia caspica Annenkova, 1929a: 18–20, pl. 3, fig. 1–4, pl. 4, fig. 10–12. Zenkevitsch. – 1935: 198–201. – Hartman 1951: 389. – Bick et al. 2024: 449–455, pl. 2–5.

Material examined. Russia, Caspian Sea: 43°57.733’N, 48°37.840’E, depth 28.7 m, coll. 12.10.2018, 23 specimens (ZMMU WS 19181 (12 specimens) and ZSRO-P2695 (11 specimens)), 43°57.733’N, 48°840’E, depth 28.7 m, coll. 12.10.2018, 35 specimens (ZMMU WS 19182 (17 specimens) and ZSRO-P2696 (18 specimens)), 43°57.733’N, 48°18.100’E, depth 25.7 m, coll. 10.10.2018, 14 specimens (ZMMU WS 19183 (9 specimens) and ZSRO-P262697 (5 specimens)).

Description. Total length of specimens, including radiolar crown, between 1.8 and 4.2 mm; width between 0.15 mm and 0.22 mm; length of radiolar crown between 0.22 mm and 0.40 mm; ratio between length of radiolar crown and body length, without radiolar crown, between 0.1 and 0.2; body cylindrical, slender, slightly tapering posteriorly (Fig. 13A, D).

Radiolar crown with three pairs of radioles and one pair of unbranched vascularized ventral filamentous appendages; surface of radioles and ventral filamentous appendages wrinkled (Fig. 13A, B); first dorsal radioles branch off from the branchial lobe, median and ventral radioles arise from a common base (Fig. 14B–E); ventral and median radioles asymmetrical branched or pectinated, dorsal radioles unbranched; ventral radioles with 5–6 branches, median radioles with 2–3 branches (Fig. 14B–D); no morphological differences in the structure of the branches, except ventral filamentous appendages (Fig. 14B, C); ventral filamentous appendages and branches of radioles end at about same height (Fig. 13B); branches of radioles and vascularized ventral filamentous appendages rectangular to square in cross-section, extension about 20–28 x 24–40 µm and 32–40 x 38–56 µm, respectively (Fig. 14B); vascularized ventral filamentous appendages with blood vessel (Fig. 14B–E), connecting to corresponding dorsally located branchial heart (Fig. 14F); epidermis of radioles and vascularized ventral filamentous appendages medially with ciliated and laterally with non-ciliated epithelial cells; ciliated cells constitute a shallow food groove (Fig. 14B, C); center of branches of radioles occupied by a large cavity bordered by a narrow band of cells; radioles with cylindrical cells adorally (Fig. 14B, C); dorsal lips as triangular lobes, rounded at upper margin, with ciliated epithelium all around (Fig. 14D, E); about 30–40 µm long; dorsal lips connect dorsal radioles with vascularized ventral filamentous appendages.

Peristomium slightly longer than first chaetiger, about as long as chaetiger 2, with anterior and posterior rings; anterior peristomial ring distinctly shorter than posterior ring (Fig. 13A, B); anterior margin developed as low membranous collar ventrally, narrowly separated mid-dorsally (Figs 13B, 14A, F); ciliated band ventrally on posterior margin of anterior peristomial ring (Fig. 13B); border between anterior and posterior peristomial rings clearly visible all around (Fig. 13A, B); one pair of rounded black peristomial eyes present.

Faecal groove dorsally deeply sunken in peristomial region (Fig. 14F), becoming considerably lower on first and following chaetigers; faecal groove shifts from dorsal to ventral mid-line at border between thorax and abdomen (Fig. 13D).

Metanephridia in peristomium and chaetigers 1 and 2. First thoracic chaetiger shorter than peristomium and second chaetiger; chaetigers 2–7 successively longer; chaetiger 8 shorter than chaetiger 7 (Fig. 13A, B); first 3 thoracic chaetigers wider than long, chaetiger 4–8 distinctly longer than wide (Fig. 13A); first abdominal chaetiger long, about twice as long as second one; abdominal chaetiger 3 slightly shorter, about as long as pygidium; pygidium tapered (Fig. 13A, D); pygidial eyes absent; borders between anterior thoracic and abdominal chaetigers clearly visible, but difficult or impossible to identify on chaetigers in females with brood chamber, i.e., particularly between chaetigers 6 and 7 (Fig. 13A, C).

First chaetiger with about 2–4 short and 3–5 elongate, narrowly hooded notochaetae, neuropodial uncini absent; notopodia of chaetigers 2–5 superiorly with 3–5 (rarely 6) elongate, narrowly hooded and inferiorly with 2–4 pseudo-spatulate chaetae (Fig. 13E, F); notopodia of chaetigers 6–8 superiorly with 3–5 elongate, narrowly hooded and 2–4 short, narrowly hooded chaetae; neuropodia of chaetigers 2–5 (female) or 2–8 (male) with 2–4 (rarely 5) uncini with main fang, apical with about 4–6 rows of progressively smaller teeth (Fig. 13G); females with 3–8 transitional uncini on chaetigers 6–8, different from thoracic uncini on chaetigers 2–5; transitional uncini longer than regular thoracic uncini, without main fang, with a multitude of rows of small, equal-sized teeth (Fig. 13I); abdominal neuropodia with 2–3 elongate, and 1–2 short, narrowly hooded chaetae (Fig. 13J); abdominal notopodia with 12–17 uncini on chaetiger 9, 8–15 on chaetiger 10, and 7–12 on chaetiger 11, respectively; uncini with about 6 rows of equal-sized teeth, about 4–6 teeth per row (Fig. 13K); manubrium about 2–4 times as long as dentate region.

Females with brood chamber on chaetigers 6 and 7, formed by wing-like protrusions of the integument of these chaetigers (Fig. 13A, C); boundaries of these chaetigers barely visible here (Fig. 13A).

Fixed specimens usually without colour and translucent, only anterior peristomial ring sometimes slightly pigmented.

Tubes, significantly longer than worms they inhabit it, consist of mucus and fine sediment particles (Fig. 13H).

Remarks. The brackish water species M. caspica most closely resembles the freshwater species M. danubialis Băcescu, 1948 but there are also small differences. In M. caspica, thoracic chaetigers become continuously longer from chaetiger 2 to chaetiger 7, whereas in M. danubialis chaetiger 5 is shorter than chaetigers 4 and 6. Manayunkia caspica has 2–5 (rarely 6) thoracic uncini, whereas M. danubialis has 6–8 (rarely 4) uncini. In M. danubialis, the thoracic uncini are sometimes arranged in a slightly offset double row. The thoracic and abdominal uncini differ also slightly. In M. caspica there are about 4–6 rows of increasingly smaller teeth above the main fang in the thoracic uncini, whereas in M. danubialis there are a maximum of 4 rows. The number of teeth per row is also somewhat lower in the freshwater species. The abdominal uncini in M. caspica have a maximum of 6 rows of teeth, in M. danubialis there are 6–8 rows. Furthermore, the transitional uncini of M. caspica have considerably more apical teeth than those of M. danubialis .

The distribution area and habitat are also useful for distinguishing both Manayunkia species. Manayunkia caspica has been described on silty bottoms in the Caspian Sea at depths between 17 m and 64 m and salinities between 1.3 and 13.2 psu (Annenkova 1929a, b). Manayunkia danubialis inhabit freshwater habitats, does not tolerate large fluctuations in salinity, prefer oxygen-rich and rocky habitats at shallow depths (Popescu-Marinescu 2008).

The genetic distance between M. caspica and M. danubialis, based on the 18S rDNA sequences, including gaps is 0.110 and without gaps 0.058. For comparison, the intraspecific genetic distance in M. aestuarina, including gaps, is 0.035. This leads to the conclusion that M. danubialis and M. caspica are two closely related but different species (see Fig. 24).

There is a slight difference in the number of thoracic and abdominal uncini between smaller and larger individuals. Smaller individuals have fewer uncini in general.

The presence of transitional uncini in mature females was also found in several other Manayunkia species (see Remarks for other species). The presence of a brood chamber in females has already been described previously (Bick et al. 2024).

Geographic distribution. So far only known from the Caspian Sea. Records outside the Caspian Sea in southeast European rivers are questionable.

Biology. In the material we received from October 2018, there were females with eggs in chaetiger 4, deposited eggs in tubes (Fig. 13H) and juvenile specimens with a length of slightly more than 1 mm. The eggs had a size of about 200–300 µm x 70–96 µm. It is therefore not possible to make a reliable statement about the reproduction period of this species.

Ecology. This species was first found on silty bottoms from depths between 17 and 64 m and salinities between about 1.3 to 13.2 psu (Annenkova 1929a, b).