Manayunkia danubialis Băcescu, 1948

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 : 27-28

Manayunkia danubialis Băcescu, 1948

( Figures 15–16 )

Manayunkia danubialis Băcescu, 1948 : Bick et al. 2024: 455–461 , pl. 6–9.

Manayunkia caspica n. sbsp. Băcescu, 1944: 152.

Manayunkia caspica danubialis Băcescu, 1948: 246 (foot note 2). – Dumitrescu 1957: 120.

Manayunkia caspica fluviatilis Băcescu, 1948 : pl. 2, fig. 6 (without any description).

Manayunkia caspica Annenkova, 1929 . – Marinov 1977: 216, pl. 30, fig. 1a–e. – Jakovčev-Todorović et al. 2006: 35, fig. 1. – Popescu-Marinescu 2008: 23–31. – Atanacković et al. 2020: 621, fig. 1. – Pavel et al. 2021: 6–7, fig. 5.

Not Manayunkia caspica Annenkova, 1929a: 18–20 , pl. 3, fig. 1–4, pl. 4, fig. 10–12. – Zenkevitsch 1935: 199–201. – Hartman 1951: 389.

Material examined. Austria, Danube : Hainburg , 48°09.37’N, 16°59.39’E, coll. 15.02.2022, 1 specimen ( ZSROP2671 ), Oberloiben, 48°23.20’N, 15°31.22’E, coll. 15.02.2022, 5 specimens ( ZSRO-P2672 ), Jochenstein, 48°31.16’N, 13°42.06’E, coll. 09.02.2022, 1 specimen ( ZSRO-P2673 ), Nussdorf, 48°15.39’N, 16°22.12’E, coll. 15.02.2022, 7 specimens ( EMZ Rostock 9511), Enghagen, 48°14.25’N, 14°30.35’E, coll. 15.02.2022, 3 specimens (NHMW-ZOO-EV-21535) .

Description. Total length of specimens, including radiolar crown, between 2.2 and 3.8 mm; width between 0.15 mm and 0.26 mm; length of radiolar crown between 0.21 mm and 0.38 mm (exceptionally 0.53 mm) ( Fig. 15A–C ); 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. 15A–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. 15A–C ); ventral filamentous appendages and all branches of radioles end at about same height ( Fig. 15B, C ); first dorsal radioles branch off from the branchial lobe, median and ventral radioles arise from a common base ventral and median radioles asymmetrical branched or pectinated, dorsal radioles unbranched; ventral radioles with 5, median radioles with 4 branches ( Fig. 16B–D ); no morphological differences in the structure of the branches, except ventral filamentous appendages ( Fig. 16B, C ); branches of radioles and vascularized ventral filamentous appendages rectangular to square in cross-section, extension about 16–30 x 24–32 µm and 35–40 x 43–56 µm, respectively ( Fig. 16B ); vascularized ventral filamentous appendages with blood vessel, connecting to corresponding dorsally located branchial heart ( Fig. 16B–F ); epidermis of radioles and vascularized ventral filamentous appendages adorally with ciliated and aborally with non-ciliated epithelial cells; ciliated cells constitute a shallow food groove of radioles ( Fig. 16B, C ); center of branches of radioles occupied by a large cavity bordered by a narrow band of cells; radioles with cylindrical cells aborally Fig. 16B, C ); dorsal lips as triangular lobes, rounded at the upper margin, with ciliated epithelium all around ( Fig. 16C–E ), about 20–30 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 one ( Fig. 15B, C ); anterior margin of anterior peristomial ring developed as low membranous collar ventrally, narrowly separated mid-dorsally ( Fig. 15A–C ); ciliated band present ventrally on posterior margin of anterior peristomial ring ( Fig. 15C ); border between anterior and posterior peristomial rings clearly visible ( Fig. 15B ); one pair of rounded black peristomial eyes.

Faecal groove dorsally deeply sunken in peristomial region, considerably lower on first and following chaetigers ( Fig. 15A–B ); shifting from dorsal midline to ventral midline at border between thorax and abdomen.

Metanephridia located in peristomium and first 2 chaetigers. First chaetiger slightly shorter than peristomium and second chaetiger, chaetigers 2 to 4 successively longer, chaetiger 5 slightly shorter than chaetiger 4, chaetigers 6 and especially 7 significantly longer, chaetiger 8 shorter again; first 5 thoracic chaetigers wider than long, last 3 thoracic chaetigers distinctly longer than wide ( Fig. 15A, B ); borders between thoracic chaetigers usually clearly visible, except between last 3 thoracic chaetigers, especially in females with brood chamber ( Fig. 15A ); abdominal chaetigers short; abdomen, including pygidium, in total about as long as chaetiger 8 ( Fig. 15A, D ); pygidium about same length as chaetiger 10, terminating as rounded lobe ( Fig. 15D ); pygidial eyes absent.

First chaetiger with 2–3 short, and 3–5 elongate, narrowly hooded notochaetae, neuropodial uncini absent ( Fig. 15E ); notopodia of chaetigers 2–5 superiorly with 4–6 elongate, narrowly hooded and inferiorly with 2–4 pseudo-spatulate chaetae ( Fig. 15F ); notopodia of chaetigers 6–8 superiorly with 4–6 elongate, narrowly hooded and 3–4 short, narrowly hooded chaetae ( Fig. 15G ); neuropodia of chaetigers 2–5 (female) or 2–8 (male) with 6–8 (rarely 4) uncini with main fang and towards apical with about 4 rows of progressively smaller teeth ( Fig. 15H ); sometimes uncini in a slightly offset double row; females with 3–5 transitional uncini on chaetigers 6–8, different from thoracic uncini on chaetiges 2–5, slightly longer than regular uncini, without main fang but with a multitude of rows of small, equal-sized teeth ( Fig. 15I ); abdominal neuropodia with 3–5 elongate, and 2–4 short, narrowly hooded chaetae ( Fig. 15J ); abdominal notopodia with 13–18 uncini on chaetiger 9, 8–12 on chaetiger 10, and 5–12 on chaetiger 11; abdominal uncini with about 6–8 rows of equal-sized teeth, about 5–6 teeth per row ( Fig. 15K ); manubrium about three to four times as long as dentate region.

Mature females with brood chamber on chaetigers 6–8, formed by wing-like protrusions of the integument, segment boundaries barely visible here ( Fig. 15A ).

Fixed specimens translucent, however radiolar crown, anterior intestinal region and nephridia are brownish in colour.

Remarks. In the redescription of M. danubialis by Bick et al. (2024), the year of description by Băcescu, was given as 1944. This information is corrected here. It has been brought to our notice that the correct date of the original description is 1948. In his 1944 paper, Băcescu, only mentioned the presence of a subspecies of M. caspica in the Danube, without mentioning a name and without any description. It was not until 1948 that the name of the subspecies M. caspica danubialis was given by Băcescu and a very brief description was added as a footnote to the name. It is also irritating that the captions of the two plates have been interchanged in this paper and the subspecies is referred to here as M. caspica fluviatilis . This name was also used by Marinov (1977) in his synonym list of M. caspica .

Manayunkia danubialis most closely resembles M. caspica . Morphological differences and genetic distances between both species are described in the Remarks on M. caspica . These differences are not great. A comparison of specimens of comparable size also shows that the number of thoracic uncini is greater in M. danubialis . Individuals longer than 3 mm have on average 6–8 thoracic uncini, while those smaller than 2.5 mm have only 2–4. In contrast, specimens of M. caspica that are longer than 3 mm have on average only 3–4 thoracic uncini and those that are smaller than 2.5 mm have also 2–4 uncini.

Geographic distribution. So far only known from the Danube ( Romania to Austria, Germany?). A further invasion of the upper Danube and its tributaries is likely. This means that migration beyond the Danube into other river systems is likely to continue.

Biology. Manayunkia danubialis reaches abundances of thousands of specimens per one square meter in suitable habitats in the Romanian Danube. Females have oocytes in chaetiger 5 and males have sperm in chaetigers 6–8. Eggs are about 0.14–0.23 mm in diameter. Chaetigers 6–8 in females are elongated and form a brood chamber. The peak of reproduction is in May. More than 40 per cent of specimens are than egg-carrying females. Another peak of egg laying is assumed to occur autumn (September–November). The egg-laying and development of larvae takes place in the maternal tube. Three to 15 specimens (mean number 8) in various stages of development were found simultaneously in the tubes of the females ( Popescu-Marinescu 2008).

In specimens we examined, two early developmental stages of 200 µm length were found in a maternal tube in February, indicating an onset of the reproduction in early spring.

Ecology. Manayunkia danubialis is a species that does not tolerate large salinity fluctuations, prefer oxygen-rich and stony habitats mainly in the riparian area ( Popescu-Marinescu 2008).

Figures

FIGURE 15. Manayunkia danubialis Băcescu, 1948, SEM micrographs. A. Total view of complete female, dorso-lateral view. B. Anterior end, dorso-lateral view. C. Anterior end, lateral view. D Posterior end, lateral view (note: transitional uncini on chaetiger 8). E. Notopodium of chaetiger 1 with short, and elongate, narrowly hooded chaetae. F. Pseudo-spatulate chaetae of chaetiger 3. G. Inferior thoracic notochaetae of chaetiger 7. H. Thoracic uncini of chaetiger 5. I. Transitional uncini of chaetiger 8. J. Abdominal neurochaetae of chaetiger 9. K. Abdominal uncini of chaetiger 9. Scale bars. A 200 µm, B 100 µm, C 20 µm D 50 µm, E 10 µm, F, J 10 µm G, I, K 2 µm, H 1 µm.

FIGURE 16. Manayunkia danubialis Băcescu, 1948 semithin sections (1 µm) of the radiolar crown. A. SEM micrograph of the radiolar crown with estimated position of the section level in figures B–F. B. Branches of radioles and ventral vascularized filamentous appendages, all separated. C. Branches of ventral and median radioles start to fuse; left dorsal lip visible (100 µm basal of section B). D. Fusion of branches of ventral and median radioles continues, dorsal radioles still separated; dorsal lips visible (44 µm basal of section C). E. Dorsal laying branchial heart and ventral anterior peristomial ring lobe visible (56 µm basal of section E). F. Basis of the radiolar crown, junction of the blood vessel from the dorsal branchial heart to the ventral vascularized filamentous appendages visible (60 µm basal of section E). Scale bars. A–F 100 µm (abbr: aprl anterior peristomial ring lobe, bh branchial heart, dl dorsal lips, dr dorsal radiole, mr median radiole, np nepropore, v aprl ventral anterior peristomial ring lobe, vr ventral radiole, vvfa ventral vascularized filamentous appendages,).