Manayunkia danubialis Băcescu, 1944 (Bick & Oberrisser & Bastrop, 1944)

Bick, Andreas, Oberrisser, Philipp & Bastrop, Ralf, 2024, Redescription of Manayunkia caspica Annenkova, 1929 and M. danubialis Băcescu, 1944 n. comb. (Fabriciidae, Sabellida, Polychaeta), Zootaxa 5477 (4), pp. 445-464 : 455-460

Manayunkia danubialis Băcescu, 1944 n. comb.

( Figures 6–9 )

Manayunkia caspica danubialis B ă cescu, 1944: 153.

Manayunkia caspica danubialis B ă cescu, 1948: 7 (foot note 2).

Manayunkia caspica fluviatilis B ă cescu, 1948: pl. 1, 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.— Zenkewitsch 1935: 199–201.— Hartman 1951: 389.

Diagnosis. Eight thoracic and three abdominal chaetigers. Pectinated radioles present. Unbranched vascularized ventral filamentous appendages present. Dorsal lips erect, rounded. Anterior margin of anterior peristomial ring developed as a low membranous collar with distinct ventral lobe; anterior peristomial ring narrowly separated mid-dorsally; ciliated band ventrally on the posterior margin of the anterior peristomial ring. Border between anterior and posterior peristomial ring clearly visible. Inferior thoracic notochaetae on chaetigers 1 and 6–8 narrowly hooded, and pseudo-spatulate on chaetigers 2–5. Thoracic uncini of chaetigers 2–5 or 2–8 (sexual dimorphism?) with main fang and towards apical with about 4 rows of progressively smaller teeth. Where these uncini absent (females?), i.e., segments 6–8, transitional uncini without main fang and a multitude of rows of small, equal-sized teeth present. Dentate region of abdominal uncini with about 6–8 rows of equal-sized teeth, manubrium about three to four times as long as dentate region. Peristomial eyes black, pygidial eyes absent. Females with pigmented spermathecae and brood chamber.

Material examined. Austria, Danube : Hainburg , 48°09.37’N, 16°59.39’E, date 15.02.2022, 1 specimen ( ZSRO-P2671 ), Oberloiben, 48°23.20’N, 15°31.22’E, date 15.02.2022, 5 specimens ( ZSRO-P2672 ), Jochenstein, 48°31.16’N, 13°42.06’E, date 09.02.2022, 1 specimen ( ZSRO-P2673 ), Nussdorf, 48°15.39’N, 16°22.12’E, date 15.02.2022, 7 specimens ( EMZ Rostock 9511), Enghagen, 48°14.25’N, 14°30.35’E, date 15.02.2022, 3 specimens (NHMW-ZOO-EV-21535) .

Description. Specimens with eight thoracic and three abdominal chaetigers ( Fig. 6A, E ); 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. 6A–C, E ); 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. 6A, D, F, H, I ).

Radiolar crown with three pairs of radioles and one pair of unbranched vascularized ventral filamentous appendages ( Fig. 8A–C ); surface of radioles and ventral filamentous appendages wrinkled ( Fig. 6A–C ); ventral filamentous appendages and all branches of radioles end at about same height (6A–C, E); radiolar lobes completely separated from one another ( Fig. 8D–F ); first dorsal radioles branch off from the branchial lobe, median and ventral radioles arise from a common base ( Fig. 8B–D ); ventral and median radioles asymmetrical branched or pectinated, dorsal radioles unbranched ( Fig. 8A–D ); ventral radioles with 5 (left and right lobe), median radioles with 4 (left and right lobe) branches (only one specimen examined) ( Fig. 8B–D ); no morphological differences in the structure of the branches, except ventral filamentous appendages ( Fig. 8A, B ); 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. 8A ); vascularized ventral filamentous appendages with blood vessel ( Fig. 8A–F ), connecting to corresponding dorsally located branchial heart ( Fig. 9A ); 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 of radioles ( Fig. 8A–C ); center of branches of radioles occupied by a large cavity bordered by a narrow band of cells; radioles with cylindrical cells laterally; extracellular matrix not visible between epithelia ( Fig. 8A–C ). Dorsal lips as triangular lobes, rounded at the upper margin, with ciliated epithelium all around ( Fig. 8B–F ); about 20 to 30 µm long, dorsal lips connect dorsal radioli with vascularized ventral filamentous appendages; between base of branchial lobes and base of the dorsal lips, ciliated food groove forms as a continuation of the ciliated epithelial cells of the radioli, which extends ventrally to the mouth opening; mouth opening bounded ventrally by the anterior peristomial ring. Ventral lips or lip-like processes absent.

Peristomium slightly longer than first chaetiger, about as long as chaetiger 2, consisting of anterior and posterior ring; anterior peristomial ring distinctly shorter than posterior one ( Fig. 6A–C ); anterior margin of anterior peristomial ring developed as low membranous collar ventrally, narrowly separated mid-dorsally ( Fig. 6A–C ); ciliated band present ventrally on posterior margin of anterior peristomial ring ( Figs 6C , 9A ); border between anterior and posterior peristomial rings clearly visible ( Fig. 6B, C ); one pair of rounded black peristomial eyes present. Females with pigmented spermathecae.

Faecal groove dorsally deeply sunken in peristomial region, considerably lower on first and following chaetigers ( Figs 6B , 9B ); shifting from dorsal midline to ventral midline at border between thorax and abdomen ( Fig. 6A, D, F ).

Metanephridia located in peristomium and first two chaetigers; nephridial duct first paired, then unpaired; nephropore dorsally between base of radiolar lobes ( Fig. 9A, B ).

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. 6A, E ); borders between thoracic chaetigers usually clear visible, except between last 3 thoracic chaetigers ( Fig. 6A ), especially with females, if a brood chamber is present ( Fig. 6E ); abdominal chaetigers short; abdomen, including pygidium, in total about as long as chaetiger 8 ( Fig. 6A ); pygidium about same length as chaetiger 10, terminating as rounded lobe ( Fig. 6A, D, F, H, I ); pygidium with fields of small epidermal glandular openings ( Fig. 6I ); pygidial eyes absent.

First chaetiger with 2–3 short and 3–5 elongate, narrowly hooded notochaetae, neuropodial uncini absent ( Fig. 7A ); notopodia of chaetigers 2–5 superiorly with 4–6 elongate, narrowly hooded and inferiorly with 2–4 pseudospatulate chaetae ( Fig. 7B, C ); notopodia of chaetigers 6–8 superiorly with 4–6 elongate, narrowly hooded and 3–4 short, narrowly hooded chaetae ( Fig. 7D ); neuropodia of chaetigers 2–6 or 2–8 with 6–8 (rarely 4) uncini with main fang and towards apical with about 4 rows of progressively smaller teeth ( Fig. 7F, G ); sometimes uncini in a slightly offset double row ( Fig. 7G ); in a number of specimens, 3–5 transitional uncini on chaetigers 6–8, different from thoracic uncini on chaetigers 2–5; apical region of transitional uncini slightly longer than those of regular thoracic uncini, without main fang but with a multitude of rows of small, equal-sized teeth ( Fig. 7H ); abdominal neuropodia with 3–5 elongate and 2–4 short, narrowly hooded chaetae ( Fig. 7E ); abdominal notopodia with 13–18 uncini on chaetigers 9, 8–12 uncini on chaetiger 10, and 5–12 uncini on chaetiger 11, respectively; abdominal uncini with about 6–8 rows of equal-sized teeth, about 5–6 teeth per row ( Fig. 7I ); manubrium about three to four times as long as dentate region.

Females with brood chamber on chaetigers 6–8. Fixed specimens translucent, however radiolar crown, anterior intestinal region and nephridia are brownish in colour.

Remarks. Manayunkia danubialis n. comb. most closely resembles M. caspica described above. The differences between the two species are described in the Remarks on M. caspica . These differences are not very conspicuous. The character states also overlap. This is mainly because diagnostic characters, such as the number of thoracic and abdominal uncini, vary depending on the size of the specimens ( Bick 1995). However, a comparison of specimens of comparable size shows that the number of thoracic uncini is greater in M. danubialis n. comb. Individuals of M. danubialis n. comb. 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.

As the Danube population has already been described as subspecies M. caspica danubialis , an erection to species status becomes necessary, M. danubialis n. comb.

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

Biology. The following information on the biology of the species comes from Popescu-Marinescu (1964, 2008). Manayunkia danubialis n. comb. 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 of females are elongate and form a brood chamber. The peak of reproduction is in May. More than 40 per cent of individuals are then 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. A comparable reproductive period was described for M. aestuarina from the Baltic Sea ( Bick 1996).

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

Ecology. Manayunkia danubialis n. comb. 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 6. Manayunkia danubialis Băcescu, 1944 n. comb., SEM micrographs.A. Total view of a complete specimen, lateral view. B. Anterior end, dorso-lateral view. C. Anterior end, ventral view. D Posterior end, lateral view; arrow pointing to the shift of the position of the faecal groove. E. Total view of a female, dorso-lateral view. F. Posterior end with anal opening and faecal groove, ventral view. G. Chaetigers 7 and 8; note the orientation of the thoracal uncini. H. Posterior end; lateral view. I. Posterior end of pygidium; arrows pointing to fields of epidermal gland openings. Scale bars. A, E 200 µm, B–D, F, G 100 µm, H, I 20 µm.

FIGURE 7. Manayunkia danubialis Băcescu, 1944 n. comb., SEM micrographs. A. Notopodium of chaetiger 1 with short and long, narrowly hooded chaetae. B. Notopodium of chaetiger 2 with pseudo-spatulate and long narrowly hooded chaetae. C. Pseudo-spatulate chaetae of chaetiger 3. D. Inferior thoracic notochaetae of chaetiger 7. E. Abdominal neurochaetae of chaetiger 9. F. Thoracic uncini of chaetiger 2. G. Thoracic neuropodium of chaetiger 3; note the slightly offset arrangement of uncini. H. Transitional uncini of chaetiger 8. I. Abdominal uncini of chaetiger 9. Scale bars. A, B 20 µm, C–E, I 5 µm, F, H 2 µm, G 10 µm.

FIGURE 8. Manayunkia danubialis Băcescu, 1944 n. comb., semithin sections (1 µm) through radiolar crown. A. Branches of radioles and ventral vascularized filamentous appendages, all separated. B. Branches of ventral and median radioles start to fuse; left dorsal lip visible (100 µm basal of section A). C. Fusion of branches of ventral and median radioles continues, dorsal radioles still separated; dorsal lips visible (28 µm basal of section B). D. Left ventral and median radioles fused, dorsal radioles still separated; left ventral vascularized filamentous appendage and dorsal lip fused (16 µm basal of section C). E. Radioles fused, and connected to ventral vascularized filamentous appendage by dorsal lips (40 µm basal of section D). F. Branchial heart dorsally, and ventral anterior peristomial ring lobe visible (16 µm basal of section E). Scale bars A–F 100 µm (abbr: bh branchial heart, dl dorsal lips, dr dorsal radiole, mr median radiole, v aprl ventral anterior peristomial ring lobe, vr ventral radiole, vvfa ventral vascularized filamentous appendages).

FIGURE 9. Manayunkia danubialis Băcescu, 1944 n. comb., semithin sections (1 µm) through base of radiolar crown. A. Base of radiolar crown, junction of the blood vessel from dorsal branchial heart to ventral vascularized filamentous appendages visible. B. Cross-section through the peristomial region lobe (43 µm basal of section A). Scale bars A, B 100 µm (abbr: aprl anterior peristomial ring lobe, bh branchial heart, fg faecal groove, np nephropore, oe oesophagus, vvfa ventral vascularized filamentous appendages).