Synchaeta arcifera Xu, 1998

Rougier, Claude & Pourriot, Roger, 2006, On some rare and new species of rotifers (Digononta, Bdelloida; Monogononta, Ploima and Flosculariaceae) in the Kaw River estuary (French Guiana), Zoosystema 28 (1), pp. 5-16 : 12-15

Synchaeta arcifera Xu, 1998

(Fig. 6)

MATERIAL EXAMINED. — 10 ♀♀ in an Eppendorf tube ( MNHN AM 867 ) ; 6 ♀♀ in an Eppendorf tube (laboratory Écosystèmes lagunaires, University Montpellier II).

DIAGNOSIS. — Easily distinguished from the other members of the genus by the conical and acute dorsal extension of the integument and by the two dorsal hornlike appendages. Contracted animals take the shape of a crescent.

MEASUREMENTS (in µm). — Body length ≈ 150 (more or less contracted animals). Trophi: total length 66,

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fulcrum 35, manubrium 44, ramus 16, uncus 14. Subitaneous egg with thin shell and short spines: 42 × 39.

OCCURRENCE. — Estuary station (E, Fig. 1), mud flat station (V, Fig. 1), mangrove swamp station (M, Fig. 1).

DESCRIPTION

Generally, contracted females appear crescent-shaped with three dorsal conical points: two located in the front part of the animal and the third in the hind part. In some rare extended specimens, the paired horn-like appendages (L = 36 µm, W max = 13 µm) are located below the ciliated auricles and can be pointing forward as in S. bicornis Smith, 1904 . The third point of the crescent is made by a basal, globular appendage tapering conically and strongly. In some animals, the small globular foot was extruded ventrally to this postero-dorsal appendage (see Fig. 6D). The foot has two tiny pedal glands (2.5 µm) and terminates with two small appressed toes (L = 5 µm).

The head, with a ciliated corona, was mostly retracted, but apparently convex judging from three more or less extended specimens. The species has a double cerebral eye and tubular lateral tentacles located below the third part of the trunk, as in S. bacillifera Smirnov, 1933 .

Trophi: virgate of the Synchaetidae type; mastax large, conical, filling about one third of the anterior part of the body. Unci composed of a well separated hook preceding three teeth joined into a small plate, followed by three (left uncus) or two (right uncus) distinctly separated teeth. A last and small tooth of each uncus looks like a gingko leaf.

DISCUSSION

The crescent shape of fixed animals, the acute postero-dorsal appendage, and the gingko leaf-shaped tooth of both unci agree with the description of S. arcifera by Xu Youqin (1998) in a succinct Chinese publication, although his description differs particularly by the absence of the foot and toes. Xu clearly states (p. 167) that “no foot and toes can be observed whether living or in formalin”: that is not the case in our specimens. This Synchaeta needs careful comparison with S. bicornis , S. fennica Rousselet, 1909 and S. bacillifera , species with horn-like appendages (as shown in Table 3).

REMARKS

In view of the scarcity of S. arcifera and of the very succinct and probably incomplete original description, it seemed useful to deposit specimens of Guianan animals and to describe anew this very rare species.

REMARKS ON SPECIES ECOLOGY

Of the five species described in this paper, only Synchaeta arcifera , a true planktonic species, was encountered during both the wet season (in 4 samples/18) and the dry season (in 20 samples/32, with a highest density of 12 ind./l).

The genus Synchaeta is widespread in fresh, brackish and sea water (Ruttner-Kolisko 1974), with at least half of its 34 species occurring in marine/ brackish waters ( Hollowday 2002). Some of them are euryhaline and show varying tolerance limits to salinity, as described for a Mediterranean lagoon ( Rougier et al. 2000). In the Kaw River estuary, the conductivity varies between 25 and 40 mS during the dry season, and between 0.1 and 6 mS during the wet season, with the temperatures being similar between the two periods (27-30°C). During the dry season, a coastal type plankton develops with rotifers ( Synchaeta arcifera , S. vorax Rousselet, 1902 , S. cecilia Rousselet, 1902 , S. neapolitana Rousselet, 1902 , Trichocerca marina Daday, 1890 ), tintinnids ciliates ( Tintinnopsis spp. and Codonellopsis sp. ) and copepods ( Paracalanus crassirostris Dahl, 1894 , Oithona hebes Giesbrecht, 1891 ). The phytoplankton is mainly composed of Bacillariophyceae and Dinophyceae. During the wet season, S. arcifera was the only member of Synchaetidae encountered, except once when it was accompanied by S. cecilia .

Unlike S. arcifera , the four other species described from the Kaw estuary, Dissotrocha guyanensis n. sp., Epiphanes desmeti n. sp., Floscularia curvicornis n. sp. and Testudinella haueriensis , were encountered only during the rainy season, suggesting that they were probably carried away with the freshwater flow. Like Synchaeta , Epiphanes desmeti n. sp. is planktonic, while the three others are tychoplanktonic. These species were accompanied by other rotifers (Bdelloida Hudson, 1884, Colurellidae Bartos, 1959 , Lecanidae Bartos, 1959 , Trichocercidae Remane, 1933 , Notommatidae Remane, 1933 , etc.), cladocerans ( Chydoridae Dubowski & Grochovski, 1894 , Macrothricidae Norman & Brady, 1867 , Ilyocryptidae Smirnov, 1976 , Sididae Baird, 1850 ), and some copepods. The densities of this continental plankton type were very low (8-10 ind./l, about 50 times lower than the density observed during the dry season). The phytoplankton was dominated by the Chlorophyceae (> 80%).