Synchaeta oblonga, Ehrenberg, 1832

Bininda-Emonds, Tanja Wilke Wilko H. Ahlrichs Olaf R. P., 2018, A comprehensive and integrative re-description of Synchaeta oblonga and its relationship to Synchaeta tremula, Synchaeta rufina and Synchaeta littoralis (Rotifera: Monogononta), Organisms Diversity & Evolution 18, pp. 407-423 : 419-422

Morphology of S. oblonga compared to Synchaeta littoralis and Synchaeta rufina

To underscore the species status of S. oblonga , we compared the suite of characters delineating it (see Tables 2, 3 and 4) to all remaining members of this genus (based on personal observations and/or the literature descriptions), bearing in mind the relative quality or subjectivity of the features. In so doing, we found no convincing differences between S. oblonga and either of S. rufina or S. littoralis ( Fig. 6 ) based on morphological descriptions of the latter from the literature, although both are recognised as valid species by Jersabek et al. (2015). Synchaeta rufina is regarded to be a Lake Baikal endemic and was re-discovered only once since its initial description by Kutikova and Vasiljeva (1982) by Sheveleva et al. (1995), where it was mentioned in a species list. By contrast, S. littoralis was first described from a brackish habitat in Dundee, Scotland ( Rousselet 1902), with further reports from the Bay of Kiel (Voigt 1956–1957) and brackish inland lakes ( Althaus 1957) in Germany; Fyrsjöen, Spitzbergen ( Amrén 1964); and presumably also from freshwater billabongs in Australia ( Shiel and Koste 1993). It has long been known to be morphologically similar to S. oblonga (Voigt 1956–1957; Donner 1959; Koste 1978; Shiel and Koste 1993), and, because the necessary differentiation from S. oblonga (Arndt et al. 1990; citing a pers. comm. from Koste) has never been adequately provided, it is often ranked as a species of doubtful status (species inquirendae; Hollowday 2002; Segers 2007).

The species status of S. littoralis was initially justified by the presence of two frontal eyespots (frontal aggregations of pigment granules after Remane (1929)) connected to the cerebral eye by a stream of pigment granules ( Rousselet 1902; Hollowday 1949; Amrén 1964), although it was later established that the latter can also be absent ( Buchholz 1952, as cited in Voigt 1956–1957; Althaus 1957). However, we would argue that this character complex alone is insufficient to support S. littoralis as a species. As mentioned above, assessing its status as present or absent can depend on the intensity of pigment colour, which itself can be strongly influenced by nutrition and might therefore be dependent on environmental factors ( Birky 1964). Moreover, the character complex is also variably present in S. oblonga and also overlaps with the condition in S. rufina where the streams are described as being always present (see Table 4).

Indeed, the morphological intraspecific variability present in S. oblonga tends to obscure many purported differences between it and S. littoralis or S. rufina . For example, for those characters that are highly variable in S. oblonga (see Table 4), we found that the literature data for S. rufina and S. littoralis lie entirely within the range of its intraspecific morphological variability ( Table 4). The only exception is for body size in S. rufina . However, S. rufina itself is described as being variable in body size (200–380 μm) and, although its maximum size does surpass that of S. oblonga (195–260 μm), the body size range of the latter does fall almost entirely into that of the former. Likewise, the more constant, but slightly ambiguous features in S. oblonga (e.g. convexity of the apical field, distance of apical receptors and the elevation of lateral styles; see Table 3) are all congruent to the states found in S. littoralis and S. rufina .

More importantly, this general tendency also extends to the robust and non-variable characters that, in combination, are specific for S. oblonga (see Table 2), thereby calling the species status of S. littoralis and S. rufina into doubt. For instance, all three species exhibit a neck region with three distinct transverse folds as well as lateral antennae that occur mid-ventrolaterally in the caudal third of the trunk. An additional similarity between S. rufina and S. oblonga for the latter occurs in that the lateral antennae in both species are surrounded by papillary folds. The same feature might also apply to S. littoralis , where the antennae were described as being “prominent” ( Rousselet 1902). Because the lateral antennae generally consist of otherwise inconspicuous cilia, we construe that their prominent nature in S. littoralis might derive from a prominent epidermal fold surrounding the cilia basally. Importantly, as far as we know, “papillary” lateral antennae do not occur in any other species of Synchaeta . Instead, most remaining members of this genus exhibit only a very low epidermal fold surrounding the cilia, and three— Synchaeta bacillifera Smirnov, 1933 ; Synchaeta johanseni Harring, 1921 ; and Synchaeta pachypoida Kutikova and Vasiljeva, 1982 —have distinct “tubular” sheaths ( Hollowday 2002) that appear to be more extensive than the papillary ones of S. oblonga and S. rufina and possibly S. littoralis as well. In addition, each of these three species is readily distinguishable from all of S. oblonga , S. rufina and S. littoralis in their possession of wattle-like ventral outgrowths, a tubular frontal antenna and a massive foot with needle-like toes, respectively ( Hollowday 2002).

The morphology of the foot, which otherwise clearly distinguishes S. oblonga from S. tremula , also speaks towards the strong similarity between S. rufina and S. oblonga , where it is carried partly retracted in both species and contains pedal glands that are shorter than the foot and have a distinctive form (i.e. spherical proximally followed by an abrupt constriction before widening slightly distally). To our knowledge, only Synchaeta baltica Ehrenberg, 1834 exhibits this suite of features (pers. obs.; contra pedal glands of the same length as the foot after Hollowday (2002)), but this species is clearly distinguished by its marine habitat preferences, the distinct comb-like teeth on the uncus and, secondarily, its larger body size. Unfortunately, the position of the foot and the morphology of the pedal glands for S. littoralis are not specified explicitly and many discrepancies are apparent among the drawings and LM images of this species. For instance, it is uncertain if the foot is outstretched or withdrawn in the specimen illustrated in Rousselet (1902), but it appears to be partly retracted in the LM image in Jersabek and Leitner (2013) as indicated by the anal pseudosegment partly overlapping the foot together with the pedal glands appearing to extend into the body. Moreover, the pedal glands appear to be short in the drawing of Rousselet (1902), slender and long in Althaus (1957) and morphologically similar to those of S. oblonga in the LM image in Amrén (1964).

Finally, the overall similarity among the three species is further supported by the morphology of the trophi. For instance, the peculiar shape of the fulcrum in S. oblonga —machete-like (with lateral longitudinal ridges) and an oblique distal end—is also apparent in the illustration of the fulcrum of S. littoralis depicted by Althaus (1957). More importantly, the uncus of all three species is characterised by distinctly incised teeth separated into two groups by a deep sulcus. Although Hollowday (2002) reported that the sulcus is only shallow in S. rufina, Kutikova and Vasiljeva (1982) instead emphasised the deep indentation in their initial description of this species. An uncus of similar form also occurs in S. tremula , Synchaeta tremuloida Pourriot, 1965 (Wilke et al. 2017), S. bacillifera and Synchaeta bicornis Smith, 1904 ( Hollowday 2002), but these species are distinguishable from all of S. oblonga , S. rufina and S. littoralis by possessing lateral antennae located near the base of the foot in the former two species or by the presence of horn-like outgrowths behind the auricles in the latter two.

Finally, the habitats of all three species also show some degree of overlap. Synchaeta littoralis was regarded initially as occurring in brackish water only, whereas S. oblonga (e.g. Voigt 1956–1957) and S. rufina ( Kutikova and Vasiljeva 1982) were considered to be freshwater species. However, S. oblonga does tolerate brackish water ( Koste 1978; Arndt et al. 1990) and its co-occurrence with S. littoralis in brackish habitats was reported by Arndt et al. (1990) and Rozanska (1963, as cited in Telesh and Heerkloss 2002). Conversely, S. littoralis appears to have been found in freshwater habitats as well (“ S. cf. littoralis ”, Shiel and Koste 1993 ).

Naturally, we cannot exclude that all these similarities do indeed represent true, shared similarities between S. oblonga and S. rufina and S. littoralis . However, we have also failed to find any characters or suite of characters that distinguish either S. rufina or S. littoralis from S. oblonga and both of the former species also share the specific suite of robust and unambiguous morphological features that otherwise delineate S. oblonga from all remaining members of the genus. Therefore, to clarify the species status of both S. littoralis and S. rufina and to ascertain whether or not they might be synonymous with S. oblonga , we strongly recommend that comprehensive re-descriptions of both

depicted as circles with their size representing the number of individuals found per haplotype. Hypothetical, missing haplotypes are shown as black squares. Dashes represent the number of mutations separating the haplotypes

species should be performed from topotypes sampled from their type localities (Dundee, Scotland, and Lake Baikal , Russia, respectively), including molecular information if possible, and with a particular focus on trying to differentiate both from S. oblonga .

Figures

Fig. 6 Illustrations of a, b S. rufina and c S. littoralis. Drawings of S. rufina and S. littoralis are from Hollowday (2002) and represent original drawings (a, b) by Kutikova and Vasiljeva (1982) and (c) by Rousselet (1902)