Synchaeta oblonga Ehrenberg 1832
Creators: name: Bininda-Emonds, Tanja Wilke Wilko H. Ahlrichs Olaf R. P.
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Rotifera
Eurotatoria
Ploima
Synchaetidae
Synchaeta
Synchaeta oblonga

description: Distinguishing between Synchaeta oblonga and Synchaeta tremula   
 A good example for a species of Synchaeta defined by a suite of characters is the cosmopolitan and abundant S. oblonga, where previous morphological descriptions in the literature often show discrepancies to one another, especially as concerns the morphology of the foot and trophi (Hollowday 2002). Indeed, because of the high degree of intraspecific variability in S. oblonga (see Ruttner-Kolisko 1972), doubt can arise as to whether given specimens of S. oblonga really belong to that species, either because they might belong to another (undescribed) species (e.g. as Hollowday (2002) suspected of specimens of S. oblonga described by Rousselet (1902)) or because of the high similarity between S. oblonga and S. tremula, with the resulting confusion over these two species often being emphasised in the literature (e.g. Voigt 1956–1957; Donner 1959; Koste 1978). In the latter case, despite both S. oblonga and S. tremula being considered valid species by Jersabek et al. (2015), their similarity means that they are often clustered together (as Synchaeta gr. tremula-oblonga sensu Ruttner-Kolisko 1972; Obertegger et al. 2006; Dokulil and Herzig 2009; Waervågen and Andersen 2017) with several authors having even gone so far as to question their status as distinct species (Ruttner-Kolisko 1972).   
 Our molecular sequence data, however, clearly support the species status of both S. oblonga and S. tremula with respect to one another. Both the more slowly evolving 18S as well as the more quickly evolving COI markers revealed large interspecific distances, with the 10 mutations between S. oblonga and S. tremula for the former being distinctly above the one-mutation threshold typically applied for supporting the species status using this marker (see Raupach et al. 2010). In addition, the molecular data also support S. oblonga being a single species despite the relatively high level of intraspecific morphological variation in this taxon. COI haplotypes taken from three distinct temporal populations of S. oblonga diverged at most by only two mutations from one another. By contrast, the less morphologically variable S. tremula showed much higher levels of molecular divergence, with three widely separated COI-haplotype clusters. Admittedly, our sequence data for S. oblonga derives from only a single locality (albeit from three distinct sampling episodes), whereas that for S. tremula is obtained from two (18S) or more (COI). As such, we have undoubtedly underestimated the intraspecific molecular diversity of both species; however, it is unlikely that increased sampling of either individuals or localities would close the large molecular distance gap between these two taxa appreciably.   
 Nevertheless, the morphological intraspecific variability present in S. oblonga (Table 4) causes many of its morphological features to overlap with those of S. tremula, which, in the absence of any readily accessible diagnostic character(s) for S. oblonga, makes differentiating between these two species difficult. Especially variable features in S. oblonga include the body size and shape, the morphology of the eye as well as the number of uncus teeth and therefore comprise characters that are ecologically and environmentally influenced as well as presumably more genetically determined (e.g. the number of uncus teeth). Among the former set are the variable body size and shape, which are largely dependent on age or nutrition and/or the presence of developing eggs, respectively. In addition, the variability in eye morphology and frontal and streams of pigment granules might also be tied in part to nutrition because food can affect the colour intensity of the pigments (Birky 1964), which, in turn, could influence the interpretation of the features (e.g. frontal aggregations of pigment granules as being present or absent). Indeed, in examining the status of S. oblonga and S. tremula, both Pejler (1957, as cited in Donner 1959) and Donner (1959) rejected body size and shape as well as eye morphology as being suitable discriminatory characters in this context.   
 Instead, Donner (1959) held it to be necessary to consult features presenting a distinct difference between the concerned species and proposed the apical field as being potentially suitable given that is more convex in S. oblonga. Although we can confirm that this difference is indeed consistently present, we would add that an additional, important criterion in this context is that the interpretations of such features should also be relatively objective in nature. This is not the case for the shape of the apical field nor for several other readily accessible features (e.g. the size and orientation of the auricles, the separation of the apical receptors, the elevation of the dorsolateral styles, the elevation of the dorsal antenna and the shape of the lateral ends of the hypopharynx; see Table 3). Thus, despite their potential discriminatory power, we nevertheless recommend caution in applying these features for demarcating between S. oblonga and S. tremula because of their oft-times subjective interpretation.   
 However, our comprehensive re-description of S. oblonga did reveal several non-variable and robust features that clearly delineate this species from S. tremula (see Table 2). This includes Donner’ s (1959) second discriminatory feature—that the lateral antennae are located in the caudal third of the trunk in S. oblonga but at the caudal end in S. tremula —to which we would add that the antennae are always situated mid-ventrolaterally and surrounded by a papillary fold in S. oblonga (Fig. 2a, c; la; also Donner 1959), whereas they are always located nearer to the transverse axis and lack any papillary fold in S. tremula (Fig. 5a, c; la). Another character is the morphology of the neck, which possesses three distinct transverse folds to delimit the head and trunk regions in S. oblonga (Fig. 2a) but lacks any distinct folds in S. tremula such that the head transitions gradually into the trunk (Fig. 5a).   
 Additional robust features delimiting S. oblonga from S. tremula (see also Table 2) come from the foot, where the former species usually carries the foot at least partly retracted while swimming, thereby leading to the impression that the pedal glands, which are only maximally two thirds of the overall foot length, instead seem to be of the same length as the foot (as in Hollowday 2002) or even extend into the body (Fig. 2a; pg). In addition, the pedal glands are spherical proximally and decrease abruptly in width in the middle of the gland before widening again distally (Fig. 2a; pg). By contrast, the pedal glands in S. tremula are indeed as long as the foot and not spherical proximally, such that they decrease gradually in width moving distally (Fig. 5 a; pg). Furthermore, the foot is never withdrawn in S. tremula and bends dorsally (Fig. 5c; fo), whereas it is coplanar with the longitudinal axis of the body or bends slightly ventrally in S. oblonga (Fig. 2c; fo). Although Hollowday (2002) reported a foot with two pseudosegments in S. oblonga, we found only a single pseudosegment and suspect that the second might mistakenly be the distinct anal pseudosegment that overlaps the foot in S. oblonga when the latter is partly withdrawn.   
 Less accessible, but, nevertheless, still robust characters include the location of the gland openings of the retrocerebral organ (dorsal to the apical receptors in S. oblonga (Fig. 2b; go) but lateral to and in line with those receptors in S. tremula (Fig. 5b; go)) and various features of the trophi. Indeed, the latter shows good utility for distinguishing between species of Synchaeta (Segers 2004; Obertegger et al. 2006) and is generally regarded as being species specific across rotifers (De Smet 1998). However, existing descriptions of the morphology of the trophi in S. oblonga show numerous discrepancies to one another (see Hollowday 2002), such that unambiguous descriptions of the trophi are needed for this species (Donner 1959; Obertegger et al. 2006). Our investigation revealed that the trophi of S. oblonga are indeed morphologically similar to those of S. tremula (see also Obertegger et al. 2006), with the uncus, which typically represents the most important element of the trophi in the context of species discrimination, being essentially identical between the two species. Instead, the fulcrum is more informative in this context (Table 2), being macheteshaped in lateral view with an oblique distal end in S. oblonga (Fig. 2e; fu), but blade-shaped and with a rounded distal end in S. tremula (Fig. 5e; fu). Furthermore, the longitudinal striae at the distal end of the fulcrum, which were found here to be consistently present in S. oblonga but, by Wilke et al. (2017), to be absent in S. tremula, might represent another informative distinguishing feature between these species so long as they do not derive from preparation artefacts. As such, the robustness of this character requires further investigation.   
 Finally, although swimming behaviour can be highly important for species identification in rotifers, it is generally not considered for species demarcation although we note clear differences between S. oblonga and S. tremula in this feature. For instance, S. tremula often anchors itself to objects by a mucus thread together with a twisting movement about the longitudinal axis, a behaviour that we did not observe for S. oblonga. Furthermore, the latter species swims in a slightly coiled forward motion together with a slow rotation about the longitudinal axis, whereas S. tremula swims straight forward with a quick rotation about its longitudinal axis.   
 Although our data for the cosmopolitan S. oblonga stem from only a single locality, it is based on monthly sampling of this locality over a course of 2 years. As such, we have likely sampled many different, temporal populations in addition to the population break that occurred annually in December and/ or January. Importantly, our morphological analyses of upwards of 65 specimens showed a range of variability that was similar to that present in the literature for this species, which derives from many more, diverse localities. Thus, although we cannot exclude that additional sampling of new localities might result in more intraspecific variation for S. oblonga and thereby a decreased distinctiveness between it and S. tremula, we remain confident that the morphological and especially the molecular gaps between these species that we have documented are large enough to support the species status of both taxa.
description: Published as part of 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), pp. 407-423 in Organisms Diversity & Evolution 18 on pages 417-419, DOI: 10.1007/s13127-018-0380-8, http://zenodo.org/record/13322305