Synchaeta tremula, Muller, 1786

Wilke, Tanja, Ahlrichs, Wilko H. & Bininda-Emonds, Olaf R. P., 2017, A comprehensive and integrative re-description of Synchaeta tremula (Müller, 1786) and the newly rediscovered Synchaeta tremuloida Pourriot, 1965 (Rotifera: Synchaetidae), Zootaxa 4276 (4), pp. 503-518 : 512-513

Trophi of S. tremula and S. tremuloida

The following is a morphological description of the trophi of both S. tremula and S. tremuloida , which are virtually identical except for peculiarities of the frontal hook and the extension of the cauda tip. The trophus elements comprise a single elongated and dorsally bending fulcrum, extensive paired ventral rami, an unpaired ventral hypopharynx, two extensive paired dorsolateral manubria with elongated cauda, and paired apical unci. The trophi of both species are of the virgate type, which Koste (1978) has further subdivided within Synchaeta into the Synchaeta-pectinata- type, which exhibits unci with a frontal hook and no further teeth, and the Synchaeta-tremula - type, which exhibits unci with at least one frontal hook and several distinctly incised teeth. The trophi of both S. tremula and S. tremuloida belong to the Synchaeta-tremula -type.

Unci. The paired unci are located immediately behind the mouth. They are connected to the rami and manubria by ligaments and are characterized by exhibiting hooks and several unci teeth. Two frontal hooks are present per uncus, of which the dorsal one is noticeably larger ( Fig. 4 E, J; dfh). A triangular platelet of numerous piliform and sclerotized processes connects each dorsal frontal hook of an uncus with the ipsilateral manubrium ( Fig. 4 E, J; tp). The ventral frontal hook is slightly broader and distinctly blunter in S. tremuloida than in S. tremula , where it is thin and sharp ( Fig. 4 E, J; vfh). A distinct gap is present between the ventral frontal hook and the unci teeth. The unci are slightly asymmetric, with seven and six teeth being typical of the right and the left uncus, respectively ( Fig. 4 D, I; un). The teeth are all distinctly incised and separated by a deep sulcus into dorsal and ventral groups of three except for the dorsal group on the right uncus which has four teeth. Occasionally (ca. 10%), the unci of either species also possess 5–6 teeth on the right side and 4–5 on the left. In such cases, the ventral tooth-group that normally consists of three teeth is fused into two teeth or one large, blunt tooth.

Rami. Because no evidence of any chamber openings was observed, the rami would appear to lack the basal and sub-basal chambers that are otherwise typical of the rotifer ramus. Instead, like the rami of other Synchaeta species, those of S. tremula and S. tremuloida are built up of two extensive lamellae that converge distally at the proximal end of fulcrum. The inner side of each ramus is raised and robust. Both inner reinforcements of the rami jointly form a triangular shape that demarcates the median interspace between both lamellae ( Fig. 4 A, F; rir). An additional robust ridge originates from each uncus to span the median portion of each lamella ( Fig. 4 A, F; rmr). Another strong, short ridge is located distally, reinforcing the distal part of each lamella ( Fig. 4 A, F; rdr). The remaining parts of the rami are more finely structured. The margin of each ramus is rounded outwardly and exhibits a distinct indentation distally at the proximal end of the fulcrum.

Hypopharynx. The hypopharynx is an unpaired trophus element that is located ventral to the apical part of ramus. The apical part of the hypopharynx is composed of a horizontal joist that is raised centrally ( Fig. 4 C, H; hyp), is of medium width, and exhibits pointed lateral ends. The caudal part of hypopharynx comprises a semicircular and fine structure attached to apical joist ( Fig. 4 A, F; hyp).

Fulcrum. The fulcrum is an unpaired element that arises ventrally from the distal ramus and bends towards the dorsal side of the animal at an angle of 135° ( Fig. 4 C, H; fu). It is blade-shaped in lateral view and decreases slightly in width moving distally. Distinct longitudinal ridges, as sometimes found in the fulcra of other Synchaeta species (e.g. Synchaeta grandis ; pers. obs.), were never observed.

Manubria. The paired manubria, like those of other Synchaeta species, apparently lack the dorsal, median and ventral manubrial chambers. Each manubrium is composed of an extensive, round, but fine lateral clava ( Fig. 4 B, G; cl), a distinct dorsoventrally curved and robust cauda ( Fig. 4 C, H; ca) and a medially directed, triangularly shaped toothlike clava lamella ( Fig. 4 B, G; clt); all these components encompass the lateral and, in part, the dorsal trophus regions ( Fig. 4 B & C, G & H). The cauda extends beyond the clava distally by one-third of the overall cauda length. In S. tremula , this extension of the cauda tip is blunt and narrow, whereas it is broadened in S. tremuloida ( Fig. 4 C, H; ca).

Molecular analysis. Genetic distances for both COI and 18S revealed patterns of little to no intraspecific variation within each of S. tremula and S. tremuloida compared to large interspecific variation between the two species (see Table 2). Results from the popDiversity.pl analyses revealed that the maximal observed intraspecific variation for either species never overlapped with the minimal interspecific variation, with the minimal bar-coding gap being 2.9x (COI for S. tremula versus the interspecific distance). This pattern is likewise found in the haplotype network for each gene, with the two species being separated by either 10 mutations for the more slowly evolving 18S or 80 mutations for the more quickly evolving COI ( Fig. 5 ). In most cases, only a single haplotype was present for both S. tremula and S. tremuloida for each gene with the exception of S. tremula for COI where eight haplotypes were divided into three linearly arranged clusters separated by a high number of mutations (38 and 56). Finally, all three species demarcation analyses ( Fig. 6 ) supported that both species form well separated, reciprocally monophyletic clades with respect to one another for each gene. Again, evidence for additional, “significant” sequence divergence within S. tremula is indicated for COI, with the different methods indicating the existence of up to five separate species within this clade.

Figures

FIGURE 4. SEM photographs of the trophi of A – E S. tremula and F – J S. tremuloida (A, F, ventral; B, G dorsal; C, H lateral with the ventral rami pointing upwards; D, I unci, apical view with the ventral side of the trophus pointing upwards; E, J unci, frontal hooks and triangular platelet, latero-apical detail with the ventral side of the trophus pointing upwards). ca, cauda; cl, clava; clt, toothlike clava-lamella; dfh, dorsal frontal hook; hyp, hypopharynx; fu, fulcrum; ra, ramus; rdr, ramus distal ridge; rir, ramus inner ridge; rmr, ramus median ridge; tp, triangular platelet; un, uncus; and vfh, ventral frontal hook.

FIGURE 5. Medium-joining network of individuals of Synchaeta tremula and Synchaeta tremuloida from Haarenstau (HS), Haarenniederung (HN), Schlossteich (ST) and Löschteich (LT) for 18 S (A) and COI (B). Individual haplotypes are represented as circles, the size of which expresses the number of individuals found per haplotype; black squares represent hypothetical, missing haplotypes. The number of mutations separating haplotypes is given by the dashes. All sequences and positions were included in each analysis, regardless of their length and amount of missing or ambiguous data (i. e. gaps or IUPAC codes).

FIGURE 6. Maximum-likelihood trees derived from the four-times analyses (Birky et al. 2010) for 18 S (A) and COI (B). Bootstrap frequencies (n = 1000) are shown above the major nodes. Bars to the right of the tree indicate the different species inferred by the four-times rule, the ABGD method at the highest cut-off criterion, and the maximum-likelihood based bPTP analyses, respectively. Provenance of the samples is abbreviated as in Fig. 5. Trees were rooted using Polyarthra remata. The scale bar indicates the average number of substitutions per site per unit time.