Distinction among the species of the Parotoplana jondelii group is possible on the basis of differences in number and morphology of the copulatory spines (Fig. 3) and presence/absence of bursal spines.
The most obvious combinations of characters which are diagnostic for the species of the complex are as follows:
P. jondelii: presence of bursal spines and high number (18) of copulatory spines. G1 similar to D2 ; G3 similar to V, which are provided with a nearly straight apex, and an obtuse distal tip. The spines of the girdle are provided with small, subtriangular apophyses. With bursal spines.
P. axi: overall similar to P. jondelii, but with fewer (16), shorter and slender spines. It differs also for D1 similar to D2 , and both provided with recurved distal tips; G1 with falcate apices, and without any spine pair clearly corresponding to the peculiar G3 of P. jondelii. The species possesses unique V spines, provided with a proportionally very long, whip-like distal tip. Without bursal spines.
P. pulchrispina: with the highest number of copulatory spines in the group (20-22). D1 distinctly thinner than D2 ; G7 and G8 spines very characteristic, with pointed apices and short apophyses; the rest of the girdle spines with distinctive, obtuse apices and slender, upturned apophyses. Unique V spines, similar to the girdle spines, with reduced apophyses and obtuse distal tips. Without bursal spines.
P. obtusispina: with 14 spines, most of which have obtuse distal tips. G1 and G2 are straight, slightly inflated subapically, without apophyses. V spines nearly straight, with blunt apices and small, flap-like apophyses. Without bursal spines.
P. ichnusae: with 16 spines. Girdle spines are broad; G1 have very small apophyses, and are similar in morphology to D2 . G2 have long, falcate apices. The species has slender V spines, with small falcate apices, and short, narrow, upturned apophyses. Without bursal spines.
P. crassispina: with 14 copulatory spines. G1 are short, nearly straight, with very small apophyses, G2 - G4 are broader than in any species of the group, with comparatively short, broad, flat apophyses. The V spines are characteristic, with long and flat apophyses, and a more or less straight distal tip, variable in morphology in the populations studied. With bursal spines.
P. carthagoensis: with 14 spines; similar to P. crassispina, but with a distinctive shape of G2 ; G1 are proportionally much longer and with more developed apophyses. The rest of the G spines are distinctly slimmer than the corresponding spines of P. crassispina. V spines with much shorter apophyses. With bursal spines.
P. gracilispina: with 14 spines;similar to P. carthagoensis, but with overall slender spines. D2 are thin, almost as broad as D1 ; G2 proportionally even longer and thinner distal tip than in P. carthagoensis; G1 , G3 , G4 and V are provided with much longer and thinner apices. Bursal spines not observed.
Attribution of allopatric populations with distinct morphology of V spines to the single species P. crassispina (see Appendix), may be debatable. The conservative approach adopted reflects the lack of information from geographically intermediate areas. Morphological divergence among the similar species-pair P. carthagoensis – P. gracilispina appears much more marked, involving almost any pair of the spines of the set, and is interpreted here as supporting their specific distinction.
Karyotype, when available, revealed a very similar morphology in the species of the complex, with, at the most, small differences in the values of centromeric indices. The only species recognisable by features of karyotype alone appears to be P. jondelii. In this species each of the three smallest pairs is isobrachial. In the other species, at least one pair is markedly heterobrachial.
Monophyly and interrelationshipsnext section
The presence of a complete or partial girdle of copulatory spines is not a rare feature within the genus Parotoplana. However, the arrangement most commonly found in the genus differs from what is described above for the P. jondelii group. In the type species, Parotoplana capitata Meixner 1938andin P. varispinosa Delogu, Casu and Curini-Galletti 2008, two pairs of spines lie distinctly above the other spines of the set. The rest of the spines form a crescent-shaped, semicircular girdle. The distal tips of these spines range in morphology from needle-shaped to falcate, and the ventral-most pair is indistinguishable from the surrounding spines (Ax, 1956, Figs 140-143, p. 717; Delogu et al., 2008, Fig. 2B, p. 167). In the observations carried out on living, semi-squashed specimens, the dorsal group appears distinct and obliquely set, with a different orientation than the other spines (Ax, 1956, Fig. 139, p. 717).
Similarly, in the P. turgida Ax and Ax, 1974 group, which includes one species from Galápagos and four species from Bermuda (Ax and Ax, 1974; Ax and Sopott-Ehlers, 1987), one or two dorsal spines, according to the species considered, lie above a semicircular girdle. In this species group, all spines have a similar morphology. Moreover, the dorsal spines are visible in living, semi-squashed specimens as a distinct group, obliquely oriented with respect to the rest of the set.
The complex sclerotised apparatus of P. terpsichore Delogu and Curini-Galletti, 2007 is formed by a complete girdle of numerous (46) spines. In addition, a pair of spines, dorsal to the girdle, is present. In this species, most spines are fine and slender and lack apophyses (Delogu and Curini-Galletti, 2007).
A comparable arrangement to that described above for the P. turgida group is found in Parotoplanella progermaria Ax, 1956 (Ax, 1956; Figs 178-183, p. 733) – a possible sister-group of Parotoplana.
In most of the other species of Parotoplana, the ejaculatory duct is connected distally to a stylet, formed in general by two gutter shaped spines, often fused basally, or even for their entire length. The rest of the spines are lateral to the stylet (e.g., P. procerostyla Ax, 1956; see Ax, 1956; Fig. 175, p. 728), or form a semicircular girdle (e.g., P. macrostyla Lanfranchi, 1978; see Lanfranchi, 1978; Fig. 1A, p. 250).
There are thus no indications that the sclerotised set found in the P. jondelii group, i.e. a circular, complete girdle with the dorsal groups aligned with the rest of the set and with spines arranged in symmetrical pairs with different morphologies, is the plesiomorphic condition for the genus Parotoplana. This arrangement is thus assumed here to define a monophyletic group within the genus.
Species of the P. jondelii group present other unique characters:
- the wide bursal canal, bifurcated at least in P. crassispina, lined by a peculiar, furrowed, infranucleated epithelium is not known for other species of the genus. In most of them, in fact, the canal is narrow and elongated and lined by a flat epithelium with intraepithelial nuclei (see e.g., Ax, 1956; Fig. 150, p. 718);
- besides the P. jondelii species-group the karyotypes of other eight Parotoplana species are known. P. macrostyla, P. cucullata Delogu, Casu and Curini-Galletti 2008, P. pythagorae Delogu and Curini-Galletti 2007 and P. renatae Ax 1956 share the same haploid chromosome number (n = 6) with the P. jondelii group. However, only two pairs of larger chromosomes are present in the karyotypes of these species, while the remaining pairs are distinctly smaller (Curini-Galletti et al., 1984; Delogu and Curini-Galletti, 2007; Delogu et al., 2008; Martens et al., 1989a). On the contrary, all species of the P. jondelii group have a karyotype with three large isobrachial chromosomes and three very small pairs, almost one third the length of the previous ones.
However, the lack of information about the above characters for some of the species of the P. jondelii group makes us refrain from considering them as further synapomorphies for the complex.
A further peculiar character, found in many species of the P. jondelii group, namely the presence of bursal spines, is on the contrary fairly widespread in species of Parotoplana and even present in the type species of the genus (see Ax, 1956; Fig. 143, p. 717; Fig. 150, p. 718). The occurrence of ‘bursal nozzles’ in apparently unrelated species of Parotoplana (Delogu et al., 2008) suggests that the character may indeed be plesiomorphic for the genus. Furthermore, the bursal spines may be easily overlooked in semi-squashed observations and in sections, and their presence even more widespread than reported at present. Exemplary in this respect is the case of P. rosignana Lanfranchi and Melai, 2008, where the bursal nozzles, which in this species constitute a very obvious tube with a flaring opening (Delogu and Curini-Galletti, 2009), are not even mentioned in the original description of the species (Lanfranchi and Melai, 2008).
Although the present state of knowledge does not allow inferences to be drawn on the polarity of the characters seen in the P. jondelii group, the sister-taxon relationship between P. carthagoensis and P. gracilispina is apparent. In fact, they share a peculiar morphology of G2 spines not found in any other species of the group. These two species have the same number and similar overall morphology of copulatory spines of P. crassispina, which may indeed be closely related to them.
Distribution and biodiversity
Species of the P. jondelii complex are common constituents of meiofaunal communities of medium-grained sediments in shallow water. The group appears restricted to the central and eastern Mediterranean; none of the species were found in the BIOIMPA samples obtained from the northern Adriatic or along the Spanish coasts. In most areas, only one species of the group appears to be present. Exceptional in this regard is a beach in Sardinia, Le Bombarde, where three species of the complex (P. crassispina, P. pulchrispina, P. ichnusae) were found sympatrically in sediment samples taken at about 2 m deep. However, only P. crassispina was consistently present in the samples taken in shallower water, up to the intertidal zone.
The distributional pattern of the species of the group is indeed remarkable. P. crassispina is in fact the only species known for a wide range across the north-western Mediterranean. However, morphological divergence among populations is noticeable. P. ichnusae has been found in two localities on the west coast of Sardinia, about 50 km apart. The rest of the species are at present only known from their type locality, and have not been found even in apparently suitable substrates in adjacent areas. This limited distribution appears unusual even for the genus Parotoplana, where at least a few species present wider ranges across the central Mediterranean (Delogu and Curini-Galletti, 2009).
Owing to the present limited state of knowledge on biogeography of interstitial flatworms, the distributional data should be interpreted with caution. In fact, they depend on the scale of samplings performed so far, which is spotty at best. It is, however, significant that in most of the areas sampled during the BIOIMPA campaigns, different species of the complex were found, suggesting that ranges of these species may indeed be limited. The combination of narrow, mostly non overlapping ranges and divergence among populations, as observed in P. crassispina, is suggestive of poor dispersal and speciation acting by allopatric processes. The distribution of the two sister species P. carthagoensis and P. gracilispina from southern Mediterranean coasts (Tunisia and Israel) is indeed not conflictual with their origin from a common ancestor by allopatry.
Speciation processes, responsible for the peculiar geographical scale of speciation of the P. jondelii complex, have been plausibly enhanced by biological features of interstitial flatworms (i.e. strong thigmotactism of adults, direct development, short generation times). It is thus plausible that the study of the BIOIMPA material will reveal that congruent distributional and speciation patterns are widespread in mediterranean proseriates.