Polycladida, Lang, 1884 [viz. Faubel, 1983/, 1984
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publication ID |
https://doi.org/ 10.1007/s13127-017-0327-5 |
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persistent identifier |
https://treatment.plazi.org/id/9F2E1669-C732-F215-9D88-F92AAF70C716 |
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treatment provided by |
Felipe |
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scientific name |
Polycladida |
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Polycladida View in CoL character evolution
The origin of Polycladida is not much debated in the literature, but Ax (1995) stated that these worms are the first B real^ platyhelminths, as they are remarkably flat. An early, persistent hypothesis was that Polycladida may have originated from a Ctenophore-like ancestor ( Lang 1884; Laidlaw 1902), which is corroborated by ontological and anatomical features. One possible reason for the lack of debate is that most of the potentially closely related groups have very distinct lifestyles, being meiofaunal Platyhelminthes, while polyclads can be classified as B macroturbellarians^ of benthic habit. This means that morphological convergence related to habitat is very common and makes it hard the diagnosis of homologous characters between groups. The closer outgroups, from the order Prorhynchida ( Fig. 3 View Fig ), also present a seminal vesicle closely associated to the prostatic vesicle, in an almost interpolated arrangement (Palombi 1928). The same group can also present stylet (like the order Macrostomida ) and gonopore insemination ( Marcus and Marcus 1951; Janssen et al. 2015), suggesting that these are the plesiomorphic states of such characters in Polycladida . The Prorhynchida , as Polycladida , lacks the regenerative ability, common in other groups of Rhabditophora ( Marcus and Marcus 1951), like Macrostomum ( Fig. 3 View Fig ). Concluding from the distribution of characters within Polycladida , the ancestral flatworm may have possessed cerebral and marginal eyespots but lacked tentacles ( Table 2 and Fig. 3 View Fig ). The character that is traditionally used to divide Polycladida basally in two suborders, a sucker, can have different muscular arrangements, and in polyclads might have different positions ( Faubel 1984; Rawlinson and Litvaitis 2008). Some acotyleans present an adhesive structure but it is either lateral to the median line or anterior to the female gonopore ( Faubel 1984). An adhesive structure behind the female gonopore might have evolved once, thus being an autapomorphy of Cotylea . Then, the adhesive structure might also have evolved to a true sucker once in the tree ( Fig. 3 View Fig ), in the ancestor of pseudocerotids, prosthiostomids, and euryleptids.
Quiroga et al. (2015), based on 12 polyclad species from 11 families, showed that Cotylea presents a less developed nervous system (thinner nerve chords, lacking globuli cell masses, and not bilobed brain capsule) than Acotylea. However, cotyleans evolved special external sensory structures, like pseudotentacles and a more concentrated eyespot arrangement, also leading to cephalization. These different morphological traits might be related to different strategies or behaviors, as Acotylea tend to be cryptic and Cotylea less so. Marginal eyespots could have (a) originated three or four times in the tree and be lost two times or (b) evolved once, early in the tree, and have been lost three times ( Fig. 3 View Fig ), which seems more parsimonious. The family Prosthiostomidae is one of the three cotylean groups that have marginal eyespots ( Table 2). In this suborder, eyespot arrangement apparently evolved from an ancestral cotylean condition showing cerebral, marginal, and tentacular eyespots (like in Pericelis ; Table 2) to the division in the following two groups: one with cerebral and marginal eyespots (loss of tentacular clusters) and the other with cerebral and tentacular (loss of marginal clusters) eyespots ( Figs. 3 View Fig and 8 View Fig ). In Acotylea, Adenoplana and Phaenocelis group share cerebral, nuchal, and marginal eyespot character with the tentaculated group ( Table 2 and Fig. 3 View Fig ), which points to a plesiomorphic trait. Eyespot groupings thus appear less reliable than previously thought ( Lang 1884; Prudhoe 1985), possibly having evolved independently throughout acotylean genera.
Lang (1884) hypothesized that nuchal and marginal tentacles have different origins, while Laidlaw (1903c) believed them to have a common origin and therefore be homologous. Our results support Lang’ s hypothesis as more parsimonious, since for tentacles to have originated once, they would have been lost at least four times in the tree ( Fig. 3 View Fig ). The alternative being marginal tentacles evolving in the ancestor of Pericelis and all other cotyleans, and being lost once, in Prosthiostomidae . And in Acotylea, nuchal tentacles appear to have evolved one time ( Fig. 3 View Fig ), in the ancestor of Stylochidae , Planoceridae , and Hoploplanidae clade ( Table 2 and Fig. 7 View Fig ). Some hypotheses about the origin of this kind of tentacles are (a) nuchal tentacles are homologous to dorsal tentacles of some ctenophores ( Lang 1884) and (b) are peculiar for Polycladida and originated from marginal tentacles that shifted to lie dorsally ( Laidlaw 1903c). The results presented here clearly support the latter hypothesis. According to Laidlaw (1903c), the movement of tentacles to the dorsal surface also carried the eyespots related to that structure; thus, nuchal eyespots would be homologous to tentacular or pseudotentacular eyespots. The tree presented here shows that acotyleans without tentacles also have nuchal eyespots, a trait that does not occur in Cotylea and may point that the character is not homologous. Also, there is no intermediary condition of tentacles and corresponding eyes at an intermediary position between margin and post-cerebral area; thus, both types of tentacles appeared independently. However, there is a need for more evidences (i.e., evolutionary developmental biology) to make further assumptions about tentacles and nuchal and tentacular eyespots. Marginal tentacles either (a) have originated once and have been lost two times ( Figs. 3 View Fig and 8 View Fig ), once in Prosthiostomidae and once in Theama and Chromyella clade; or (b) have originated three times, and in one of them evolving into highly complex structures (in the Pseudocerotidae + B Euryleptidae^ group), or suffering reduction (in Cycloporus ); or even (c) have originated six times, once in Pericelis , then in Boninia , then in Cycloporus + Prosthecereus + Maritigrella clade, and finally in Pseudocerotidae . In general, our molecular tree and preliminary reconstruction of character evolution suggest a trait of anteriorization of sensory structures, from the plesiomorphic acotylean body plan, over the somewhat intermediate condition in B acotylean^ taxa here assigned to Cotylea , to the traditional cotylean gross morphology.
Among cotyleans, Thysanozoon appeared as a sister group of both the Monobiceros - Phrikoceros and Pseudobiceros, Yungia , and Maiazoon clusters; this points to a single origin of multiplication of reproductive structures ( Fig. 3 View Fig and Table 2). Lang (1884) considered the type of pharynx a character with B more weight^ than others, and Faubel (1984) designated a superfamily based on that character. However, no Euryleptidae grouped with Prosthiostomidae in the trees presented in this study. Thus, the tubular pharynx might have originated (a) once and then have been lost in Pseudocerotidae (b) or evolved at least three times independently ( Fig. 8 View Fig ). We consider (a) more parsimonious. A much more severe conflict concerns the type of prostatic vesicle, as it presents a high variability and is clearly subject of parallel evolution; thus, this character should not be used as a defining trait between groups. Traditionally used morphological characters related to reproductive structures also showed convergence in other Platyhelminthes groups such as Macrostomorpha ( Janssen et al. 2015). The penis and the cirrus structures that evolved in some groups seem to be homologous, as was found in Macrostomorpha ( Janssen et al. 2015), and transitions from one structure to the other might have happened multiple times in different platyhelminths, but this still needs to be tested in polyclads. The type of vagina differs in general terms between the two suborders, being more often looping in Acotylea and short in Cotylea . This is most likely related to the presence of hypodermic impregnation in cotylean polyclads and other close-related Platyhelminthes (Rawlinson et al. 2008; Janssen et al. 2015); in this case, the vagina is mainly for egg deposition and spermatophores are injected anywhere in the skin ( Bock 1922).
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