Phylogeny and biogeography of the Cavernicola (Platyhelminthes: Tricladida): Relicts of an epigean group sheltering in caves?

Highlights

• First molecular phylogeny of a rare (11 species) planarian suborder, Cavernicola.

• Confirms monophyly, Maricola sistership and their status as a distinct suborder.

• Freshwater and epigean in origin, colonized both epigean and hypogean habitats.

• Disjunct distribution may be due to Gondwana origin, and loss of most epigean diversity.

• Genetic diversity and new findings foreshadow more species to be discovered.

Abstract

The planarian suborder Cavernicola Sluys, 1990 was originally created to house five species of triclad flatworms with special morphological features and a surprisingly discontinuous and broad geographic distribution. These five species could not be accommodated with any degree of certainty in any of the three taxonomic groups existing at that moment, viz., Paludicola Hallez, 1892, Terricola Hallez, 1892, and Maricola Hallez, 1892. The scarce representation of the group and the peculiarities of the morphological features of the species, including several described more recently, have complicated new tests of the monophyly of the Cavernicola, the assessment of its taxonomic status, as well as the resolution of its internal relationships. Here we present the first molecular study including all genera currently known for the group, excepting one. We analysed newly generated 18S and 28S rDNA data for these species, together with a broad representation of other triclad flatworms. The resulting phylogenetic trees supported the monophyly of the Cavernicola, as well as its sister-group relationship to the Maricola. The sister-group relationship to the Maricola and affinities within the Cavernicola falsify the morphology-based phylogeny of the latter that was proposed previously. The relatively high diversity of some cavernicolan genera suggests that the presumed rarity of the group actually may in part be due to a collecting artefact. Ancestral state reconstruction analyses suggest that the ancestral habitat of the group concerned epigean freshwater conditions. Our results point to an evolutionary scenario in which the Cavernicola (a) originated in a freshwater habitat, (b) as the sister clade of the marine triclads, and (c) subsequently radiated and colonized both epigean and hypogean environments. Competition with other planarians, notably members of the Continenticola, or changes in epigean habitat conditions are two possible explanations -still to be tested- for the loss of most epigean diversity of the Cavernicola, which is currently reflected in their highly disjunct distributions.

Introduction

Between 1946 and 1983 five species of planarian flatworms (Platyhelminthes, Tricladida) had been described that consistently defied the taxonomic schemes developed by planarian systematists. Four out of these five species (Opisthobursa mexicana Benazzi, 1972; O. josephinae Benazzi, 1975; Balliania thetisae Gourbault, 1978; Novomitchellia sarawakana (Kawakatsu & Chapman, 1983)) usually had been assigned to the marine triclads of the Suborder Maricola Hallez, 1892. The fifth species, Rhodax evelinae Marcus, 1946, was considered to belong to the freshwater triclads or Paludicola Hallez, 1892. It should be noted that the Suborder Paludicola is no longer valid; its representatives, together with terrestrial planarians –the now obsolete Suborder Terricola Hallez, 1892- are currently classified in the Suborder Continenticola Carranza et al., 1998 (Sluys et al., 2009, Riutort et al., 2012). In all cases, however, some doubt was expressed about the taxonomic assignments of these five species (Ball, 1974, Sluys, 1990). At long last, Sluys (1990) resolved the taxonomic confusion surrounding these five species by showing, on the basis of morphological characters, that they formed a monophyletic group that represented a new and different clade in the phylogenetic tree of the triclad flatworms. At that time three major clades, at the level of suborder or infraorder, were recognized within the Tricladida Lang, 1884, viz., Paludicola, Maricola, and Terricola. For his new, fourth branch on the tree of the planarian flatworms Sluys (1990) erected a new taxon for which he coined the name Cavernicola Sluys, 1990, presently being ranked as a Suborder (Sluys et al., 2009). Although most of its constituent species had a hypogean habitat and exhibited adaptations to life in caves (unpigmented body, lack of eyes), Sluys (1990) stressed the notion that the name of the new taxon had no ecological connotation.

With respect to the phylogenetic position of the new suborder within the Tricladida, Sluys (1990) suggested a possible close relationship between the Cavernicola and the Paludicola, based on the fact that the cavernicolan Opisthobursa josephinae exhibits one of the three presumed autapomorphies of the Paludicola, viz., sperm transfer by means of a spermatophore. However, he considered that character distribution as too weak to formally propose presence of a spermatophore as a synapomorphy for the Cavernicola and the Paludicola. Relationships within the Cavernicola were analysed also by Sluys (1990). The fact that the species possess a mixture of primitive features (Marcus, 1946, Sluys, 1990) greatly complicated resolution of their phylogenetic affinities.

After this, it took a long time before the number of species for the Cavernicola started to increase slowly. Two new species and one new genus were described in recent years, viz., Hausera hauseri Leal-Zanchet & Souza, 2014 from Brazil, and Novomitchellia bursaelongata Harrath, Sluys & Riutort, 2016 from Africa; both species live in a hypogean habitat (Leal-Zanchet et al., 2014, Harrath et al., 2016). In addition, Laumer and Giribet (2014) reported 18S and 28S rRNA sequences for a new, undescribed species of Cavernicola. It was only recently that this new species acquired its proper taxonomic designation when it was described as the new genus and species Kawakatsua pumila Sluys, 2019 (Sluys and Laumer, 2019). It is noteworthy that this species was found in a basically terrestrial habitat. Addition of these new species to the Cavernicola made even more evident a conspicuous feature of this small group of species, i.e., their highly disjunct distributions (Fig. 1).

The present study is the first to include molecular data for all cavernicolan taxa, excepting Balliania Gourbault, 1978. In our analyses we have incorporated also representatives of 15 genera of triclads belonging to the other two suborders, thus allowing us to test for the first time the previously hypothesized monophyly of the Cavernicola, to analyse its relationships within the Tricladida, as well as the affinities between its constituent taxa.