Ceratopera, den Hartog, 1964

Diez, Yander L., Monnens, Marlies, Wuyts, Arlien, Brendonck, Luc, Reygel, Patrick, Schmidt-Rhaesa, Andreas & Artois, Tom, 2023, Taxonomy and phylogeny of Dalytyphloplanida Willems et al., 2006 (Platyhelminthes: Rhabdocoela), with the description of a new family, a new genus, and sixteen new species from Cuba and Panama, Organisms Diversity & Evolution (New York, N. Y.) 23 (4), pp. 631-681 : 675

Ceratopera

The morphological similarities between C. gracilis ( Graff, 1882) Den Hartog, 1964 , and C. paragracilis ( Ehlers & Ax, 1974) are also reflected in the phylogenetic relationships of both species, which form a fully supported clade. Specimens of C. paragracilis collected in Cuba are similar in overall morphology to those in other populations of this species. However, some intraspecific variability has been reported ( Karling, 1978), to which we here add new data. With a length of ~ 51 µm, the stylet of the Cuban specimens is longer than that reported in populations from Bermuda (40 µm; Karling, 1978) and the Galapagos (36 µm; Ehlers & Ax, 1974). These measurements recorded in literature considered the stylet as a whole, including the mantle in all instances. The bursal appendage also varies in length; in the specimen from Bermuda, it is 90 µm long (the bursal part being 60 µm long; the tubular part 30 µm) ( Karling, 1978), while Ehlers and Ax (1974) mention a total length of 45 µm in the specimens from the Galapagos.

Also, the populations in Cuba differ substantially from each other as to the length of the bursal appendage, ranging from~ 62 µm in the population from Bahía Larga to 105 µm in the specimen from Bueycabón. Moreover, the specimen from Bueycabón shows a bursal appendage with two distal tubes, as was also mentioned for the specimens from Bermuda ( Karling, 1978). In the populations from Bahía Larga and Galapagos Islands ( Ehlers & Ax, 1974), only one distal tube is present. Considering these size differences and the wide distribution of the species, it might be that C. paragracilis comprises a species complex. Also in C. axi , a variable number of distal tubes is reported, which even lead to the recognition of two different species: C. axi (1 distal tube) and C. bifida Ehlers & Ax, 1974 (2 distal tubes). However, Willems et al. (2004a) considered both species to be one and the same ( C. axi ), a view we follow here. Yet, Willems et al. (2005b) and other authors (e.g. Van Steenkiste & Leander, 2018) also expressed the possibility that C. axi is a species complex. Following this logic, and because the number of specimens is low for all sampled populations, we opt to keep all populations of C. paragracilis in one species for the time being. To bring more clarity, an extensively sampled dataset including sufficiently fast-evolving markers and representing specimens of all species from different localities is needed.