4.2. Two morphologically distinguishable species
The genera Cnemidocarpa is described within the Styelidae (Ascidiacea) family (WoRMS, Shenkar et al., 2020) which presents highly variable morphological characters (Monniot, Monniot, & Laboute, 1991). The Cnemidocarpa genus include solitary ascidians with thin but leathery tunic; gonads that could be elongated, tubular and occasionally ramified, always united in a compact mass contained within a membrane and attached to the body wall (Kott, 1985; Rocha, Zanata, & Moreno, 2012). To distinguish the species within the genus one of the most used characteristics is the number of gonads at each side of the body, however, C. verrucosa presents a high variability from 1 to 4 gonads and in this work no significant variation was found in this character. A high variability on colour and shape of warts was observed in the studied specimens and in the field (Fig. 4), no other new or already described characters were found to discriminate between genetically different species. However, we found that the presence/absence of basal disc could be a possible diagnostic character for identifying two genetic species in Potter Cove (where both species coexist): all C. verrucosa sp. A specimens showed no basal disc, while C. verrucosa sp. B presented this structure. To define “basal disc” we followed Kott (1971) descriptions of C. verrucosa . In her work, it is reported a “stalk” posteriorly expanded basally where the animal is attached to the substrate, she described that the body wall prolongs into a muscle-free jelly-like extension that expands into a basal plate in the base of the stalk, this structure increase in thickness to form a sort of spherical rhizome, constricted off from the rest of the body. Tatián et al. (1998) already reported differences in stalk (basal disc in this study) development in C. verrucosa and Molgula pedunculata from Potter Cove. In their work, it was suggested that different substrate fixing requirements could determine the greater development of the stalk diameter in specimens of C. verrucosa from soft bottoms over those from hard bottoms. It was addressed in the literature that morphological differentiation of species depends on ecological/environmental factors and time since divergence (Fišer, Robinson, & Malard, 2018; Harmon, Schulte  II, Larson, & Losos, 2003; Losos, 2008; Schluter, 2000). Our results suggest that the presence of the basal disc could be more related to genetically determined morphological differentiation than to phenotypic plasticity of the species. Nonetheless, to confirm this hypothesis it is necessary to address the genetic, morphological and spatial pattern of C. verrucosa sensu lato in a wider sampling range.
Morphological species delimitation is key, especially for recognition in the field when species are distributed in sympatry. Cnemidocarpa verrucosa was described initially by Lesson (1830), the type specimen was collected in Malvinas/Falklands Islands, in this and others descriptions the species was reported presenting high variability in shape, colour and size (Kott, 1971; Tatián, Antacli, & Sahade, 2005; Turon et al., 2016), a pattern shared with others ascidians (Dias, Abreu, Silva, & Solferini, 2008; Viard, Roby, Turon, Bouchemousse, & Bishop, 2019; Wiernes, Sahade, Tatián, & Chiappero, 2013). However, a character (morphological, molecular) may only appear to be polymorphic when two or more species are mistakenly treated as a single one. It is not uncommon that the apparent degree of polymorphism is strongly reduced once the cryptic or pseudo-cryptic species have been correctly identified (Dietz et al., 2015; Janosik & Halanych, 2010; Korshunova, Martynov, Bakken, & Picton, 2017; Montano, Maggioni, Galli, & Hoeksema, 2017).