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).