On the identity of Neritina baetica Lamarck, 1822 and Nerita meridionalis Philippi, 1836 (Gastropoda: Neritidae) from the Iberian Peninsula

When Mermod (1953) depicted the types of Theodoxus baeticus of Lamarck’s collection his photo did not display a pseudo-apophysis (peg). This led recent authors believe, that Th. baeticus is a synonym of Th. fluviatilis and Theodoxus meridionalis is the valid species (Alba et al. 2016: 55). In this paper I studied the syntypes of Th. baeticus as well as the syntypes of Th. meridionalis and demonstrated that in Th. baeticus a pseudo-apophysis exists and the opercula of Th. baeticus and Th. meridionalis are similar. Thus Th. baeticus is a good species and Th. meridionalis is a younger synonym of it. Therefore I can conclude that three Theodoxus spp., i.e., Th. fluviatilis , Th. baeticus and Th. valentinus occur in the Iberian Peninsula.


Introduction
Many species have formerly been described on the basis of shell morphology and especially on the patterns or the color of the shells. Many of these nominal taxa belong at least to Theodoxus fluviatilis. For instance in Central Europe Th. fluviatilis has patterns of drop shape, while it has in Spain zigzag lines, and in the Balkans we find a combination of both patterns (Glöer & Pešić 2015).
The morphology of the operculum may be the best morphological character which can be used to differentiate Theodoxus species. It is suggested to be more conserved intraspecifically yet more variable interspecifically for Theodoxus (Glöer and Pešić 2015, Alba et al. 2016, Anistratenko et al. 2017, with the apophysis (rib) and the pseudo-apophysis (peg), and in some species the rib-shield, rib-pouch, and the callus being the most important characters ( fig. 1).
In the 21 th century other authors listed Theodoxus baeticus as a threatened species (Moliner et al. 2001, Martinez-Ortí 2008, 2011 or listed it in their species lists for the Balearic Islands and mainland of Spain (Beckmann 2007, Bank 2011).
Bunje & Lindberg (2007) have been the first who showed that there is a relationship between Th. meridionalis and Th. valentinus, maybe an intraspecific relationship. Interestingly Bunje & Lindenberg (2007) also included specimens they attributed to Th. baeticus in their phylogeny.
Zettler & Van Damme (2010) believe that Theodoxus meridionalis is restricted to Sicily and Tunisia while Welter-Schultes (2012) listed Th. baeticus from S-Spain in addition to Th. meridionalis (Sicily and rare in Tunisia) and Th. elongatulus (Morelet, 1845)  This paper is intended to show that Theodoxus baeticus is a good species distinct from Th. fluviatlis and Th. meridionalis is a synonym of Th. baeticus.

Material and Methods
To be sure which species occur in the Iberian Peninsula I borrowed the syntypes of Theodoxus baeticus and Th. meridionalis. With the help of Dance (1966), who listed where the collections of former malacologists are housed, the syntypes could be found in the Natural History Museum of Geneva (NHMG, Th. baeticus of Lamarck's collection) and Natural History Museum Berlin (NHMB, Th. meridionalis of Philippi's collection).
The shells of Th. meridionalis are dried up and to get the operculum it was processed in a solution of KOH for some minutes to remove the operculum. The photos of Th. meridionalis have been made with a Leica digital camera system.

Results
At the operculum some adductors are attached which closes the shell to be save against exsiccation and predators. The prominent feature of the operculum is the apophysis, sometimes named "rib", and in some species the pseudo-apophysis, also named "peg" or "knob". The two latter names are appropriate for danubian Theodoxus spp. but not for species from the Mediterranean, because Th. baeticus and Th. meridionalis have a diagonal pseudo-apophysis which links the basis of the operculum with the apophysis (figs. 5-6). Some Theodoxus spp. have a rib shield, which forms together with the apophysis the rib pouch (figs. 3-4). Abbreviations: ca = callus, eo = embryonic operculum, la = left adductor, ap = apophysis, pa = pseudo-apophysis, ra = right adductor, rp = rib pouch, rs = rib shield.
When Mermod (1953: p.155, Fig. 169) published the types of Lamarck's collection in Geneva he depicted in addition to the black shell a photo of the operculum of Theodoxus baeticus (Lamarck, 1822) ( fig.  1). The quality of this photo was very poor and led some authors believe that Th. baeticus is a synomy of Theodoxus fluviatilis Linnaeus, 1758(Vidal Abarca & Surez`1985 because a pseudo-aopophysis was not visible. Mermod depicted only one operculum of the two syntypes ( fig. 7), possibly because the second one was broken ( fig. 9). But on this operculum the pseudo-apophysis is visible, in the depicted operculum it was broken. New high quality photos of the operculum of the syntypes of Th. baeticus showed that this species has a diagonal pseudo-apophysis, like Th. meridionalis (figs. 8-9), while in Th. fluviatilis a peg is missing ( fig. 2). For the sake of completeness I borrowed the syntypes of Nerita meridionalis Philippi, 1836, which are housed in the Museum für Naturkunde Berlin. The type series consists of 40 specimens, predominant empty shells in addition to a few specimens with operculum. All specimens show slimmer or broader violet zig-zag lines on light or dark ground (figs. 10-15). The diagonal peg of the operculum shows that it is conspecific with Th. baeticus. The rib shield is small and thus the rib pouch is short.
Already Bunje & Lindberg (2007) found that Theodoxus samples from Spain, Sicily, Tunisia, and Greece build one cluster (clade D in their paper). Recent molecular data confirm that there are only three Theodoxus spp. which occur in the Iberian Peninsula: Th. fluviatils, Th. meridionalis, and Th. valentinus (Martinez-Ortí et al. 2015, Alba et al. 2016.
The comparisons of the type material showed that Theodoxus baeticus is a good species and Th. meridionalis is a younger synonym of it.