Aplysia parvula, IN

Golestani, Haleh, Crocetta, Fabio, Padula, Vinicius, Camacho-García, Yolanda, Langeneck, Joachim, Poursanidis, Dimitris, Pola, Marta, Yokeş, M. Baki, Cervera, Juan Lucas, Jung, Dae-Wui, Gosliner, Terrence M., Araya, Juan Francisco, Hooker, Yuri, Schrödl, Michael & Valdés, Ángel, 2019, The little Aplysia coming of age: from one species to a complex of species complexes in Aplysia parvula (Mollusca: Gastropoda: Heterobranchia), Zoological Journal of the Linnean Society 187, pp. 279-330 : 323-325

APLYSIA PARVULA IN THE MEDITERRANEAN

The recent invasion of the Mediterranean by tropical or subtropical species has been facilitated by the ‘tropicalization’ or ‘meridionalization’ of the region and it is a direct consequence of climate change ( Francour et al., 1994; Bombace, 2001; Morri et al., 2001). Because of the location of the Mediterranean Sea between the Atlantic and Indian Oceans, and the economic importance of this region, there is a high number of possible introduction vectors for non-native species ( Galil & Zenetos, 2002; Galil, 2009, 2012; Zenetos et al., 2010, 2012); these include, among others, maritime traffic, transport of aquaculture animals and plants and aquarium trade. Three sea hare species, namely Bursatella leachii de Blainville, 1817 , Aplysia dactylomela Rang, 1828 and Aplysia parvula have been reported in the Mediterranean as non-native species and their spread has been considered a consequence of the ‘tropicalization’ process. While the invasion pathway for A. dactylomela and B. leachii have been determined using genetic data (Valdés et al., 2013; Bazzicalupo et al., 2018), in the case of A. parvula there is rampant speculation on the origin of the Mediterranean populations. The case of A. parvula , in particular, has been a focus of debate because of inconclusive morphological similarities with the native species A. punctata ( Bebbington, 1970; Eales, 1970; Bebbington & Brown, 1975). Eales (1970) reported A. parvula from Cyprus based on specimens most likely collected in 1968 and indicated that A. parvula invaded the Mediterranean from the Red Sea via the Suez Canal, but gave no reasoning or justification behind this claim. Bebbington (1970) doubled down on the Red Sea origin hypothesis, while reporting additional specimens collected between Malta and Sicily. Bebbington (1977) re-examined Mediterranean specimens with the typical black parapodial marginal line of A. parvula and found that they had the single penial retractor muscle, which is characteristic of A. punctata . Bebbington (1977: 95) concluded that ‘these animals may be colour variants or may be hybrids and further investigation will have to be made’. Bebbington (1977) contradicted, in the same publication, the previous statement, indicating that A. parvula is either a relict from the Tethys Sea or a recent migrant into the Mediterranean, which could have arrived via the Gibraltar Strait or the Suez Canal. More recently, authors have suggested caution because of the controversial identity of Mediterranean animals ( Zenetos et al., 2005; Sciberras & Schembri, 2007).

Contrary to published reports, we have found no evidence of widespread presence of A. parvula s.l. in the Mediterranean Sea. Multiple specimens identified as A. parvula collected across the entire Mediterranean basin, as well as adjacent areas in the eastern Atlantic, were sequenced and examined morphologically. Whereas externally those specimens possess a dark parapodial margin and are similar to animals previously identified as A. parvula , in phylogenetic analyses they cluster with animals identified as A. punctata and collected as far away as northern Europe. Anatomically, the penial morphology of all Mediterranean animals examined is also consistent with that of A. punctata , having only one retractor muscle. Sequence data from the nDNA gene histone H3 provided similar results: A. punctata is distinct from all specimens of A. parvula s.l. sequenced in this study and the genetic makeup of the great majority of specimens from the Mediterranean and the eastern Atlantic is consistent with that of A. punctata . However, two specimens collected in the Azores, Portugal (HG20) and Gallipoli, Italy (HG19) were heterozygotes with one A. punctata allele and one allele from A. ghanimii (Supporting Information, Fig. S1). One plausible explanation is that these two specimens are hybrids and if this is confirmed it would imply asymmetric genetic introgression of A. ghanimii into A. punctata , because the mtDNA of these two specimens is consistent with that of A. punctata .

While we found no evidence of the proposed presence of A. parvula s.l. in the Mediterranean or the north-eastern Atlantic, with the exception of two nDNA sequences, it is clear (based on a review of the literature) that the A. parvula phenotype, namely small Aplysia punctata specimens with a dark parapodial edge, have become common in the Mediterranean. We cannot provide a definitive explanation for this observation, but there are several possibilities. Because the colour pattern of A. parvula s.l. and A. punctata is so variable, it is possible that it is influenced by environmental factors such as temperature or diet. If that is the case, the tropicalization of the Mediterranean ( Francour et al., 1994; Bombace 2001; Morri et al., 2001; Golani et al., 2002; Gofas & Zenetos, 2003) or the arrival of non-native seaweed species (e.g. Verlaque et al., 2015; Zenetos et al., 2017) may have resulted in the A. punctata phenotype becoming more similar to that of A. parvula s.l. One problem with this hypothesis is that the A. punctata phenotype persists in the Mediterranean. One of us (MS) has observed hundreds of egg-laying A. punctata in Croatia over the years; these animals are large (up to 200 mm when crawling) and display the typical A. punctata phenotype. These animals co-exist with smaller (~ 20 mm), sexually mature animals displaying the A.parvula phenotype, as observed by another one of us (JL). Another possibility is a more widespread introgression of A. parvula s.l. genes coding for pigmentation into A. punctata . Such an introgression would not have been detected in a study exclusively targeting a histone gene. Testing these hypotheses will require additional research, including more in-depth sequencing of Mediterranean animals using RAD-seq technology and/or ‘common garden’ experiments to investigate the influence of the environment on colour pattern and size. Both are well beyond the scope of this paper.

The results of this and other recent papers highlight the need for careful molecular and anatomical studies to understand the origin and dispersal of non-native species. The only three alien species of sea hares found in the Mediterranean ( B. leachii , A. dactylomela and A. parvula ) were hypothesized to have a Red Sea origin. Two of them, B. leachii and A. dactylomela , were found to have an Atlantic origin (Valdés et al., 2013; Bazzicalupo et al., 2018) and the case of A. parvula is most likely an example of mistaken identity.