Various anthropogenic and environmental factors determine the dispersal and establishment of nonindigenous mollusc species in the Mediterranean Sea (UNEP-MAP RAC/SPA 2010). Indo-Pacific introductions via the Red Sea through the Suez Canal and Atlantic introductions via the Gibraltar straits follow the natural water circulation patterns of dispersal in the Mediterranean Sea (Valdés et al. 2013). These introductions are also aided by human-induced factors, such as fouling on private and commercial ships, untreated ballast water dumping, macrolitter rafting, aquaculture, and the aquarium trade (Katsanevakis et al. 2013; Ulman et al. 2019). Among the dominant aiding factors for the successful establishment of nonindigenous species (NISs), especially in the eastern Mediterranean basin, is the increase in sea surface temperature (SST) due to global warming. In fact, satellite observations from 1985 to 2008 revealed a significant SST increase of approximately 1°C in the area, with the highest peak for the Aegean Sea reported regularly in August (on average 1.23°C) (Nykjaer 2009; Raitsos et al. 2010). This process is expected to intensify in the coming decades since the Mediterranean Sea is the fastest warming sea globally, with sea surface temperatures increasing 20% faster than the global average (Givan et al. 2017; Peleg et al. 2019; Albano et al. 2021a).
The steady increase in SST has changed biotic interactions and affected temperate marine systems through a process known as “tropicalization of marine environments” (Scuderi and Viola 2019). Through this process, thermophilic species expand their natural range to novel areas/higher latitudes (Scuderi and Viola 2019). Similarly, there is an increased occurrence of warm water biota in the Mediterranean Sea, where approximately 1,000 alien species have been recently recorded (Zenetos et al. 2022). Moreover, infralittoral zones are more exposed to thermal variability, and they are more affected by sea warming and tropicalization (Verges et al. 2014). In some cases, ectothermic malaco-fauna react accordingly, and new southern thermophilic species are established, whereas indigenous species flee to cooler localities (Albano et al. 2021a). Interestingly, the shift from assemblages composed of Mediterranean species to those dominated by tropical NISs in shallow Israeli reefs (Albano et al. 2021a) highlights that the transformation of subtidal marine ecosystems could have significant effects on the marine biota in the eastern Mediterranean basin. Similar thermal conditions occur in the South Aegean Islands (Greece), which are part of the fast-warming Levantine Basin. Concurrently, local water circulation patterns, such as those in the mid-Mediterranean Jet, create a complex system of cyclonic-anticyclonic eddies and fronts in the area, favouring larval transport (Valdés et al. 2013). Such conditions result in a high recruitment of nonindigenous species on the southern Dodecanese Islands, turning the area into a hot spot for their establishment (Crocetta et al. 2017).
To date, 173 nonindigenous molluscs have been reported in the Mediterranean Sea, representing an increase of 38% during the last 20 years (Zenetos et al. 2022). The establishment of sessile bivalve NISs is further facilitated locally by both the availability of suitable substrata and increases in water temperature, especially in coastal habitats. Despite the magnitude of bivalve invasions in the Levantine basin, the ecological effects on the benthic community have not been thoroughly studied (Diga et al. 2023). Six species of nonindigenous sessile bivalve taxa, including members of the genus Isognomon (Lightfoot 1786), have been reported in southern and warmer localities of Greece, such as the southern Dodecanese Islands (Crocetta et al. 2017). They prevail in infralittoral sessile communities and alter hard substrata assemblages (Albano et al. 2021a).
Sessile epibyssal bivalves of the genus Isognomon are widespread in the intertidal and infralittoral zones of the tropics and subtropics (Benthotage et al. 2020). They have a pearly interior and highly irregular shapes (Reeve 1858; Printrakoon 2015). Recent molecular studies have shown that they belong to the family Pteriidae [Gray, 1847 (1820)] (Tëmkin 2010). The genus Isognomon is easily distinguished from the rest of the Pterioidea [Gray, 1847 (1820)] by its multivincular ligament (Stenzel 1971). There are 16 valid species belonging to the genus Isognomon (Benthotage et al. 2020). However, the diagnostic characteristics used to discriminate these species are particularly problematic because they are mainly based on descriptive features of the shell, such as the outline of the shell margin, obliquity, extent of the auricles, and various aspects of its shape and colour (Tëmkin 2006; Ubukata 2003). Four of them, Isognomon australicus (Reeve, 1858), Isognomon isognomum (Linnaeus, 1758), Isognomon legumen (Gmelin 1791), and the Isognomon nucleus (Lamarck, 1819), were recorded in the Red Sea (Vine 1986), which made them Lessepsian immigrant candidates due to their geographic proximity. Moreover, the Isognomon ephippium was recorded in Israel (Mienis 2004) as a non-Indo-Pacific immigrant; however, this record was not confirmed.
Only two species, Isognomon legumen (Gmelin, 1791) and Isognomon australicus, have been recorded in the Mediterranean Sea, the first from Israel, Greece, Turkey and Italy (Mienis 2016; Micali et al. 2017; Ovalis and Zenetos in Stamouli et al. 2017; Scuderi and Viola 2019), and the latter from Astypalaia, Saronicos, and Plakias Crete in Greece (Angelidis and Polyzoulis 2018; Albano et al. 2021b; Manousis et al. 2021a). In the literature, I. legumen is acknowledged as a well-established species in the Mediterranean Sea; however, there is skepticism about the establishment of I. australicus (Zenetos et al. 2018; Crocetta et al. 2017; Garzia et al. 2022). Very recently, a third species, Isognomon bicolor (CB Adams, 1845), was recorded in southern Italy (Garzia et al. 2022). Isognomon bicolor was originally described from Jamaica (Caribbean Sea, western Atlantic), and it has become a widespread invasive species across the Brazilian coast, having a negative impact on the local biota (Domaneschi and Martins 2002; Breves-Ramos et al. 2010; López et al. 2014).
Hence, our knowledge of the genus Isognomon in the Mediterranean Sea remains controversial. The sole use of a morphological approach for species identification has been proven unreliable due to the overall similarity of morphologies and the lack of solid diagnostic characters to distinguish species of the genus (Benthotage et al. 2020). In this study, we attempt to elucidate the establishment of Isognomonidae in the South Dodecanese Islands, Greece, through an integrated approach based on DNA barcoding, morphology, and field observations.