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The New Intertidal Record of Goniobranchus annulatus (Eliot, 1904) (Chromodorididae) from Mersin Bay, Northeastern Mediterranean, Turkey

One individual of Goniobranchus annulatus (Eliot, 1904), which is known as a tropical species, was found at the intertidal zone among the rocks near the Tasucu Bay. It was 3 cm in length and remarkable with its bright colors. After it was photographed and measured, the specimen was released back to the sea. This Heterobranchs are widely distributed in the Indian Ocean and is also being found in the Red Sea. It was first listed as “casual” alien species in CIESM in 2005, but today, it is established in the Levantine Basin.


Introduction
Goniobranchus annulatus (Eliot, 1904) was first described in 1904 as "Chromodoris annulata". In a study conducted in 2012, the name of the species was accepted as "Goniobranchus annulatus", and it was included in the Heterobranchia subclass (Johnson & Gosliner, 2012;Schrödl & Stöger, 2014). G. annulatus is a gastropod that belongs to the order Nudibranchia. Nudibranchs are brightly colored mollusks. They are widely distributed in seagrass beds, coral reefs, mangroves, rocky, muddy, and sandy regions (Carpenter et al., 1997). G. annulatus, known as a tropical species, is found mostly in the Red Sea and the Indian Ocean (Daskos & Zenetos, 2007;Rudman, 1987). It is also listed as casual alien species in CIESM 2005 (Zenetos et al., 2005). Some researchers see the presence of G. annulatus in shallow and tidal areas as a strategy to reduce the pressure of predators on them. However, it is also thought that other Nudibranchs are a more significant threat to G. annulatus than predator fish (Lüttmann et al., 2006).
It has a white body covered with yellowish-orange dots. Rhinophores and gills are located in two purple rings. Also, a purple-continuous line surrounds the body from the sides. Rhinophores and gills are white, the tip and base parts are purple (Rudman, 1987;Nithyanandan, 2012). Species belonging to the family Chromodorididae are known to feed on sponges (Rudman, 1987;Sachidhanandam et al., 2000).
In this study, a specimen of G. annulatus was photographed in a rocky area containing some algae species (Ulva spp. and Enteromorpha spp.) at 1 m depth in Mersin Bay.

Material and Methods
One specimen of Goniobranchus annulatus was photographed at the intertidal zone of the Taşucu Bay (36°09'34.2"N 33°41'14.4"E) on 04 June 2019, at a depth of 1 m among rocks together with Ulva spp., Enteromorpha spp. and other algae on the shore (Figure1). Species identified according to Rudman (1987).

Results
In this study, a 3 cm long G. annulatus was observed at a depth of 1 m. In this example, typical G. annulatus morphological characteristics were found. Body-color was bright white and body surface covered with orange-yellowish spots. The trunk was covered with a continuous line in dark purple on the sides. The base and tips of rhinophores and gills were dark purple. Table 1 shows the recordings, lengths, and depths of G. annulatus in the Mediterranean.

Discussion
For the Mediterranean, G. annulatus was first recorded in Greece in 2004, which was observed at a depth of 12 m and in a rocky environment. It was carrying the coloration seen in Arabian waters. A purple line interconnected the purple rings surrounding the rhinophores and gills and the body covered with only a few yellow spots (Daskos & Zenetos, 2007). The records from Antalya and Iskenderun were the samples with Lessepsian coloration (there is no connection between Table 1 shows that the largest specimens of G. annulatus (14-20 cm) were found in the Gulf of Iskenderun up to a depth of 15 m (Özcan et al., 2010). The second-largest record given in the same table was Antalya (Gökoğul & Özgür, 2008) and Israel (Pasternak et al., 2011) with a length of 4 cm between 1-5 m depth. The smallest sample recorded is 1 cm long and is from the Kaş region (Yokeş et al., 2009).
The absence of direct geography to allow the migration of G. annulatus between Greece and the Arabian Peninsula supports the idea that the migration was by transport rather than the Lessepsian migration mentioned in Daskos & Zenetos (2007). In all other records in the Mediterranean and the Suez Canal, it is understood that G. annulatus prefers the relatively coastal and rocky areas as habitats. Thus, it can be said that there is a possible dispersion pathway for G. annulatus on the continental shelf on the mainland.
G. annulatus individuals have been previously reported from Antalya and İskenderun. The specimen observed in this study shows a color pattern similar to the pattern of the Red Sea population. Although the sample reported in Daskos & Zenetos (2007) shows a different feature like having an interconnection between rhinophores and gills, our finding supports the idea of the Lessepsian migration path for this species. This study is the new intertidal record of G. annulatus for the Levantine basin and the first record for the Mersin Bay.