INTRODUCTION
Okenia aspersa (Alder & Hancock, Reference Alder and Hancock1845) was first described from the British waters as Idalia aspersa; the original description stated that the species is reddish or yellowish in colour, freckled with orange and reddish-brown speckles, having four long anterior filaments, two close to the base of each tentacle. Moreover, it possesses two or three lateral short filaments on each side of the body, and ten branchial plumes. It was considered similar to Okenia quadricornis (Montagu, Reference Montagu1815) which, according to the short original description, is white and brown in colour, with only two long anterior filaments, one at the base of each tentacle.
Alder & Hancock (Reference Alder and Hancock1845) expressly reported that the number of filaments is a constant and specific morphological feature, and reported both species for the shallow water area, which they called the ‘coralline (sic) zone’: Okenia aspersa was considered distributed in the north-eastern and western British Islands, and Okenia quadricornis in the southern coast of Devonshire, as described by Montagu (Reference Montagu1815).
In 1974, the International Commission on Zoological Nomenclature (ICZN), with Opinion 1014, voted for the suppression of the taxon Okenia quadricornis, so the junior synonym Okenia aspersa has been used since then (ICZN, 1974). Nevertheless, Thompson & Brown (Reference Thompson and Brown1984) and Gosliner (Reference Gosliner2004) considered them to be different species, and in particular the latter, after phylogenetic analyses, considered both species valid and required a comparison between Atlantic and Mediterranean specimens. Moreover, Schmekel & Portmann (Reference Schmekel and Portmann1982) and Valdés & Ortea (Reference Valdés and Ortea1995) considered Okenia quadricornis a valid species and Okenia aspersa a junior synonym.
In the Mediterranean Sea, an adult specimen of Okenia aspersa was recorded as Okenia quadricornis (evidently considering it again a valid name) for the Maltese Islands (Sammut & Perrone, Reference Sammut and Perrone1998). In addition, Valdés & Ortea (Reference Valdés and Ortea1995) found a juvenile 3.5 mm long in Aci Trezza (Sicily, Italy), and Schmekel & Portmann (Reference Schmekel and Portmann1982) recorded two juveniles, 1.1 and 2 mm long, in the Gulf of Naples. Outside the Mediterranean, the distribution of this species was found as quite continuous from Norway to the Cadìz Gulf (Thompson & Brown, Reference Thompson and Brown1984; Just & Edmunds, Reference Just and Edmunds1985; Platts, Reference Platts1985; Cervera et al., Reference Cervera, López-González and García-Gómez1991, Reference Cervera, Calado, Gavaia, Malaquias, Castaño, Ballesteros, Megina Martínez and García-Gómez2004; Urgorri et al., Reference Urgorri, Díaz-Agras, Besteiro and Montoto2011). Morse (Reference Morse1972) described Okenia ascidicola from Massachusetts, USA, now considered a synonym of Okenia aspersa.
Okenia aspersa is reported (sometimes as Okenia quadricornis) to feed on Ascidiella spp. (Thompson, Reference Thompson and Crisp1964; Thompson & Brown, Reference Thompson and Brown1976; Todd, Reference Todd1981) and Molgula spp. (Holt, Reference Holt1891; Thompson & Brown, Reference Thompson and Brown1976; Just & Edmunds, Reference Just and Edmunds1985; Picton & Morrow, Reference Picton and Morrow1994; Valdés & Ortea, Reference Valdés and Ortea1995).
Doto koenneckeri Lemche, Reference Lemche1976 was first described from southern Norway, at about 60° latitude, and from Eire, around 53° latitude; its main distinguishing features are having a body that is darker than most of its conspecifics and the presence of some elongated or ‘comma-shaped’ dots on the tubercles of the cerata. Evertsen & Bakken (Reference Evertsen and Bakken2005), in a survey of the opisthobranch fauna of Norway, only found it in the same area of the original description and considered it a rare species. Afterwards, it was also described from a large part of the Iberian coasts: the north-eastern coasts of Spain, Portugal, Ceuta, Eastern Andalucia and Catalonia (it has not been recorded in the central portion of the Mediterranean Spanish coast); it was also found in the Azores (Ortea & Urgorri, Reference Ortea and Urgorri1978; Fernández-Ovies, Reference Fernández-Ovies1981; Fernández-Ovies & Ortea, Reference Fernández-Ovies and Ortea1981; Urgorri & Besteiro, Reference Urgorri and Besteiro1983, Reference Urgorri and Besteiro1984; Ballesteros, Reference Ballesteros1984, Reference Ballesteros1985, Reference Ballesteros2007; Templado et al., Reference Templado, Luque and Moreno1988; García-Gómez et al., Reference García-Gómez, Cervera, García, García-Martín, Medina and Burnay1991; Calado et al., Reference Calado, Urgorri, Gaspar and Cristobo1999, Reference Calado, Malaquias, Gavaia, Cervera, Megina, Dayrat, Camacho, Pola and Grande2003; Sánchez Tocino et al., Reference Sánchez Tocino, Ocaña and García2000; Calado, Reference Calado2002; Cervera et al., Reference Cervera, Calado, Gavaia, Malaquias, Castaño, Ballesteros, Megina Martínez and García-Gómez2004; Ortea et al., Reference Ortea, Moro, Ocaña and Bacallado2009). Along the coast of France, this species has only been recorded by recreational divers in the western part of the French Mediterranean coast; in particular, in Port-Leucate in May, 2005 (Poddubetskaia Ossokine, Reference Poddubetskaia Ossokine2006) and in Banyuls in March, 2010 (Horst, Reference Horst2010). It is known to feed on many thecate hydroids, in particular Thecocarpus myriophyllum and different species of the genus Aglaophenia (Lemche, Reference Lemche1976; Picton, Reference Picton1978; Brown & Picton, Reference Brown and Picton1979; Picton & Brown, Reference Picton and Brown1981; Todd, Reference Todd1981; Urgorri & Besteiro, Reference Urgorri and Besteiro1984; Just & Edmunds, Reference Just and Edmunds1985; Templado et al., Reference Templado, Luque and Moreno1988; Thompson, Reference Thompson1988; Picton & Morrow, Reference Picton and Morrow1994).
MATERIALS AND METHODS
Noli Cape is a rocky promontory situated in the western part of the Ligurian Sea (Italy), about 500 m outside the southern borders of the Site of Community Importance (SCI) IT1323271 ‘Fondali Noli-Bergeggi’, and a few kilometres south from the Marine Protected Area (MPA) ‘Isola di Bergeggi’. The whole area between Bergeggi and the Cape is regularly monitored by the authors, and recently they were able to record the presence of the alien nudibranch Godiva quadricolor inside the SCI (Betti et al., Reference Betti, Cattaneo-Vietti and Bava2015). During a scuba diving survey conducted on 14 March 2015, in front of the northern side of the Noli Cape beach (WGS 84 coordinates: 44°11′58″N 8°25′11″E; Figure 1), Okenia aspersa and Doto koenneckeri were recorded. Underwater pictures of the specimens, for further checking of the records, were taken with a Nikon D7000 camera and Nikkor 60 mm micro lens in Isotta housing equipped with two Sea & Sea external strobes. A sample of the hydrozoans on which D. koenneckeri was crawling was taken for further identification with a stereomicroscope, while the identification of the nudibranchs was made solely in situ using pictures, and further molecular analysis may be requested in the future for a complete taxonomic identification of the species.
Map of Okenia aspersa and Doto koenneckeri distributions in the Mediterranean Sea. In red, the new records from Noli Cape, in white, the previous records of O. aspersa, in yellow, the previous records of D. koenneckeri.
RESULTS
One adult specimen of Okenia aspersa and two of Doto koenneckeri were observed during the survey. Okenia aspersa followed precisely the original description by Alder & Hancock (Reference Alder and Hancock1845); it was 2 cm long, crawling on large pebbles at a depth of 10 m. Two specimens of what appeared to be D. koenneckeri were found at a depth of 5.5 m, on a big stone about 30 × 30 cm wide, completely covered by fertile colonies of the thecate hydroid Aglaophenia tubiformis, which was later identified in the laboratory and is known to be a prey of D. koenneckeri (Templado et al., Reference Templado, Luque and Moreno1988). On the hydroids, many caprellids belonging to the alien invasive Caprella scaura Templeton, 1836 and the two nudibranchs were moving. Hydroids are known to host a rich and various epifaunal community, comprising both nudibranchs and caprellid amphipods (Bavestrello et al., Reference Bavestrello, Cerrano, Cattaneo-Vietti and Sarà1996), and the latter may show an aggressive behaviour towards nudibranchs, which are forced to retreat from the hydroid (Caine, Reference Caine1998); in this case, the molluscs and crustaceans were apparently pacifically cohabiting the colonies of A. tubiformis. Three of the newly produced egg coils of D. koenneckeri were also found (Figure 2). Both the animals and the eggs perfectly followed the features of the original description.
Underwater pictures of the specimens reported in the paper: (A) Okenia aspersa on pebbles; (B) Doto koenneckeri on the hydroid Aglaophenia tubiformis; (C) egg mass of D. koenneckeri on the same hydroid.
DISCUSSION
Despite the ICZN decision, it seems that there is still some confusion regarding the use of the name Okenia quadricornis, and according to Gosliner (Reference Gosliner2004), further studies are required to better understand if Okenia aspersa and Okenia quadricornis are two different, valid species; nevertheless, at present, we have to consider it a synonym of Okenia aspersa and, because of that, the present record of this species is to be considered the first from the Ligurian Sea, and the second one of an adult specimen for the Mediterranean Sea. Doto koenneckeri represents the first record from the Italian seas.
The coast of Noli Cape presents a steep slope, ranging from one to several meters in depth, mainly constituted by steep stripes of fine sand, alternated with bands of pebbles and gravel; the area is subjected to upwellings of cold deep waters, which bring nutrients to shallow habitats (Brasseur et al., Reference Brasseur, Beckers, Brankart and Schoenauen1996; Casella et al., Reference Casella, Molcard and Provenzale2011; Betti, Reference Betti2013). During the months of February and March, the area reaches its lowest peaks in temperature, with a monthly average surface temperature of 12.6°C in February and 13.9°C in March (ISPRA, n.d.). It is plausible that cold water species, such as the two here recorded, are able to colonize the Ligurian Sea in the colder season, when the water temperatures are similar to the ones found in their typical areal of distribution. The presence of Okenia aspersa in the Mediterranean Sea may, however, be considered as occasional, because there are only two records of adult specimens from here and they were found at a great distance from each other, both spatially and temporally.
Noli Cape is a few kilometres away from the important harbour of Savona – Vado Ligure, which had a cargo throughput of more than 13 million tonnes of goods in 2013, and represents one of the 24 Italian harbours considered to have international or national relevance (Valleri et al., Reference Valleri, Lamonarca and Papa2006). Because of that, the harbour is considered to be responsible for the ingression into the Mediterranean Sea of the alien nudibranch Godiva quadricolor (Betti et al., Reference Betti, Cattaneo-Vietti and Bava2015) – shipping may also be a possible vector for the ingression of this species. This theory is also supported by the fact that Malta, Sicily and Naples (where the juvenile forms were found) are historically known to have been receiving a large amount of cargo from the UK since WWII. Furthermore, it is important to note that no previously known prey of Okenia aspersa have ever been recorded in the area; the ascidians found in the Noli Cape area were: Microcosmus sp., Halocynthia papillosa, Phallusia mammilata, Botryllus schloesseri, Diplosoma spongiforme and Polysyncraton lacazei (F. Betti, personal observation). Because of this, further records are required in order to understand if the presence of the nudibranch may be considered as more than just occasional. Regarding Doto koenneckeri, the presence of the two specimens, the egg coils and their food could lead to it being considered as a more stable presence in this area. It is suggested that the species is slowly extending its distribution to the Italian coasts, in a progressive ingression in the Mediterranean Sea through the Gibraltar Strait; a possible negative interaction with Caprella scaura was not observed in the present work, and it is not considered to have any significant effect on the spread of the nudibranch.
