The point on Opaliopsis atlantis (Gastropoda: Epitoniidae) distribution: new data from the Mediterranean and implications

Specimens of the rare amphi-Atlantic epitoniid Opaliopsis atlantis have been recorded in the Strait of Messina (central Mediterranean) from a hydrozoan stylasterid-rich habitat. The record, which adds a new site to the sporadic occurrences of this prevalently deep-water species, may be considered the first contextualized report from Mediterranean Sea. Opaliopsis atlantis displays a planktotrophic larval development functional for long-range colonization of favorable habitats. Its discontinuous distribution all over its broad geographic range highlights the potential role of Atlantic seamounts as stepping-stones for transoceanic dispersal. Although no conclusive information is yet available upon the feeding requirements of O. atlantis all over its range, we suggest that this cnidarian-ectoparasitic prosobranch could adapt to different hosts, as a strategy that may enhance its wide biogeographic distribution. Key-Words. Epitoniidae; Amphi-Atlantic; Mediterranean; Bathyal; Biogeography.


INTRODUCTION
Opaliopsis atlantis (Clench & Turner, 1952) belongs to a kind of epitoniids provided with a multispiral and ribbed planktotrophic larval shell. This morphologically distinct deep-water group, previously placed in the subfamily Nystiellinae Clench & Turner, 1952, was then elevated to family level by Nützel (1998), until subsequent biomolecular studies did not validate their separation from Epitoniidae (Takano & Kano, 2014;Bouchet & Rocroy, 2017). The planktotrophic development, involving a high dispersal ability (Bouchet & Warén, 1986), accounts for the number of "nystiellids" displaying an amphi-Atlantic distribution (Lima & Christoffersen, 2013). Originally described upon few specimens from Cuba and Florida, O. atlantis was reported also off Brazil (Andrade et al., 2011;Lima & Christoffersen, 2013). To the east, O. atlantis is known from Azores, Madeira and Gibraltar (Bouchet & Warén, 1986;de Frias Martins et al., 2009;Segers et al., 2009;Gofas et al., 2014) up to the Mediterranean (Smriglio & Mariottini, 1999;Mifsud, 2009;Manousis et al., 2018). O. atlantis is one of the most uncommon epitoniids, known thus far from a handful of live specimens and a few empty shells all over its range. The finding of two specimens from the Strait of Messina (central Mediterranean) improves our knowledge on the areal of this rare species and suggests some considerations on its ecology.

MATERIAL AND METHODS
Samplings were carried out in July-August 1995 by means of modified Van Veen grab (0.25 m² sampling surface) and triangular dredge (60 cm mouth opening), according to the expected sea-floor nature. Samples were washed on board, under a gentle seawater flow throughout 1 mm mesh sieve, and residues fixed in alcohol 70%. The living fauna was sorted in laboratory under a stereomicroscope, while the residual sediment was dried at room temperature. A first specimen of O. atlantis (Messina University; repository code: BEL145POP'95FRAN1) was found in a crab sample from a steep canyon head belonging to the Gioia Basin system (Longhitano, 2018)

DISCUSSION
The scant information available on O. atlantis mostly refers only on a generic geographic location. For instance, general information about depth and geographical coordinates has been provided for holotype and paratypes, with the indication "off Lantana in 83 fathoms" of McGinty collection paratype. Nothing is reported about habitat and specimen condition, although the original description, reporting operculum features, suggests that the type at least was collected alive (Clench & Turner, 1952). Other West Atlantic reports, only providing depth and geographical coordinates, concerned empty shells from Brazil (Andrade et al., 2011). To the east, the statement "The species may be still living in the E. Atlantic" by Bouchet & Warén (1986) about their record from Azores implies that the specimen was not collected alive (some inaccuracy on the BALGIM 153 position was later clarified in Bouchet & Taviani, 1989). Further records of dead specimens and fragments by Brenke (2002), Segers (2009), de Frias Martins et al. (2009, were occasionally supported by depth and/or geographic position (Table 1).
In the Mediterranean, more information is given by Bouchet & Warén (1986) which hypothesize that their contextualized records from Gibraltar and Alboran pertain to last glacial (Würmian) submarine deposits. Sporadic dead specimens have been reported as associated with continental slope bioconstructions at 500-600 m depth (Smriglio & Mariottini, 1999) and Leptometra community (Gofas et al., 2014; see figure in Rueda et al., 2019), respectively from the Tyrrhenian Sea and the Alboran Sea. Depth and generic indications of "rocky and coralliferous" habitats are given by Manousis et al. (2018), about occasional records of one living specimen and five shells from North Aegean. More important, the occasional record of a living specimen from Malta's continental slope with "fossil" deep water corals (Mifsud, 2009) documents that O. atlantis in the Mediterranean is ectoparasitic on the actinarian Actinauge richardi (Marion, 1882).
Such host-parasite relationship, however, seems limited to the situation observed in the Strait of Messina, where the habitat of A. richardi (Pierdomenico et al., 2019) overlaps that of O. atlantis, whilst this actinarian is unknown in the west Atlantic fauna. A potential implication is that O. atlantis is not strictly species-specific, suggesting that, similarly to other ectoparasites, it is not tied to a peculiar habitat, but could exploit other cnidarians in its geographic range. Nothing however is known about the West Atlantic habitat of O. atlantis, except for the bathymetric range that agrees with the Mediterranean records (Table 1).
Opaliopsis atlantis is also known as fossil in lower circalittoral to bathyal Pleistocene deposits facing the Strait of Messina (Crovato & Taviani, 1985), as well as in nearby north-eastern Sicily (Di Geronimo et al., 2005). As far as concerns the amphi-Atlantic distribution of O. atlantis, first comments are given by Crovato & Taviani (1985), which favor the hypothesis of a Mediterranean settlement by vertical migrating larvae rather than through a step-by-step bottom colonization. Such a hypothesis is consistent with the recognized ability of some tropical gastropod species to ensure an amphi-Atlantic distribution by means of long-living teleplanic larvae. The planktotrophic development, moreover, has been indicated as a functional adaptation in prosobranch gastropods inhabiting transient habitats such as sunken wood and hydrothermal vents, more in general seeming most suited for exploitation of patchy but widespread habitats (Jablonski & Lutz, 1983).
At least two Amphiatlantic-Mediterranean coralliophilinae ectoparasitic on deep water corals, Coralliophila richardi (Fischer, 1882) and Babelomurex sentix (Bayer, 1971), similarly, illustrate the role of the seamounts as stepping-stones in transoceanic dispersal (Oliverio & Gofas, 2006). The physiographic setting of the seamounts, in this respect, plays a fundamental role, due to little sedimentary input and extensive hard bottoms in the bathyal, similarly to the Messina Strait environment (Longhitano, 2018). Regarding C. richardi, Taviani et al. (2009) suggest its status of permanent resident in the western Mediterranean, in accordance with "its prolonged, albeit not necessarily continuous, presence  , 2020; v.60: e20206059 5/7 in this basin since the Early Pleistocene". A situation somehow analogous for O. atlantis, whose presence in the Strait of Messina area is documented since the Pleistocene (Crovato & Taviani, 1985).
The scattered records of O. atlantis, covering a wide bathymetric range, concern different benthic assemblages that include the here reported oyster beds and bathyal stylasterids Errina aspera, a peculiar habitat that the Strait of Messina shares with Gibraltar and, throughout the vicariant E. dabneyi (Pourtalès, 1871), with the East Atlantic Azores islands (Braga-Henriques et al., 2011).

CONCLUSION
The present records in the Strait of Messina confirm further the amphi-Atlantic distribution and basinal-wide occurrence of O. atlantis in the Mediterranean Sea. The report, which after Bouchet & Warén (1986) and Gofas et al. (2014) for Gibraltar and Alboran, is the first to arise from a planned investigation, rather than from occasional records, suggests that this species is not an occasional visitor from the Atlantic Ocean as it has been hypothesized for most deep-sea gastropods (see Bouchet & Taviani, 1989). Opaliopsis atlantis, instead, should be considered a component of the Mediterranean fauna since long (Pleistocene), as testified by its fossil record in this basin.
The planktotrophic larval development in principle permits the distribution of this species over a wide geographic range accounting for its presence in the western Atlantic. The few eastern Atlantic records refer to seamounts underlining their role as stepping-stones for prosobranch amphi-Atlantic dispersal (Leal & Bouchet, 1991;Brenke, 2002), up to include the Mediterranean Sea (Gofas et al., 2014). Ectoparasitic prosobranchs, like O. atlantis, could be able to exploit such corridors adapting to different habitats and communities. This situation might imply a certain plasticity with respect to feeding requirements which could contemplate different hosts.