Endemicity in the Gulf of Gabès: the small mussel drill Ocinebrina hispidula is a distinct species in the Ocinebrina edwardsii complex (Muricidae: Ocenebrinae)

Abstract

The Gulf of Gabes (Tunisia) is a shallow gulf on the North African coast, in the Sicily Channel, characterized by weak currents, low-energy waves and high salinity (37.5–39.25‰) and temperature (13.2–26.58C) (Ktari-Chakroun & Azouz, 1971). It hosts very large meadows of Posidonia oceanica and is known as an important nursery area for demersal resources; nevertheless, studies on the benthic communities are scattered and more are needed (e.g. De Gaillande, 1970; KtariChakroun & Azouz, 1971). The Gabes populations of many animals show morphological features strikingly different from the norm in the rest of their ranges. This is particularly well known in gastropods, to the extent that many such populations have been given full specific rank and are considered to be endemic to the Gulf. In a recent review of the molluscan fauna of the Gulf of Gabes, 33 putative endemics were listed (7% of the total molluscan fauna; Cecalupo, Buzzurro & Mariani, 2008). This enormous number of presumed endemics in a restricted area has raised interest, because of the very young age of the Gulf of Gabes (Sabelli & Taviani, 1981, 2013). During the last glacial maximum (18,000 years before present), sea level reached a minimum of c. 2120 m, and the shallow-water areas of the Gulf of Gabes were completely exposed. It can reasonably be assumed that the Gabes ecosystem, as we know it today, is not older than 6–8 ka, when sea level rose sufficiently (see Morhange & Pirazzoli, 2005 and references therein). Therefore, some possible hypotheses should be tested for each distinct presumed endemic. Is it (1) a distinct species that has originated elsewhere and is now restricted in the Gulf; (2) a distinct species that has originated elsewhere, has colonized the Gulf and is also present elsewhere in the Mediterranean; (3) a distinct species that has originated in the Gulf very recently; or (4) is the Gabes population an ecophenotype characteristic of the extreme conditions in the Gulf? We examined the case of the small mussel drills of the genus Ocinebrina Jousseaume, 1880 (Muricidae: Ocenebrinae), which comprises c. 17–18 currently accepted species in European and West African waters (Houart, 2001; Bonomolo & Buzzuro, 2006; Cecalupo, Buzzurro & Mariani, 2008; Crocetta et al., 2012). (Note that Pacific species sometimes included in Ocinebrina are now ascribed to other genera, including Urosalpinx and Ocenebra). Their taxonomic status has been debated and the use of molecular data is required to stabilize the taxonomy of this group. Some of these taxa are known from single or restricted localities, and are often regarded as narrow-range endemics. Crocetta et al. (2012) recently described a new species in the O. aciculata group, based on subtle morphological features and clear genetic data (COI barcode sequences). Ocinebrina hispidula was originally described (Pallary, 1904: 232, pl. 7, fig. 18) as a variety of O. edwardsii from the Gulf of Gabes. Its status has long been debated, with some authors considering it as a distinct species and others as a mere variety of O. edwardsii. It is diagnosed from O. edwardsii by a narrower shell, a longer siphonal canal and more spiny sculpture with fewer primary spiral cords. It is currently (Houart, 2001) considered as a distinct species with a restricted distribution in the Gulf of Gabes, although shells with a similar morphology have been recorded from other Mediterranean localities (e.g. Cyprus: Houart, 2001; Sicily and Croatia: Giuseppe Bonomolo,