Not all that clear cut: intraspecific morphological variability inSqualus blainville(Risso, 1827) and implications for identification of the species

Abstract

Of the three species of the genus Squalus that occur in the Mediterranean Sea, S. blainville and S. megalops are very difficult to distinguish. This study assesses the variability in morphological features that have been used to differentiate between these species. Squalus were collected from stations within the 25-nautical mile Fisheries Management Zone around the Maltese Islands; 349 specimens were dissected and categorized into male and female, mature and immature, and individuals were randomly selected from each category to make up a sample of 169 specimens. For each individual, total length and first dorsal fin parameters were measured, and morphology of denticles isolated from the laterodorsal area, of the upper and lower teeth and of the chondrocranium was analysed. The first dorsal spine was shorter that the fin base in 93% of the specimens, which is typical of S. megalops; this character was not related to either gender or maturity. Chondrocrania with one lateral process (typical of S. blainville) and two lateral processes (typical of S. megalops) were present. Teeth from the same individuals showed morphological features that overlap between S. blainville and S. megalops. Both unicuspid (typical of S. megalops) and tricuspid denticles (typical of S. blainville) were observed on the same individuals. Twelve specimens (six having one and six having two lateral chondrocranial processes) were analysed genetically by sequencing of the mtDNA marker Cytochrome Oxidase Subunit I (COI). All resulted to be S. blainville showing that intraspecific variability in supposedly diagnostic morphological features is large enough to render these unreliable to tell apart these two species, especially in the field.

Investigation on the genus Squalus in the Sardinian waters (Central-Western Mediterranean) with implications on its management

“…Both present and previous genetic data confirm that this taxon is widely distributed in the Mediterranean (Serena, 2005;Bat et al, 2005;Serena et al, 2009;Landi et al, 2014;Bonello et al, 2016;Kousteni et al, 2016;Cariani et al, 2017;Veríssimo et al, 2017).…”

“…Moreover, it is reported that some common diagnostic morphological features, such as dermal denticles, teeth and dorsal fin spines could vary in shape with the onthogenetic development (White et al, 2013;Veríssimo et al, 2014). Consequently, the dermal denticles morphology should be further investigated before it can be properly used as a suitable classification tool, as also suggested by Bonello et al (2016) particularly for the genus Squalus.…”

“…These findings have been confirmed by Marouanì et al (2012) in the Gulf of Gabès (southern Tunisia, central western Mediterranean Sea) through morphometric, meristic and genetic analyses, suggesting that S. megalops could be even more common than S. blainville in these waters. On the other hand, in a recent study, S. blainville was the only Squalus species identified in the Maltese waters (Bonello et al, 2016). Indeed, the authors asserted that the species identification based only on morphological characteristics can easily lead to taxonomic misidentifications, especially when multiple anatomical characters (e.g.…”

“…Despite the fact that S. acanthias shows diagnostic characters, such as the presence of white spots on the back or narrowly round to acutely angular rear tips and inner margins of the pectoral fins, which permit an easier identification and discrimination from the other two species (Bonello et al, 2016), S. blainville and S. megalops, do show a very similar morphology. According to Muñoz-Chápuli et al (1984) and Muñoz-Chápuli & Ramos (1989), S. blainville and S. megalops can be discriminated principally based on the number of chondrocranial lateral processes, in addition to other morphological features such as teeth and dermic denticles morphology.…”

“…Indeed, the authors asserted that the species identification based only on morphological characteristics can easily lead to taxonomic misidentifications, especially when multiple anatomical characters (e.g. skull and teeth morphology) are used (Bonello et al, 2016). Moreover, Veríssimo et al (2017) reported that S. blainville and S. megalops are two names used almost interchangeably along the Eastern Atlantic and the Mediterranean Sea to identify the same species with the former mostly employed in the Mediterranean area while the latter in the Eastern Atlantic.…”