How reliably can northeast Atlantic sand lances of the genera Ammodytes and Hyperoplus be distinguished? A comparative application of morphological and molecular methods

Accurate stock assessments for each of the dominant species of sand lances in the northeast Atlantic Ocean and adjacent areas are not available due to the lack of a reliable identification procedure; therefore, appropriate measures of fisheries management or conservation of sand lances cannot be implemented. In this study, detailed morphological and molecular features are assessed to discriminate between four species of sand lances belonging to the genera Ammodytes and Hyperoplus. Morphological characters described by earlier authors as useful for identification of the genera are confirmed, and two additional distinguishing characters are added. A combination of the following morphological characters is recommended to distinguish between the genera Hyperoplus and Ammodytes: the protrusibility of the premaxillae, the presence of hooked ends of the prevomer, the number of dermal plicae, and the pectoral-fin length as a percentage of the standard length. The discriminant function analysis revealed that morphometric data are not very useful to distinguish the species of each of the two genera. The following meristic characters improve the separation of Hyperoplus lanceolatus from Hyperoplus immaculatus: the number of lower arch gill rakers, total number of gill rakers, numbers of caudal vertebrae and total vertebrae, and numbers of dorsal-fin and anal-fin rays. It is confirmed that Ammodytes tobianus differs from Ammodytes marinus by its belly scales that are organised in tight chevrons, scales which are present over the musculature at the base of the caudal fin, as well as by the lower numbers of dermal plicae, dorsal-fin rays, and total vertebrae. In contrast to the morphological data, mitochondrial COI sequences (DNA barcodes) failed to separate unambiguously the four investigated species. Ammodytes tobianus and Hyperoplus lanceolatus showed an overlap between intraspecific and interspecific K2P genetic distances and cannot be reliably distinguished using the common DNA barcoding approach. Ammodytes marinus and Hyperoplus immaculatus exhibited gaps between intraspecific and interspecific K2P distances of 2.73 and 3.34% respectively, indicating that their DNA barcodes can be used for species identification. As an alternative, short nuclear Rhodopsin sequences were analysed and one diagnostic character was found for each of the species Ammodytes marinus, Hyperoplus lanceolatus, and Hyperoplus immaculatus. Ammodytes tobianus can be characterised by the lack of species-specific mutations when compared to the other three species. In contrast to COI, the short nuclear sequences represent a useful alternative for rapid species identification whenever an examination of morphological characters is not available.