Towards the identification of the ommastrephid squid paralarvae (Mollusca: Cephalopoda): morphological description of three species and a key to the north-east Atlantic species

Abstract

Oceanic squids of the family Ommastrephidae are an important fishing resource worldwide. Although cumulative knowledge exists on their subadult and adult forms, little is known about their young stages. Their hatchlings are among the smaller cephalopod paralarvae. They are characterized by the fusion of their tentacles into a proboscis and are very difficult to identify to species level, especially in areas where more than one species coexist. Seven species are found in the north-east (NE) Atlantic. In this study, mature oocytes of Illex coindetii, Todarodes sagittatus and Todaropsis eblanae were fertilized in vitro to obtain and describe hatchlings. Full descriptions based on morphometric characters, chromatophore patterns, skin sculpture and the structure of proboscis suckers are provided based on live specimens. This information was combined with previous descriptions of paralarvae, not necessarily based on DNA or known parentage, from four other ommastrephid species distributed in the same area and a dichotomous key was developed for the identification of paralarvae of the NE Atlantic. The most useful taxonomic characters were: the relative size of the lateral and medial suckers of the proboscis, the presence/absence of photophores and the arrangement of pegs on the proboscis suckers. This key was successfully used to identify wild collected rhynchoteuthion paralarvae from the NE Atlantic. Reliable identification of wild paralarvae can foster a better understanding of the population dynamics and life cycles of ommastrephid squids.