Intraspecific morphological variability in less common species of the family Rajidae

Abstract

The rays and skates (family Rajidae) Dipturus oxyrinchus, Raja miraletus, Raja radula, Leucoraja melitensis and Leucoraja circularis are amongst the less commonly caught species that nonetheless occur regularly as by-catch in Maltese fisheries. All five have similar habitat and dietary preferences, reproductive biology, size and overall morphology. These similarities, along with intraspecific variation in colouration and morphology, greatly increase the chance of misidentification, especially in the field. The main aim of this study is to assess the degree of morphological variability in these species collected from Maltese-Sicilian waters. The key morphological characters used in identification manuals commonly used in the field and in other literature sources were compiled, compared and quantified by analyzing 179 specimens of the 5 species obtained from commercial bottom otter trawl landings between August 2018 and June 2019 and from the 2019 ‘Mediterranean International Trawl Survey’ (MEDITS) carried out annually in the General Fisheries Commission for the Mediterranean (GFCM) Geographic Sub-Area (GSA) 15. For each individual, data on external meristic (e.g. ‘number of alar, malar, pre-orbital and post-orbital thorns’), morphometric (e.g. ‘total length’, ‘distance between the eyes’ and ‘eyeball length’) and qualitative characters (including eye spot colouration, colour pattern and shape of the line between tip of the snout to the pectoral fin) was collected. Additionally, fisheries independent MEDITS survey data for GFCM GSA 15 as well as fisheries dependent data from recently published Scientific, Technical and Economic Committee for Fisheries (STECF) reports and associated data tables were obtained and analysed graphically. Group average hierarchical clustering, non-metric MDS ordination, and canonical analysis of principal coordinates (CAP) grouped the majority of the specimens into distinct species clusters corresponding to the five species, with the exception of a few anomalous outlier individuals. The variability in these outliers was not due to age differences or sexual dimorphism, but resulted from a number of morphometric characters (e.g. ‘length of snout’, ‘eyeball length’, ‘pelvic fin measurements’) that overlapped in values. These results suggest that overall there is low risk of misidentification of the species for the majority of the individuals; however, anomalous intermediate specimens do occur. This may have implications for the monitoring and managing of stocks and for conservation management, since observers on-board fishing vessels, at fish markets, or at fishing ports may misidentify such anomalous individuals. Future studies on these and other ‘less common species of Rajidae’ are necessary so as to establish the degree of intraspecific variation and whether this may affect species identification based on guides and manuals.