A preliminary study of the population, ecology and genetic characters of the Mediterranean box jellyfish (Carybdea marsupialis in the island of Malta).

Abstract

The only known Cubozoan species to inhabit the Mediterranean is Carybdea marsupialis (Linnaeus, 1758). Recently, historical reconstruction together with new genetic and morphological data suggests that it is an endemic species to the Mediterranean Sea. The main aim of this project was to identify the factors associated with the increase in numbers of C. marsupialis in Maltese waters. Hence, monitoring of the abiotic/biotic factors and individual abundance of this species was carried out every two weeks between July 2014 and July 2015. The method of choice exploited the photopositive response of Cubozoans to aggregate around artificial lights to carry out quantitative counts against a time period of 30 minutes. Individuals were collected through the deployment of a hand net and local samples were genetically compared to samples obtained from Spain and Tunisia, along with the Atlantic samples from Cadiz, in southwest Spain. DNA was extracted with phenol:chloroform:isoamyl alcohol followed by an ethanol precipitation. Genetic characterisation was carried out through the analysis of the mitochondrial 16S rDNA region and the region between the 18S and 28S of the rDNA. /n vitro experiments to document the development of C. marsupia/is. from the egg stage to that of the primary polyps were also successfully undertaken. Over the whole time period, the Birzebbuga site had an overall total of 590 caught individuals against the 324 from the Msida site. A Mann-Whitney U-Test was run for the Msida and Birzebbuga sites and a statistical difference (p-value<0.05) in the morphometric parameters of individuals from both sites was observed nearly throughout the whole study period. Furthermore, principal component analyses (PCAs) and Spearman's rank correlation coefficient (rs) highlighted the fact that populations of both sites were strongly and positively correlated with sea water temperature and negatively correlated to phytoplankton and chlorophyll levels. MegaBlast database comparison confirmed that the sequence amplified was in fact that of Carybdea sp. Nonetheless, the sample with the highest substitutions per site was that of Cadiz. All Mediterranean samples shared >98% of identical base pair sequence and the homology between them indicates close genetic homology. The clear presence of distinct bases in the Cadiz sample may reflect a number of situations. The specimen may have been misidentified at the stage of collection. If this is so, this study highlights the importance of using genetic and molecular studies in addition to traditional morphometric studies to identify C. marsupialis. Alternatively, although less likely, the specimen may represent more distantly related species of C. marsupialis. In any case, further studies are recommended. Key words: 16S rDNA, 188 and 288 rDNA, abiotic/biotic factors, Carybdea marsupialis, Cubozoa, morphometric parameters, population studies.