Zooplankton size structure in the Gulf of Tadjoura (Djibouti) during whale shark sighting: a preliminary study

Zooplankton, collected in the Gulf of Tadjoura (Republic of Djibouti) during a cruise conducted to study the feeding behavior of the whale shark Rhincodon typus, was analyzed in terms of size class, abundance and species composition. This pilot study was aimed to better understand a possible correlation between mesozooplankton composition and size spectra, collected up to 10 m depth, and different feeding behaviour of R. typus. Our data indicate that holoplankton accounted from 93 to 100% of the total community, copepods represented more than 80% of the zooplanktonic community, with 14 copepod genera and 9 species. Our preliminary results suggest an association between suction feeding activity of R. typus and high abundance of small copepod species. Passive feeding was recorded during the lowest abundance of zooplankton, mainly composed by large chaetognaths and larvae of Belbezub hansenii belonging to the family of Luciferidae (Sergestids). These first results highlight the importance of analyzing the dimensional structure of mesozooplankton in the aggregation areas of whale shark and open up new and intriguing investigative approaches to better understand the biology of R. typus.