Epizoic diatoms form an important part of micro-epibiota of marine vertebrates such as whales and sea turtles. The present study explores and compares the diversity and biogeography of diatom communities growing on the skin and shell of loggerhead sea turtles (Caretta caretta) from four different localities: Adriatic Sea (Croatia), Ionian Sea (Greece), South Africa and Florida Bay (USA) using both light and scanning electron microscopy. We observed almost 400 diatom taxa belonging to more than 100 genera. Diatom communities from Greece and Croatia showed the highest similarity and were statistically different from those recorded from South Africa and Florida. Part of this variation could be attributed to differences in sampling techniques; however, we believe that geography had an important role. In general, contrary to several previous observations from sea turtles, the presumably exclusively epizoic diatoms contributed less than common benthic taxa to the total diatom flora, which might have been related to the loggerhead feeding behavior. Moreover, skin samples differed from carapace samples in having a distinct diatom composition with a higher proportion of the putative true epizoonts. Our results indicate that epizoic diatom communities differ according to loggerhead geographical location and substrate (skin vs. carapace). The relative abundances of common benthic diatoms and putative exclusive epizoic taxa may inform about sea turtle habitat use or behavior though detailed comparisons among different host species have yet to be performed.

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During a survey of the epizoic diatom flora on carapaces of loggerhead sea turtles (Caretta caretta) from the Adriatic Sea, a relatively small population of an unknown amphoroid species was observed, presenting some morphological similarities with the genera Lunella and Catenula. Due to the small size and the lack of distinct structures visible in LM, the observations of its ultrastructure under very high magnification in the scanning electron microscopy (SEM) proved to be critical in taxon delineation and indicated that it should be separated as a new Catenula species. The new species is characterized by very small, dorsiventral valves, striae composed of 1–3 small, marginally placed areolae, a very simple raphe structure lacking terminal raphe fissures, straight simple central raphe endings, the absence of helictoglossa and a distinct central nodule, the latter often discernible in LM. Typical ghost striae, reported to be one of the main morphological features of the genus Catenula, are very faintly present and only visible in the valve interior. The new species is illustrated using both LM and SEM observations and compared with all other known Catenula and Lunella species.