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During a survey of the diatom flora attached to carapaces of loggerhead sea turtles (Caretta caretta) from the Adriatic Sea, a large population of an unknown marine achnanthoid species was observed that could not be assigned to any of the currently described monoraphid diatom genera. Detailed morphological analysis based on scanning electron microscopy observations and comparison with a large number of monoraphid genera such as Madinithidium, Scalariella, Kolbesia and Karayevia, resulted in the description of the new benthic marine genus Majewskaea gen. nov. with the newly described Majewskaea istriaca sp. nov. as generitype. The new genus is characterized by a sternum valve showing a clear recurving ridge surrounding the entire valve margin, the presence of macroareolae closed internally by perforated hymenes, separated by a longitudinal hyaline area, a well-developed sternum and vestigial raphe slits. The raphe valve has a simple straight raphe with bent terminal fissures, a very fine striation pattern composed of macroareolae cut in two by a longitudinal hyaline zone, and a very shallow mantle. The new genus is illustrated using both LM and SEM observations and compared with other resembling marine and brackish Planothidium monoraphid species.

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.

In recent years, there is a growing scientific interest in epibiotic communities on sea turtles. A broad array of animals such as crustaceans, annelids, molluscs and macroalgae are known to thrive successfully as macro- epibionts on the sea turtle body surfaces. Together with these macroscopic organisms, sea turtles also host unique and diverse micro- epibiotic diatom communities composed of a large number of genera, several of which that have been described as new to science in the past five years. Typical examples include the genera Chelonicola, Medlinella and Poulinea, known exclusively from the carapaces and skin of different marine turtle species (Majewska et al. 2015, Frankovich et al. 2016). During a survey of the diatom flora associated with the carapaces of loggerhead sea turtles (Caretta caretta) from the Adriatic Sea, a fairly diverse diatom assemblage was observed composed of several Diploneis, Fallacia and Amphora taxa together with three unknown diatom taxa that initially could not be identified even on genus level. For at least two of them, further analysis revealed that they most likely represent new genera whereas a third one could be assigned to the genus Catenula. A first unknown taxon belongs to the monoraphid diatoms based on the presence of both a raphe and rapheless valve. The rapheless valve is characterized by the presence of a large silica crest surrounding the entire valve and covering part of the valve margin. The striae consist of two large areolae, separated by a broad hyaline plate and covered externally by porous hymenes. The raphe is rather simple with bent terminal fissures and straight, simple central endings. Comparison with achnanthoid genera such as Scalariella, Kolbesia and Madinithidium yielded both clear similarities but also distinct differences (Desrosiers et al. 2014, Riaux- Gobin et al.2012). The second unknown taxon shows similarities to the members of the genera Nitzschia, Rhopalodia and Psammodictyon based on the presence of an eccentric raphe, distinct fibulae and a dorsiventral valve outline. Finally, the third taxon is most likely a new Catenula species, a small amphoroid genus with only marginal striae, a very simple raphe structure and non-perforated, narrow girdle bands. These observations show the special nature of the epibiotic flora found on loggerheads and confirm our lack of taxonomic knowledge increasing the importance of the currently ongoing research investigating microepibiotic assemblages associated with sea turtles.

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.

A new Lucanicum species, L. ashworthianum Majewska, K.Robert & Van de Vijver sp. nov., was found in multiple samples of biofilm growing on loggerhead sea turtles Caretta caretta and their symbiotic barnacles Chelonibia testudinaria collected on the beaches of Kosi Bay, eastern coast of South Africa, during the two nesting seasons (the austral summers of 2017/2018 and 2018/2019). The new species possesses several features typical of the genus Lucanicum, including the uniseriate striae composed of one large macroareola, the presence of large apical slit fields and transapically elongated rimoportulae at both poles, and a cingulum composed of numerous open, doubly perforated copulae. It differs from the only other Lucanicum species known to date, the coral-associated L. concatenatum, in having considerably smaller cells with clearly rostrate or sub-capitate apices and internal rimoportula opening composed of a C-shaped inner lip and an oval, flap-like outer lip - a feature not observed previously in Lucanicum. Based on these new observations, an amendment of the genus description is proposed.