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Species composition and abundance of diatoms inhabiting thalli of the "killer seaweed", Caulerpa taxifolia, from the Mediterranean coasts of France and Croatia

This study focused on the diatom species associated with exotic seaweed collected on the coasts of the Mediterranean and Croatian Adriatic Sea. The macroalgae is the "killer seaweed" Caulerpa taxifolia and was collected from Saint Raphaël, west of Cannes in France and the Bay of Stari Grad, Island of Hvar, Middle Adriatic, in Croatia. Preserved thalli and processed material (hydrogen peroxide treated) were studied by means of light (LM) and electron microscopy (SEM and TEM). Our examination of both uncleaned and processed samples revealed an abundant and species-rich diatom flora. Examination of uncleaned material under low magnification with the SEM demonstrated strong patchiness in the occurrence of the attached diatom flora. Aside from diatom-free patches, numerous spots with abundant diatoms were observed. Diatom abundances ranged from ca. 2000 to 11500 individuals per mm². In C. taxifolia from France much higher abundances were observed and adnate forms (Cocconeis spp. and Mastogloia spp.) dominated. In processed samples, a total of ca. 330 taxa have been identified. The difference in species composition between the sites was significant. Bray-Curtis analysis of similarity prepared for both sites showed two distinct clusters, though the level of similarity amounted to ca. 50%. This similarity results from the dominance of a small, previously unknown Cocconeis species which accounted for 67% of the diatom abundance in France and 26.5% in Croatia (in LM). In Croatia, the group of taxa with increased abundances included Berkeleya fennica and Berkeleya rutilans. The abundant taxa are well known as epiphytes. Included also in this group are representatives of Berkeleya, Cocconeis, Licmophora, Hyalosynedra, Mastogloia and Navicula. It seems that despite its notorious reputation and the common name "killer seaweed" Caulerpa taxifolia is a suitable host for epiphytic diatoms. Especially interesting is the dominant Cocconeis species, which we are going to describe as new for science. Study of the physiology of this new species is justified since it is able to colonize Caulerpa taxifolia in such great abundance ; the host plant is known to be lethal to other organisms.

Diatoms; Caulerpa taxifolia; Mediterranean Sea; France; Croatia

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