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Diatom community structures at locations under the influence of different invasive Caulerpa species ; C. taxifolia and C. cylindracea, were compared in a two-year interval. Samples for taxonomic study were taken from coastal rocks at three stations (Island of Hvar, island of Mljet and Dubrovnik area) at eastern Adriatic Sea from November 2008 to October 2010. Light and electron microscopy examinations were used for qualitative analysis. Taxonomic analysis of diatom communities revealed altogether 388 epilithic taxa belonging in 63 genera including Hyalosynedra, Rhopalodia, Navicula, Cocconeis, Berkeleya, Tabularia, Nitzschia and Grammatophora as the most abundant. The study confirmed differences in species composition and abundance of epilithic diatoms among two Caulerpa sites. While the average number of taxa at C. taxifolia sampling site (island of Hvar) was 77 per sample, the average number of taxa at C. cylindracea sampling sites (Dubrovnik and Mljet) was lower and amounted to 64 per sample. The highest values of Shannon- Wiener Diversity Index were during autumn and were almost the same for all three sampling sites ranging from 5.26 to 5.34. As revealed by MDS ordination, location has more impact on diatom community structure than seasonal changes. ANOSIM test performed on species abundance data confirmed that the difference between diatom communities among different sampling seasons was not significant. The results indicate that the influence of invasive seaweed of genus Caulerpa that change the habitat by competing with autochthonous algae diminishing the structural complexity, richness of species and diversity evident at the macroscopic level have impact on microflora as well. Results of this study presents the first study of the taxonomic composition and distribution of epilithic diatoms from areas of invasive Caulerpa spp., and it contributes to the broader and regional knowledge of diatom diversity in these challenged environments.

This study focused on the diatom species associated with exotic seaweeds on the coast of the Croatian Adriatic Sea. The macroalgae are Caulerpa racemosa (Island of Mljet, Island of Lokrum near Dubrovnik, Southern Adriatic) and C. taxifolia (Bay of Stari Grad, Island of Hvar, Middle Adriatic). Fresh and processed material was studied by means of light (LM) and electron microscopy (SEM and TEM). Our examination of both unprocessed and processed (hydrogen peroxide treated) samples revealed an abundant and species-rich diatom flora. Examination of unprocessed material demonstrated an attached diatom flora, not incorporated into bioaggregates. A total of ca. 150 taxa have been identified. The difference in species composition between the seaweed taxa was significant, as only 26 taxa occurred on both host plants. We have identified 69 diatom taxa, representing 26 genera, associated with C. racemosa and 131 taxa, representing 32 genera, with C. taxifolia. Mode of attachment of the epiphytes differed between the two host algae. On C. racemosa surface-adhered Cocconeis or tube dwelling Mastogloia species were dominant. On C. taxifolia tube-dwelling taxa dominated. In the case of C. racemosa, the most abundant species were Cocconeis spp., while in C. taxifolia, Mastogloia spp. were most abundant. The most species rich genus included Mastogloia with 14 taxa on C. racemosa and 25 taxa on C. taxifolia. A variety of tube patterns of Mastogloia spp. were observed on thalli of both seaweed species. This was independent from preservation state of the thalli (i.e. fresh versus decaying). The abundant taxa are well known as epiphytes. Included in this group are representatives of Berkeleya, Cocconeis, Licmophora, Mastogloia and Navicula.

The framework of the author’s research project includes a description of the epiphytic diatom flora on Caulerpa taxifolia (Vahl) C. Agardh. Material was collected monthly over two years (autumn 2008 – autumn 2010) from an area influenced by Caulerpa taxifolia in the Bay of Stari Grad (Island of Hvar). Light and electron microscopy is expected to provide for the first time information on the general morphology and ultrastructure of species, and enable a determination of the taxonomy of diatoms. Taxa of genus Cocconeis Ehrenberg were the most frequent and abundant epiphytes of the Caulerpa thallus. A new species of Cocconeis, tiny Cocconeis caulerpacola Witkowski, Car et Dobosz was observed as epiphyte with patchy distribution covering the surface of Caulerpa taxifolia. The seasonal dynamics will be described on a fine time scale. Knowledge of the diatom community structure in the areas of Caulerpa spp. is important for studies of the toxic effects of the host.

We present identification and ultrastructural observation on frustular features on Mastogloia taxa found on three locations in south Adriatic Sea: Stari Grad Bay on Island of Hvar, Gonoturska Bay on Island of Mljet, and Orsula near Dubrovnik. Studied samples were taken monthly during 2009 and 2010 at the depth from 5m to 15m. The study is made in the framework of a research project dealing with exotic seaweeds invasion of Caulerpa racemosa and Caulerpa taxifolia in the Croatian part of southern Adriatic Sea. The genus Mastogloia includes a number of species living as epiphytes on seaweeds and seagrasses, together with other biraphid (i.e. Diploneis, Amphora) and monoraphid genera (i.e. Cocconeis, Achnanthes). The genus Mastogloia is highly biodiversified and quantitively significant in benthic communities. The detachment and cleaning of epiphytic diatoms have been obtained by treating macrophytes with acids and then washing with distilled water. Using light and scanning electron microscopy we detail the frustular morphology of different Mastogloia taxa. The frustular morphology within the genus Mastogloia has been observed and described by many authors since 1856 when Thwaites established this genus. The biodiversity of this genus is large with 300 published taxa. In our material more than 25 species of Mastogloia genus have been found, including: M. binotata (Grunow) Cleve, M. fimbriata (Brightwell) Cleve, M. crucicula (Grunow) Cleve, M. ovalis A. Schmidt, M. cuneata (Meister) Simonsen, M. splendida (Gregory) Cleve, M. erythraea Grunow, M. inaequalis Cleve, M. ovulum Hustedt, M. pusilla Grunow, and M. pumila (Grunow) Cleve. Preliminary results of assemblages of benthic diatoms revealed that the genus Mastogloia is very abundant and species-rich at the Adriatic Sea coast.

Rod Mastogloia Thwaites ex W. Smith uključuje brojne vrste koje žive kao epifiti na morskim cvjetnicama i algama, a zajedno s ostalim birafidnim (Diploneis, Amphora) i monorafidnim (Cocconeis, Achnanthes) rodovima dijatomeja doprinose ukupnoj raznolikosti bentoskih zajednica. Istraživanje je sastavni dio projekta o egzotičnim algama roda Caulerpa na istočnoj obali Jadrana. Uzorci su sakupljani mjesečno, tijekom 2009. i 2010. godine, na staništima alga vrsta Caulerpa racemosa (uvala Gonoturska na otoku Mljetu i Orsula kraj Dubrovnika) i Caulerpa taxifolia (Stari Grad na otoku Hvaru). Uzorci su analizirani skenirajućim elektronskim mikroskopom Hitachi S-3500 i S-550 te svjetlosnim mikroskopom Nikon E600 uz povećanje 1000x. Vrste roda Mastogloia imaju promjenjivu strukturu areola. U uzorcima je nađeno više od 25 vrsta, od toga su česte M. binotata (Grunow) Cleve, M. fimbriata (Brightwell) Cleve, M. splendida (Gregory) Cleve, M. crucicula (Grunow) Cleve, M. erythraea Grunow, M. corsicana Grunow, M. inaequalis Cleve, M. cuneata (Meister) Simonsen, M. ovulum Hustedt, M. pusilla Grunow, M. ovata Grunow, M. paradoxa Grunow, M. decipiens Hustedt, M. ignorata Hustedt, M. cyclops Voigt, M. pseudolatecostata Yohn et Gibson i M. pumila (Grunow) Cleve. Rezultati istraživanja doprinos su poznavanju taksonomiji i ekologiji roda Mastogloia u Jadranskom moru.