Abstract. The dichotomy between high microbial abundance (HMA) and low microbial abundance (LMA) sponges has been long recognized. In the present study, 56 sponge species from three geographic regions (greater Caribbean, Mediterranean, Red Sea) were investigated by transmission electron microscopy for the presence of microorganisms in the mesohyl matrix. Additionally, bacterial enumeration by DAPI-counting was performed on a subset of samples. Of the 56 species investigated, 28 were identified as belonging to the HMA and 28 to the LMA category. The sponge orders Agelasida and Verongida consisted exclusively of HMA species, and the Poecilosclerida were composed only of LMA sponges. Other taxa contained both types of microbial associations (e.g., marine Haplosclerida, Homoscleromorpha, Dictyoceratida), and a clear phylogenetic pattern could not be identified. For a few sponge species, an intermediate microbial load was determined, and the microscopy data did not suffice to reliably determine HMA or LMA status. To experimentally determine the HMA or LMA status of a sponge species, we therefore recommend a combination of transmission electron microscopy and 16S rRNA gene sequence data. This study significantly expands previous reports on microbial abundances in sponge tissues and contributes to a better understanding of the HMA-LMA dichotomy in sponge-microbe symbioses.

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The oviparous sponge Ectyoplasia ferox is commonly found in Florida and the Bahamas. Every year in August and/or September about 6 days after a full moon, E. ferox will shed embryo-containing spawning material into the seawater from which hundreds to thousands of larvae will hatch per host individual. In order to investigate vertical microbial transmission in E. ferox, 16S rRNA gene library construction and denaturing gradient gel electrophoresis was employed. Microbial symbionts from six phyla and the unknown lineage SAUL were shown to be vertically transmitted. The identification of 21 VT clusters, of which 19 were situated within sponge-specific or sponge-coral-specific clusters, indicated that a large fraction of the symbiotic microbial consortium was present in the sexual reproductive stages. Spawning led to a 50 % reduction of microbial numbers in the adult sponge mesohyl. We furthermore provide the first evidence that the symbiotic microbial consortia of E. ferox were generally metabolically active within the reproductive stages. Finally, we propose E. ferox as a model system for vertical transmission owing to the ease of experimental access to all sexual reproductive stages, and to experimental tractability in the laboratory including the possibility of rearing symbiont-free juvenile sponges.

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