Growth and dynamics of Hemimycale columella

Resumen

The study mainly focuses on the phylogeny, biology, and ecology of Hemimycale columella, a common Atlanto-Mediterranean sponge, and its symbiosis with a calcifying bacterium (calcibacteria), which results in the formation of abundant calcareous spherules. Posteriorly, additional sponge species were incorporated to the study to better understand the H. columella phylogeny and the potential relevance of its particular symbiosis across demosponges. To assess whether some observed differences between populations of H. columella were due to the existence of two morphologically cryptic species, we performed phylogenetic analyses of individuals collected across the Mediterranean Sea with several molecular markers. The results demonstrated the presence of two cryptic species among what was considered to be H. columella. The new species was described as Hemimycale mediterranea. We studied the life history, growth rates, and survival of the two species and characterised simultaneously the environmental variables at the respective species habitats. H. mediterranea had an annual cycle and its growth rates were the highest in summer. H. columella showed a multiannual span with the highest growth rates in cold months. The possible contrasting biological traits of cryptic species highlight the relevance of correct species identification to an adequate management and conservation of benthic ecosystems. Furthermore, the distribution and accumulation of calcibacteria in H. columella tissues was studied by designing a specific probe, performing CARD-FISH experiments, and observing hybridization through epifluorescence and confocal microscopes. Ultrastructure analyses through Scanning and Transmission Electron microscopes helped to localise and understand the symbiosis. The calcibacterium accumulated at the sponge periphery forming a kind of exoskeleton that protected the sponge from potential predators, as proved by field and laboratory experiments. To determine the taxonomic affiliation of the symbiotic bacteria and their geographical and taxonomical distribution across demosponges, we analysed the microbiomes of seven sponge species that harbour calcareous spherules similar to those of H. columella from warm and temperate seas. We demonstrated that calcibacteria were also present in sponge species others than H. columella, besides in two corals and seawater. The wide distribution of this symbiosis reinforces the hypothesis of a purported implication of bacteria in the skeletonization of some Early Metazoa. At least seven calcibacteria OTUs were identified forming a well-supported clade within the SAR116 group in the Alpha-proteobacteria Class. However, any of these OTUs was present in the species H. mediterranea, although it also harboured a huge amount of calcareous spherules, similar to those of H. columella. The calcibacterium of the new species H. mediterranea seems to belong to the Beta-proteobacteria Class, what indicates that other phylogentically apart bacteria living within sponges may also be able to calcify.