The hydrothermal springs (HTSs) are unique environments characterised by the emergence of geothermally heated groundwater that often releases large amounts of dissolved minerals. Despite interest in HTSs, the microbial composition of these sites remains largely under-explored, particularly concerning the interactions between marine invertebrates and microorganisms. As an example, the shallow HTSs of Trapani (West coast of Sicily, Italy) maintain a constant temperature of 31 °C throughout the year and two invertebrates and are inhabited by the nudibranch Peltodoris atromaculata (P. atromaculata) and the sponge Petrosia ficiformis (P. ficiformis). This study aimed to analyse the bacterial communities associated with the sponge P. ficiformis and the mantle and gut of P. atromaculata. Our results, based on Next-Generation sequencing of the bacterial marker gene (16S rDNA), revealed that the bacterial communities associated with the sponge P. ficiformis and the mantle of P. atromaculata are strikingly similar, suggesting a shared bacterial community, while also exhibiting distinct bacterial differences. However, signatures of bacterial families were found in the gut and mantle of P. atromaculata. These findings shed light on the bacterial composition of the nudibranch P. atromaculata and the sponge P. ficiformis, both considered bioindicators of environmental conditions and highlight the importance of investigating the microbial biodiversity in these peculiar environments to be preserved at every taxonomic level. Additionally, to date, we have provided a brief introduction to understanding the role of host-bacteria interactions in marine invertebrates and these unique environments in shaping entire communities.

The protection of the Vulnerable Marine Ecosystem identified by the presence of the critically endangered bamboo coral Isidella elongata has been recognized as an urgent task. Although some research has focused on its spatial distribution, there is no available information on this species connectivity and genetic structure. This study aims to investigate, for the first time, the genetic diversity pattern and the inter-population connectivity of I. elongata at the Mediterranean scale. A DNA-based approach was used to assess the genetic diversity of 33 I. elongata samples collected in six different areas using two mitochondrial genes (COI and MutS), and a nuclear one (ITS2). Molecular results confirmed that all samples belonged to I. elongata and showed a scarce level of intra- and inter-populational mtDNA differentiation, whereas nuDNA data showed higher genetic structuring. Furthermore, a Lagrangian model (forward-in-time simulations) was used to investigate the species larval connectivity under different sea currents conditions. Our results suggest the presence of persistent pathways, supporting the self-sustaining nature of the populations, especially in the central Mediterranean Sea. This study finding are discussed in light of the implementation of management measures encompassing both the conservation of vulnerable species and the sustainability of deep-water fisheries.