الفهرس 
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Abstract
Nudibranchs are colorful marine invertebrates having a diverse group of understudied animals. Recently, some nudibranch members have acquired some attention while others still have not. chromodoris quadricolor is a member of the Red Sea nudibranch, which did not have the chance to get significant attention. In the present study, we were concerned about its mantle-associated bacterial communities. Being essential partners of this nudibranch system, we investigated the bacterial communities’ taxonomic and functional profiles using a whole metagenomic shotgun approach after a differential pelleting procedure. In this procedure, we separated most of the prokaryotic cells from the eukaryotic host cells.
Our findings showed that the mantle-body part holds a diverse group of bacterial species relating mainly to Proteobacteria and Tenericutes phyla. Some species were not previously recorded as bacterial symbionts with nudibranchs. Those members were Bathymodiolus brooksi thiotrophic gill symbiont (23.2%), Mycoplasma marinum (7.4%), Mycoplasma todarodis (5%), and Solemya velum gill symbiont (2.6%). The presence of these bacterial species assumed a nutritional role to the host. However, some of these species were present in a high abundance, suggesting their important symbiosis with chromodoris quadricolor.
Exploring the bacterial ability to produce valuable products resulted in the prediction of diverse classes of biosynthetic gene clusters (BGCs). Polyketide BGC class was the most represented one, with mainly antibacterial activity. Other important secondary metabolites were also detected. This suggested the significant contribution of these bacterial symbionts to protect the nudibranch host against predators and pathogens. Globally, it is the first detailed study concerned with both the taxonomic diversity and functional potentials of the bacterial symbionts associated with chromodoris quadricolor mantle.
Key words
Chromodoris quadricolor; nudibranch; metagenome shotgun; bacterial symbionts; biosynthetic gene clusters.