Larvae of deep-sea invertebrates harbor low-diversity bacterial communities

Microbial symbionts are a common life-history character of marine invertebrates and their developmental stages. Communities of bacteria that associate with the eggs, embryos, and larvae of coastal marine invertebrates are species-specific and correlate with aspects of host biology and ecology. The richness of bacteria associated with the developmental stages of coastal marine invertebrates spans four orders of magnitude from single mutualists to 1,000s of unique taxa. If the developmental stages of coastal species are broadly representative of marine invertebrates then we may expect deep-sea species to associate with bacterial communities that are similar in diversity. To test this, we used amplicon sequencing to profile the bacterial communities of invertebrate larvae from multiple phyla (annelids, molluscs, crustaceans) collected from 2,500 to 3,670 meters depth in near-bottom waters near hydrothermal vents in three different regions of the Pacific Ocean (East Pacific Rise, Mariana Back-Arc, and Pescadero Basin). We find that larvae of deep-sea invertebrates associate with low-diversity bacterial communities (~30 bacterial taxa) with little specificity between larval types. Moreover, the diversity of these communities is an order of magnitude lower (~11.1x) than that of coastal invertebrate larvae. The low diversity, lack of specificity, and abundance of 'environmental' or transient microbes in these communities may imply that these deep-sea invertebrate larvae associate with few bacterial taxa and that deep-sea invertebrate larvae do not have the strong reliance on specialized contributions from their associated bacterial communities observed for many coastal larval species.