Microscopic marine invertebrate microbiomes do not reveal signatures of phylosymbiosis

Animals and microorganisms often establish close ecological relationships.
However, much of our knowledge on animal microbiomes comes from two deeply
studied groups: vertebrates and arthropods. To understand interactions on
a broader scale of diversity, we characterized the bacterial microbiomes
of close to 1,000 microscopic marine invertebrates from 21 phyla, covering
most of the remaining tree of metazoans. Samples were collected from five
temperate and tropical locations covering three marine habitats (sediment,
water column and intertidal macroalgae) and bacterial microbiomes were
characterized using 16S rRNA gene sequencing. Our data show that, despite
their size, these animals harbour bacterial communities that differ from
those in the surrounding environment. Distantly related but coexisting
invertebrates tend to share many of the same bacteria, suggesting that
guilds of microorganisms preferentially associated with animals, but not
tied to any specific host lineage, are the main drivers of the ecological
relationship. Host identity is a minor factor shaping these microbiomes,
which do not show the kind of correlation with host phylogeny, or
“phylosymbiosis”, observed in many large animals. Hence, the current
debate on the varying strength of phylosymbiosis within selected lineages
should be reframed to account for the possibility that such a pattern
might be the exception rather than the rule.