Across 85 studies, a mollusk's species shapes its microbiome more than its habitat does

The margin is small by the lead author's own account. "Environment, diet and geographical variables matter, but DNA and family matter a bit more for these mollusks," said Vergara-Fl贸rez, the graduate student who led the review ^[4][3].

A ranking across species says little about variation inside one. On a farm that raises a single species, species is a constant, so it cannot explain why one tank does better than another. Habitat, diet and other conditions can. The thing this doesn't tell you is how large the habitat effect is within one species.

Vergara-Fl贸rez's own account of the risk is environmental. "If we make a little change in where the animal lives, let's say, if we pollute their environment or increase its temperature, it can create something we call dysbiosis," the researcher said ^[14]. Dysbiosis is a drop in beneficial gut bacteria and a rise in harmful ones, potentially leading to illness ^[12].

Host filtering is the working explanation. Vergara-Fl贸rez said microbes and their hosts have evolved together for a long time, and that under what the group calls "host filtering" the animal takes the microbes it needs from the environment ^[11]. If that holds, host and habitat are not independent variables, because the species decides which local microbes get in. In our view a review of existing studies cannot separate those paths by itself. Experiments that move animals of one species between habitats could.

The press summary does not report an effect size or say whether the 85 studies were pooled statistically. We think the defensible use is narrow: record host species in every mollusk microbiome study and compare like with like. Applying the finding to aquaculture is a further step, and the researchers themselves frame the work as a call for more research to predict how microbiomes respond to environmental change ^[9].