Species are more diverse in the tropics than elsewhere in the world. Just think of all the “biodiversity hotspots” that we keep reading about. But there’s at least one major exception to this global rule: parasitic flatworms. According to a recent Ecology study, the farther marine horn snails are from the tropics, the more likely they’ll be attacked (and castrated!) by their parasites.

This decrease in biodiversity from the tropics to the poles is known as the “latitudinal diversity gradient.” Researchers studying this pattern think that it has to do with seasonal stability, complex food webs, relatively fast speciation rates and low extinction rates, or perhaps a combination of a few of these. However, because these factors influence one another, it’s hard to tease apart their effects. Luckily, host-parasite interactions are useful tools for testing these sorts of hypotheses. “Unlike free-living species, parasites must use hosts as their habitats,” Kochi University’s Osamu Miura explains. “Wide-ranging hosts provide a nearly constant habitat for the parasites, regardless of latitude.”

At least 20 different species of trematode flatworms compete to parasitize the closely-related Pacific horn snail (Cerithideopsis californica) and Atlantic horn snail (Cerithideopsis pliculosa). These two are widespread in estuaries all along the coasts of Central America and parts of the U.S. “It lets us do a real, apples-to-apples comparison when the habitat – the snails – is the same across this broad geographic region,” Smithsonian Tropical Research Institute’s Mark Torchin says in a statement. 

Torchin, Miura, and Ryan Hechinger from Scripps Institution of Oceanography collected snails and their parasites from 43 sites across five countries and 27 degrees of latitude in tropical and temperate parts of the Pacific and Atlantic oceans. Contrary to typical patterns, they found that levels of parasitism, flatworm diversity, and competition among rival flatworms increased with higher latitudes – a reversed diversity gradient. The team ruled out speciation rates as a factor affecting their distribution patterns since trematodes spread so quickly up and down the coast. Their offspring leave the snails to infect migratory sea birds, who end up transporting the parasites across vast distances. This means that local ecological factors must be shaping their diversity. 

Hurricanes, runoff from storms, and other sorts of tropical environmental instability might be decreasing the number of snails for flatworms to parasitize. In temperate zones, on the other hand, greater stability and bigger snail populations seem to sustain a higher diversity of these contrarian parasites.