Parasite species richness in marine snails increases with latitude in both the Atlantic and Pacific Oceans

Background/Question/Methods

Species richness generally increases from the poles to the equator.  This latitudinal gradient in diversity is a robust and general pattern which extends across a broad range of animal and plant taxa.  Species richness is negatively correlated with latitude and the tropics harbor more species compared to temperate regions.  However, it is often difficult to evaluate species richness in a standardized way within the same habitat type across a broad latitudinal range.   The parasites of two geminate snail species, Cerithideopsis californica (= mazatlanica) and C. pliculosa offer such a possibility.  Cerithideopsis californica and C. pliculosaare sister species which were separated by the rise of the Isthmus of Panama over 3 million years ago.  Both snails have an extremely broad geographic range, extending over 30 degrees of latitude and they share a similar suite of trematode parasites making this an ideal system to test mechanisms that shape patterns of species richness.   We quantified parasitism in over 27,000 snails from 37 locations across 5 countries in North and Central America.

Results/Conclusions

We encountered a total of 17 trematode species across the Pacific and Atlantic.  Rarified species richness was positively associated with latitude in the Pacific (R² = 0.52, P= 0.01) and in the Atlantic (R² = 0.41, P= 0.025).  We show that counter to conventional patterns of species richness, trematode richness in both snail species is positively correlated with latitude, where the tropics are relatively depauperate in trematode species compared to temperate regions.  We consider potential factors which might drive this unusual biogeographical pattern.