STOWAWAYS ON SHIPS THAT MIGHT PASS IN THE NIGHT: PHANEROZOIC TRENDS IN PARASITISM OF BIVALVES AND BRACHIOPODS

Brachiopods and bivalves, as dominant benthic groups occupying similar niches during the Paleozoic and post-Paleozoic, respectively, have been a reliable system for studying how biotic interactions may or may not have scaled up to influence macroevolutionary trends. They have been cited as a classic case of competitive displacement, described as “ships passing in the night” with distinct diversity trajectories, and yet other studies have demonstrated more complex relationships between origination and extinctions rates and between metabolic rates. Parasites, especially those with complex life cycles, are prone to extinction due to their reliance multiple taxa for the completion of their lifecycles. How then have parasites of brachiopods and bivalves fared through time? We have recorded species-occurrence observations of parasite-host interactions among marine animals as reported in the peer-reviewed literature. The data table is currently comprised of 1,380 observations, of which, we interpret 1,019 represent parasite-host interactions. 303 observations have prevalence values based upon a sample size >/= 10. There are 14 occurrences of brachiopods and 155 occurrences of bivalves with prevalence data. Median prevalence is significantly higher in the Cenozoic than in either the Mesozoic or Paleozoic for pooled taxa in the database, however temporal trends vary by host taxon. Brachiopods exhibit higher prevalence values in the Paleozoic than during the post-Paleozoic, though the sample size is currently quite small. Conversely, bivalves exhibit lower prevalence values of parasites during the Paleozoic (with a small number of occurrences, though some with substantial sample sizes) and higher median and a broader range of prevalence values in the post-Paleozoic. These results, though preliminary and with multiple caveats, suggest that parasite prevalence was higher among the more diverse and abundant host group through time. We encourage neo- and paleontologists to report instances of shell malformations that might be due to parasitism along with sample sizes in order to establish a more complete record of parasite prevalence through time.