DRILLING PREDATION IN BEACH DRIFT FAUNAS OF PUERTO RICO: SPATIAL VARIABILITY IN SHALLOW MARINE HABITATS OF A SINGLE BIOGEOGRAPHIC REGION

Fossil assemblages and Recent beach drift assemblages are often used as a source of information on the intensity of predation in contemporaneous local communities. Such assemblage-level analyses may be used to interpret spatial (e.g., latitudinal) or temporal (e.g., evolutionary) trends in predation pressure. Very few studies, however, have investigated how sampling methodology, local environmental conditions, or taphonomic processes can influence these records. This study aims to provide an assessment of spatial variation in gastropod predation in a single well-defined biogeographic area - the Caribbean island of Puerto Rico.

Beach drift assemblages were bulk sampled from 16 localities around the coastline of Puerto Rico. Replicate samples were collected from most localities to assess variation in predation at the local scale. In total, more than 40 samples were analyzed, and over 10,000 specimens surveyed for evidence of predatory drilling. In addition, samples were sieved using multiple sieve sizes to permit assessment of drilling intensity across a range of taxon size classes. Mean drilling intensity on bivalve prey across all beach sites was 15.9%, with significant variance between the beaches exhibiting the lowest (1.5%) and highest (53.7%) drilling frequencies. This range and mean drilling intensity is fairly consistent, although slightly lower than has been observed from coastal eastern North America, and somewhat higher than in a similar study from the Caribbean and Pacific coasts of Panama.

Variation in drilling frequency between localities reflects local-scale variations in environment from one beach (shore-face environment) to another. Local environmental variations (e.g., substrate or salinity) may influence the community roster such that it contains abundant taxa favored or unfavored by locally occurring predators (e.g., Cardiidae are bored in 8.7% of individuals; Tellinidae are bored in 38.2% of shells across all localities). Wave-energy and local beach morphology will influence not only the “shelliness” of the beach, but also the amount of taphonomic damage and sorting a beach sample may exhibit. Within-habitat variation, as indicated by differences in drilling intensity between replicate samples, presumably results from differential sorting of shells along each beach.