REPAIR FREQUENCY AND PREDATION RISK IN INTERTIDAL GASTROPOD POPULATIONS

Durophagous predation has influenced gastropod evolution, and assessing predation risk in modern environments is key to interpreting predation in fossil communities. Shell repair frequency has been used to assess predation on mollusks in the fossil record, but previous research has suggested that repair frequency may vary with environment. We examined modern gastropod populations in the Pacific Northwest to compare repair frequencies among species under different environmental conditions to see if repair frequency varies predictably with environment.

At each locality, up to five gastropod species representing a range of morphologies (Tegula funebralis, Nucella lamellosa, N. ostrina, N. canaliculata, and Ocenebrina sp.) were censused for morphometric data and repairs. Using multiple regression analysis, the number of repairs per individual was compared with locality, wave exposure, species identity, shell height, and shell thickness.

We predicted repair frequencies (R%) to be lower at exposed sites. Exposed habitats may be inhospitable to predators, such as cancrid crabs, reducing attack frequency. At protected localities, more frequent encounters with predators may result in higher attack rates and consequently higher R%. Repair frequency may also be higher if gastropods increase defense (shell thickness, ornament) in the presence of predators, thus reducing the success rate of attacks. However, R% may be lower in protected areas if 1) favorable conditions increase predators’ success frequency and fewer prey survive to repair, or 2) gastropods improve defenses to a point where predators switch to easier prey.

Repair frequencies were significantly higher in protected sites than in exposed sites; this difference holds for both high spired and globose morphologies. In the MRA, the combination of the variables locality+species+height explained the greatest variation (r² = 0.30, p < 0.001). Because locality itself correlates with variables such as wave exposure and species composition, the MRA was run again with locality omitted, producing an r² = 0.22 for exposed vs. protected+species+height. These results suggest that repair frequency varies predictably with environment, due to variation in predator presence, gastropod size, and differences in species present in different energy conditions.