Exploring the Past, Present, and Future of Predator-Prey Interactions Between Crabs and Their Gastropod Prey

Abstract

Interactions between predators and prey play an important role in structuring their communities and shaping evolution. However, human-induced climate change has the potential to influence both predators and prey and disrupt their interactions. The fossil record provides an enormous resource to investigate how both past and current climate change has affected organisms, their interactions, and ecosystems. In particular, scars left on prey by failed predatory attacks provide an excellent record, and often the only evidence, of predator-prey interactions in both modern and fossil ecosystems. However, as these injuries, known as repair scars, are records of failed rather than successful attacks (with successful attacks destroying the prey), it can be difficult to interpret whether repair scars signal overall attack rates, or the success/failure rate of the predator. Furthermore, the presence of repair scars can be affected by the structural integrity of the prey’s defenses, such as a gastropod shell, as well as prey selection by the predator. Here, shell-crushing crabs and their gastropod prey were used as a model system for exploring potential relationships between prey defenses, prey selection, and repair scars in the past, present, and possible future. Specifically, the goals were: 1) to use modern experiments to understand how prey defenses are affected by ocean acidification, a major by-product of carbon dioxide emissions, 2) to test patterns of prey selection by crabs, and 3) to then examine how patterns of repair scars in gastropods manifest through both space and time. Along the west coast of North America, crabs such as Cancer productus and their gastropod prey are key members of rocky intertidal communities. Exploration of the potential mechanisms by which human activity might disrupt these predator-prey interactions could provide insights to help protect these critical species and their relationships. In Chapter 2, I tested how gastropod shells are affected by both ocean acidification and predation cues in the gastropods Tegula funebralis and Nucella ostrina. After exposure to decreased seawater pH and/or predation cues for six months, both shell growth and strength in T. funebralis was drastically reduced. However only shell strength, and not growth of N. ostrina was impacted by low pH treatments, and not as severely as T. funebralis. Examination of shell composition and microstructure of both species in Chapter 3 used microCT scans, XRD analysis, and SEM imaging, indicated that the loss of shell strength was from dissolution of the outermost shell layer in both species, with the microstructural arrangement of T. funebralis shells likely causing more severe dissolution than observed in N. ostrina. Patterns of crab (C. productus) prey selection between three species of the gastropod Nucella were examined in Chapter 4 to understand how crabs attack and select prey. The experiment revealed that crabs are most likely to attack the first gastropod they detect, with a preference for the largest species most likely driven by their inability to always recognize smaller prey. I then explored patterns of repair scars in T. funebralis both geographically along the modern west coast of Canada and the U.S. (Chapter 5), as well as temporally between the Late Pleistocene and modern of southern California (Chapter 6). By measuring the size at which repair scars occur, I demonstrate that it is possible to distinguish between repair frequency and predator success, allowing more accurate comparisons of predation between samples. Crab predation in the modern also showed strong regional, but not latitudinal, variation along the west coast, with the lowest number of attacks in southern California. Furthermore, comparisons of modern and fossil repair scars in southern California indicated that there are fewer crab attacks today, suggesting that crab populations may already be affected by human activity. By studying how prey defenses, prey selection, and repair scar systems manifest, we can explore how predator-prey relationships have changed both in the past and present, and how they may continue to change due to our current climate crisis.