Micromorphy Offers Effective Defence Against Predation: Insights From The Cost-Benefit Analyses Of Micro Gastropod Predation Record

ABSTRACT

Predation, an important driver of natural selection, is studied in the fossil record using quantifiable traces like drill holes produced by gastropods and repair scars produced after durophagous attacks. Despite the abundance of such records in molluscan prey, predation records of micromolluscs (<5mm) remained unexplored. Using a Miocene assemblage of microgastropods from the Quilon Limestone, India, we established the predatory-prey dynamics with the help of cost-benefit analyses. The overall predation intensity is low (DF = 0.06, RF= 0.04) and does not depend on the relative abundance of prey groups suggesting a non-random prey selection regardless of the encounter frequency. The predation is selective in terms of taxonomy, ornamentation, and size of the prey. The smallest size class has the lowest DF and RF supporting a negative size refugia. Higher IDF in larger size class and ornamented groups implies morphological defense resulting in higher failure. Microgastropods show a lower predation intensity than their regular-sized counterparts in a global comparison of coeval records. Results of the cost-benefit analyses explain this difference; the net energy gain from predatory drilling is found to increase monotonically with increasing prey size making the small prey less beneficial. Because the predators try to maximize net energy gain from a predatory attack, the microgastropod prey characterized by relatively low net energy yield is not preferred in the presence of larger prey. Micromorphy, therefore, appears a viable strategy for the prey group to be adopted as an evolutionary response against predation, especially in resource-limited conditions that fail to support large body size.