Predation on the Tropical Freshwater Schrimp Xiphocaris Elongata: Rostrum Inducibility, Antipredator Responses and Cascade Effects
Predators affect prey indirectly through inducible defenses that are only present when the predator is present, and have a measurable cost. This dissertation is focused on the effects of the predatory fish Agonostomus monticola on the morphology, anti-predator strategies, and ecological role of the amphidromous shrimp Xiphocaris elongata, which exhibits a phenotypic polymorphism (i.e. long or short rostrum) that has been correlated with the presence/absence of fish predators (Covich et al. 2009). I answer four questions: 1) is the long rostrum inducible by A. monticola, 2) is the long rostrum an effective defense mechanism against A. montocola, 3) is the long rostrum costly in terms of downstream displacement and reproduction, and 4) does predator presence create top-down cascade effects? Inducibility of the rostrum by A. monticola was evaluated through exposure experiments to juvenile, long-rostrum adult shrimp, and short-rostrum adult shrimp. The anti-predator strategies of X. elongata against A. monticola were addressed through behavioral and mortality experiments for long-rostrum and short-rostrum shrimp. The cost of the rostrum was assessed through experiments that manipulated the flow in which downstream displacement occurred for long-rostrum and short-rostrum shrimp, and through an egg count of gravid long-rostrum and short-rostrum females. Top-down trophic cascades were examined through experiments in artificial pools where long-rostrum and short-rostrum shrimp could forage or hide in refuge in the presence or absence of predators. The data indicated that: 1) the long rostrum is inducible by the presence of A. monticola, 2) the long rostrum is an effective defense mechanism against fish in terms of choice, rejections, and handling time, 3) the long rostrum is not costly in terms of downstream displacement, but there seems to be a trend that the long rostrum may be costly in terms of reproduction, and 4) predator presence creates trait-mediated top-down cascades effects by reducing the amount of shredding by X. elongata, this decreases decomposition rates of the riparian species Cecropia schreberiana. This study demonstrates that the long rostrum in X. elongata fits the criteria of an inducible defense against predatory fish and that predator presence affects the ecological role of X. elongata.