Giant water bug (Belostoma sp.) predation on a cave fish (Poecilia mexicana): effects of female body size and gestational state

Abstract

Background: Predation is an important driver of life-history trait evolution. Fish predators may prey selectively on certain prey size classes, or pregnant females, particularly those of livebearing prey species. This selectivity ought to affect the evolutionary trajectory of the prey population. In sulphidic and hypoxic habitats, Atlantic mollies ( Poecilia mexicana ) have to spend considerable time engaging in aquatic surface respiration, exposing themselves to predation by giant water bugs ( Belostoma sp.). Compared with other females, pregnant P. mexicana experience greater oxygen demands leading to more aquatic surface respiration. Questions: Are pregnant P. mexicana females more likely to be captured by Belostoma as a result of more aquatic surface respiration and decreased flight abilities (i.e. slower fast–start responses)? Organisms and location: (1) Atlantic mollies ( P. mexicana : Poeciliidae, Teleostei) inhabiting a sulphidic cave (Cueva del Azufre) in Tabasco, Mexico. (2) A co-existing sit-and-wait predator, the giant water bug Belostoma sp. (Belostomatidae, Hemiptera), which catches surfacing fish at the water’s edge. Methods: Predation experiments inside the Cueva del Azufre. In Experiment 1, one randomly selected cave molly female was placed into a perforated bottle with one individual water bug. In Experiment 2, two size-matched females (one pregnant and one non-pregnant) were placed into a perforated bottle with one individual water bug. Results: Capture rates after 24 h in Experiment 1 were correlated mainly with female body size. But larger females were also more likely to be pregnant, making it difficult to disentangle the effects of size and pregnancy. Experiment 2 isolated the effect of pregnancy, and water bugs clearly preferred pregnant over non-pregnant prey.