Sand goby males trade off between defence against egg predators and sneak intrusions

According to life-history theory, a care-taking parent should balance investment in current and future reproduction in such a way that it maximizes lifetime reproductive success. In the sand goby Pomatoschistus minutus, a small marine fish with paternal care, nest-guarding males may lose current reproductive success to both parasitically fertilizing males and egg predators. Here, we observed sand gobies at a marine and a brackish site, two geographically distant and ecologically different habitats. In a field experiment, we found that sand gobies at the marine site suffered from severe egg predation by netted dogwhelks Nassarius nitidus, which are lacking at the brackish site. Because egg laying takes hours and several females often lay eggs sequentially in one nest, the risk of parasitic spawnings and egg predation overlaps in time during breeding activities. Hypothesizing that egg predators might influence the success of parasitic spawnings, we then simulated these natural conditions in a laboratory experiment with the presence or absence of egg predators, combined with the presence of sneaker males. As expected, in the egg predator treatment, egg-guarding males had to compromise between defence behaviours and thus had less time to devote to defence against sneaker males. Sneaker males took advantage of the situation and approached the nests more actively than in the predator-free treatment. However, the increase in approaches did not result in more successful parasitic fertilizations by sneaker males, as determined using microsatellite DNA. Nevertheless, in nature the adjustment of time budgets by the egg-guarding male are likely to have serious fitness consequences, both if the male fails to defend his paternity and if he fails to defend his offspring.