Hypoxia increases the risk of egg predation in a nest-guarding fish

Abstract

For fish with parental care, a nest should meet both the oxygenation needs of the eggs and help protect them against predators. While a small nest opening facilitates the latter, it impedes the former and vice versa. We investigated how the presence of potential egg predators in the form of shore crabs Carcinus maenas affects nest building, egg fanning, defensive displays and filial cannibalism of egg-guarding male sand gobies Pomatoschistus minutus under two levels of dissolved oxygen. In the high oxygen treatment, males retained their nest opening size in the presence of crabs, while males in low oxygen built large nest openings both in the absence and presence of crabs, despite the fact that crabs were more likely to successfully intrude into nests with large entrances. Males in low oxygen also fanned more. In the presence of crabs males increased their defensive displays, but while males in high oxygen reduced fanning, males in low oxygen did not. Filial cannibalism was unaffected by treatment. Sand gobies thus prioritize egg ventilation over the protection afforded by small nest openings under hypoxia and adopt defensive behaviour to avert predator attention, even though this does not fully offset the threat from the egg predators.

Parent–offspring cannibalism throughout the animal kingdom: a review of adaptive hypotheses

“…A number of empirical and theoretical studies based on fishes suggest that partial brood cannibalism can reduce offspring density and improve the survival of the remaining offspring when oxygen conditions are low [e.g. male beaugregory damselfish (Payne et al, 2002(Payne et al, , 2004; but see Olsson et al, 2016)]. When experimenters artificially simulated partial cannibalism by haphazardly removing eggs from broods to reduce density, survival of the remaining offspring was improved in sand gobies, raising the possibility that partial cannibalism alleviates density effects in this system (Klug et al, 2006).…”

Male and female preferences for nest characteristics under paternal care

“…For example, preference for constantly narrow nest openings might raise the risk of inadequate levels of oxygen being provided to the developing eggs. Goby males have previously prioritized successful nest ventilation regardless of the presence of predators (Jones & Reynolds, ; Lissåker & Kvarnemo, ; Olsson et al, ), suggesting a possible conflict of interests with nest protection being a priority for females but easy maintenance for males. Males might therefore build a narrow entrance to impress females, but widen the nest entrance after the female has spawned.…”

“…Narrow nest entrances and high sand cover both provide protection from predators through camouflage and increased defensive structure, but narrow entrance additionally challenges successful egg ventilation. Smaller entrances require more effort for displacement fanning to maintain high oxygen levels (Jones & Reynolds, ), so the nest holders may sometimes have to trade‐off ventilation against nest protection (Lissåker & Kvarnemo, ; Olsson, Kvarnemo, Andren, & Larsson, ). Indeed, previous studies differentiating nest cover from nest entrance width revealed females to prefer high sand cover but not narrow entrance (Svensson & Kvarnemo, ).…”

“…Indeed, previous studies differentiating nest cover from nest entrance width revealed females to prefer high sand cover but not narrow entrance (Svensson & Kvarnemo, ). Moreover, entrance width was affected more strongly than cover height by oxygen levels (Jones & Reynolds, ; Olsson et al, ) and water temperature (Olsson et al, ). The interests between the sexes might also differ: Females may prefer well‐built nests that better ensure offspring survival and indicate male quality (high sand cover and narrow entrance), whereas males may focus more on making sure that egg ventilation is not too costly.…”

“…Smaller entrances require more effort for displacement fanning to maintain high oxygen levels (Jones & Reynolds, 1999b), so the nest holders may sometimes have to trade-off ventilation against nest protection (Lissåker & Kvarnemo, 2006;Olsson, Kvarnemo, Andren, & Larsson, 2016). Indeed, previous studies differentiating nest cover from nest entrance width revealed females to prefer high sand cover but not narrow entrance (Svensson & Kvarnemo, 2005).…”