Compensatory growth and costs of molluscivory in Gambusia holbrooki

Some prey are exceptionally difficult to digest, and yet even
non-specialized animals may consume them—why? Durophagy, the consumption
of hard-shelled prey, is thought to require special adaptations for
crushing or digesting the hard shells to avoid the many potential costs of
this prey type. But many animals lacking specializations nevertheless
include hard-bodied prey in their diets. We describe several non-mutually
exclusive adaptive mechanisms that could explain such a pattern, and point
to optimal foraging and compensatory growth as potentially having
widespread importance in explaining costly-prey consumption. We first
conducted a literature survey to quantify the regularity with which
non-specialized teleost fishes consume hard-shelled prey: stomach-content
data from 325 teleost fish species spanning 82 families (57,233 stomach
samples) demonstrated that non-specialized species comprise approximately
75% of the total species exhibiting durophagy, commonly consuming
hard-shelled prey at low to moderate levels (~10–40% as much as
specialists). We then performed a diet survey to assess the frequency of
molluscivory across the native latitudinal range of a small livebearing
fish, Gambusia holbrooki, lacking durophagy specializations. Molluscivory
was regionally widespread, spanning their entire native latitudinal range
(> 14° latitude). Third, we tested for a higher frequency of
molluscivory under conditions of higher intraspecific resource competition
in Bahamian mosquitofish (Gambusia spp.). Examining over 5,300
individuals, we found that molluscivory was more common in populations
with higher population density, suggesting that food limitation is
important in eliciting molluscivory. Finally, we experimentally tested in
G. holbrooki whether molluscivory reduces growth rate and whether
compensatory growth follows a period of molluscivory. We found that
consumption of hard-shelled gastropods results in significantly reduced
growth rate, but compensatory growth following prior snail consumption can
quickly mitigate growth costs. Our results suggest that the widespread
phenomenon of costly-prey consumption may be partially explained by its
relative benefits when few alternative prey options exist, combined with
compensatory growth that alleviates temporary costs.