Exploitation of the eggs of nest associates by the host fish Pseudobagrus nudiceps

The Japanese minnow Pungtungia herzi deposits its eggs in the nests of other fish species, including the bagrid catfish Pseudobagrus nudiceps. The minnow generally spawns on days around the spawning day of the host species, probably to ensure egg care by the host. However, Pu. herzi sometimes spawns in the nest during the larvae/juvenile-guarding period of Ps. nudiceps, and in such cases, host young are known to feed on the eggs of their associates. We conducted fieldwork in the Umemoto River, central Japan, to clarify the details of the utilization of Pu. herzi eggs by Ps. nudiceps and its effects on the early life history of the latter. Host young fed on almost all of the Pu. herzi eggs available to them, stayed in nests with more of the associate’s eggs available for longer periods, and grew to larger sizes. Since a larger body size at independence is expected to improve the subsequent survival of the young, associate spawning should provide benefit to the host young. We revealed that nesting host males also exploited Pu. herzi eggs. Considering the unusual timing and low survival rate of Pu. herzi eggs, their spawning during the latter half of the host nesting period possibly suggests that the host has developed specific behavior to induce such associate spawning.