Morphological changes and reproductive costs in brooders of two mouthbrooding freshwater fishes

Parental care purportedly promotes individual reproductive success and improves offspring survival, but is thought to come at a cost to the adult carer. Reproductive costs can be physiological such as the resources parents spend on caring but also the morphological changes that are necessary to accomplish parental care activities. This study considers morphological and body condition changes in brooders of two paternal mouthbrooder fishes – which incubate eggs and guard young inside the buccal cavity – mouth almighty (Glossamia aprion) and blue catfish (Neoarius graeffei), from rivers of northern Australia. Potential costs associated with mouthbrooding were examined by quantifying body condition, feeding deterrence and changes in gut length and gill form across individuals from varying sex-breeding state category (brooder males, nonbrooder males and females). Brooders of both species exhibited significantly lower values in some condition indices, and did not feed, suggesting that body reserves were being used and not renewed during mouthbrooding. Brooders of both species had shorter gut lengths than nonbrooders, potentially to reduce tissue maintenance costs during a time of restricted food intake and requirements. Both species also exhibited changes in gill form and structure compared to nonbrooders; indicating structural adaptive plasticity to mouthbrooding by allowing more space for egg brooding in the buccal cavity. For G. aprion and N. graeffei, caring for progeny seemingly required similar trade-offs including nonphysiological activities (e.g. feeding) and responses, such as changes in condition, respiratory and digestive morphology, that potentially allowed brooders to compensate for parental care energy expenditure.