Single parenting is not easy; just ask toadfish (Porichthys notatus, also known as plainfin midshipman) fathers. These fish have a peculiar reproductive and parenting strategy where mothers leave the nests soon after laying their eggs while fathers adopt one of two different parenting styles. Guarder males care for their eggs, protecting them against marauders, whereas sneaker males fan their sperm into a guarder's nest or slip in stealthily, fertilize the eggs and make a hasty exit, leaving the guarder males to care for the young in the nest. However, males can also provide for their youngsters by producing protective antimicrobial compounds. Caring fish fathers in two other fish species make antimicrobial compounds in glands, known as accessory glands, that are projections of the testes or the sperm duct, and rub the compounds on their eggs. Intriguingly, midshipman guarder fathers collected from the field also had large accessory glands and these glands remained large while they were caring for their nests, raising the question whether midshipman are part of the same antimicrobial parenting club. Meghan Pepler and researchers from McMaster University, Canada, investigated whether midshipmen fathers’ accessory glands make antimicrobial compounds that may increase their offspring's survival.
First, the team had to figure out which type of bacteria live on midshipman eggs. After collecting healthy and unhealthy eggs from beaches in British Columbia, Pepler and colleagues grew the bacteria found on the eggs in the lab. They also collected guarder and sneaker males from British Columbia, removed their accessory glands and collected fluid from the glands with a syringe. To test whether these extracts could halt bacterial growth, the team dispensed the liquid onto the bacteria isolated from the eggs, and then measured bacterial growth.
They found that the extracts from both guarder and sneaker males targeted the bacteria that live on unhealthy eggs but did not affect the bacteria that live on healthy eggs. Additionally, the fluid that came from guarder males was far more potent with stronger antibacterial properties than the fluid that came from the sneaker males. The result is that stay-at-home guarder dads offer their eggs protection from damaging bacteria that could make their offspring sick. However, the protective guard and sneaker fathers’ secretions are harmless for the bacteria that reside on healthy eggs. And, when the team compared the bacteria growing on the healthy and sick eggs, they found that the bacteria were different. It seems that some species of bacteria might be beneficial for the development of healthy eggs, while others are not. The accessory gland fluid from caring fathers could help maintain the healthy bacteria by preventing unhealthy bacteria from growing. And, the guarder fathers’ secretions were so potent that they halted bacterial growth over a range of conditions, from brackish water to high salt and temperatures ranging from 4 to 30°C.
These antimicrobial compounds can certainly help to maintain the developing youngsters’ safety, regardless of the environmental conditions, by keeping infections at bay. The next step is to figure out why some guarder nests still succumb to infections despite the fathers’ protective attentions. Do fathers need to continue applying their protective secretions during the entire 60 day incubation period? And, if dads run out of juice, are their young almost certainly doomed? Regardless, guarder fathers try to give all the eggs in their nests a good start in life, even the cuckoos left behind by devious sneaker dads.