Rueger, Theresa, Hugo B. Harrison, Naomi M. Gardiner, Michael L. Berumen, and Geoffrey P. Jones. “Extra‐pair mating in a socially monogamous and paternal mouth‐brooding cardinalfish.” Molecular Ecology. 2019. 28-10: 2625-2635

Link to paper: https://onlinelibrary.wiley.com/doi/full/10.1111/mec.15103

In this study, we followed more than 500 Pajama cardinalfish on reefs in Papua New Guinea. Cardinalfish stay close to each other in pairs for long periods of time, often for several years. Looking at the babies they produce with genetic markers, we found that most of them do exclusively breed with that partner. However, contrary to expectations, we also saw some sneaking behaviour. When presented the chance, both males and females take the opportunity to mate with other individuals.

The findings are remarkable because the males brood the eggs in their mouths. They can’t feed during that time and their swimming ability is compromised, so brooding is very costly for them. That puts females in an advantageous position, because they can produce eggs quicker than the male can brood them and they can go and give eggs to another male. The males can offset that advantage by eating some or all of the eggs and accept eggs from another female. Also, in some rare cases males can fertilize eggs that another male is brooding, saving all the energy they would need to use to brood them themselves.

It’s a complicated mating system, which is something we can only find out by spending lots of time observing the fish and using genetic analysis to identify parentage. Our tests reveal the complex nature of social groups in fishes and how promiscuity could upturn theories for how monogamy arose. – Theresa Rueger

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