Pajama Cardinalfish

Sphaeramia nematoptera is socially monogamous, meaning that any individual male fish will form a long-term bond with just a single female. The two fish stay close to one another and mate with each other for several breeding cycles, though extra-pair fertilizations have been observed in the species. If one partner dies or is otherwise removed, the widowed fish is likely to form another bond. Both males and females prefer to mate with fish that are of similar size to themselves. Copulation occurs year-round and throughout the day, but is more common at night. While mating, the two fish may become aggressive towards conspecifics. To mate, the female will deposit eggs directly into the water column. Then, the male will quickly fertilize the eggs and scoop them into its mouth. Like other cardinalfish, male Sphaeramia nematoptera are mouthbrooders; holding the fertilized eggs in their mouths for 8–31 days. This practice occurs even if the eggs are not all related to the male (though this is uncommon). In these cases, the male will be carrying an average of 18% of genetically unrelated eggs. Both males and females have been recorded to mate outside their bonded pair, leading to males carrying eggs from multiple partners. In total, only around 70% of all egg clutches will be the result of monogamy with bonded social partners.