Genetic evidence has revealed that sexual fidelity is rather rare in supposedly monogamous animals. Pairing, whether sexually faithful or not, is very uncommon in fish and has not previously been confirmed in fish from seagrass habitats. Here, the first underwater study of seahorse reproduction reveals that males and females of an Australian species (Hippocampus whitei) form pairs that mate repeatedly and exclusively. Partners greet each other daily and eschew interactions with non-partners. Seahorses are unusual in that both sexes provide clear visual evidence of having mated (the male becomes pregnant as the female transfers hydrated eggs) allowing confidence that these fish are sexually faithful to one another. Pairs do not divorce and a pair bond only terminates when one partner disappears. Intra-sexual competition appears not to be important in maintaining pair bonds.
Vincent, A.C.J. & L.M. Sadler (1995). Faithful pair bonds in wild seahorses, Hippocampus whitei. Animal Behaviour 50:1557-1569. https://doi.org/10.1016/0003-3472(95)80011-5