The mating systems of four Caribbean jawfish species were studied in natural populations. The type of parental care in this family of fishes, male mouth brooding, limits male mating success to some extent in all four species with the result that polygyny is constrained or prohibited.
Monogamy is apparently the predominant mating system in Opistognathus maxillosus and Opistognathus aurifrons, and it occurs irregularly in Opistognathus macrognathus. Mouthbrooding precludes caring for the eggs of more than one female at a time and may be associated with
other costs due to reduced feeding rates of parental males. Mouthbrooding may also strengthen other factors, such as spawning synchrony by females and a relatively long development time of embryos, in constraining polygyny or stabilizing monogamy. In the non-monogamous species, Opistognathus
whitehursti, in which spawning is not synchronous, the long development time of the embryos limits the number of clutches that can be sequentially brooded by males; thus mouthbrooding limits male mating success in this species as well. All four species exhibit sexual dimorphism for larger
mouths in males. Larger male mouths may be important because enhanced ventilation of embryos may result from a larger brood space. The fact that development time is more rapid in O. macrognathus, which has the highest ratio of male mouth volume to clutch volume, suggests that larger
male mouths may lead to more efficient ventilation of embryos and more rapid development.
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