Mating system in a small pelagic spawner: field case study of the mandarinfish, Synchiropus splendidus

Mating systems in fishes are extremely diverse, ranging from monogamy to multiple polygamous forms, some of which include alternative male mating tactics within species. The major reproductive mode across approximately 33,000 bony fishes is pelagic-spawning with external fertilization. Despite the dominance of this mode, it is relatively little studied in the field because the greater mobility of larger species typically makes individuals difficult to follow for extensive periods, or to catch and mark. However, smaller, more sedentary pelagic spawners provide excellent opportunities to examine mating system variability in response to environmental conditions. The mandarinfish, Synchiropus splendidus, is a small sedentary pelagic spawner which is accessible to divers. Its complex body markings are distinctive and variable allowing individuals to be identified and followed repeatedly without the need for capturing and marking fish. In over 400 h of underwater surveys, we recorded movements, inter- and intra-sexual behaviors, sex ratios, and the timing and duration of spawning activity of focal fish in predetermined study areas. We concluded that the mandarinfish has a polygamous mating system with pair-spawning. Females are sequentially polyandrous, spawning once nightly or not at all, while males are polygynous, spawning up to 8 times a night. Males and females use loosely defined spawning sites on a daily basis and show flexibility in their tactics through differential movements according to dispersion of individuals, distances between spawning sites and temporally limited daily female receptivity. These results reveal the mandarinfish to be an excellent model species for studying small pelagic spawners in the field.