Dragonets (Pisces: Callionymidae) have long been recognized as exhibiting high degrees of sexual dimorphism. In the century following Darwin's (1896) suggestion that sexual dimorphism in Callionymus lyra has evolved through sexual selection, there has been no published research
on sexual selection in any species of dragonet. The present study provides a detailed description of sexual dimorphism in the spotted dragonet, Diplogrammus pauciradiarus, as well as a description of the behavior patterns involved in courtship, spawning, and agonistic interactions.
Observations in the field and laboratory revealed the context in which sexually dimorphic traits were expressed, thus providing a basis for future research on sexual selection. Like most dragonets, the males of D. pauciradiarus are larger than the females, and the former develop longer
dorsal and caudal fins. Males can also develop black pigmentation on the lower jaw (mouth bar), a black bar across the eye (eye bar) and orange pigmentation on the head, and all three pigment patterns are accentuated (if present) during courtship and agonistic encounters. Males are highly
aggressive toward each other and form dominance hierarchies. During both courtship and agonistic encounters, males perform lateral displays that involve raising the first dorsal fin. Spawning occurs during the last 2 hours of light (dusk) and involves ascending into the water column to release
pelagic eggs. Because the sexually dimorphic traits of the males are involved in both courtship and agonistic behavior patterns, they may have evolved in response to either female mate choice, male-male competition, or a combination of both selective forces. Comparisons are made with other
dragonets, though the behavior of most species remains unstudied.
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