Sexual dimorphism in an under-ice spawning fish: the burbot (Lota lota)

Sexual dimorphic characteristics arise in response to differing selective pressures on the sexes and can be used to attract mates or signal reproductive readiness. How sexual dimorphism is expressed where visual cues may be of limited use is an underexplored aspect of reproductive ecology. The burbot (Lota lota (L., 1758)) is a common boreal fish that is not overtly sexually dimorphic. It spawns mid-winter in a light-limited under-ice environment. We examined a variety of morphological and reproductive characteristics in burbot from a northern lake over one full year to assess both seasonal and sex-based variation. Spawning occurred under ice in early February. Seasonal variation was more pronounced in females for many of the traits examined. Growth, fin lengths, swim bladder mass, and liver lipid concentration did not differ between the sexes. Male burbot had significantly higher body condition, larger gas glands, and smaller livers. Males also had significantly larger gonads than females, unusual for boreal fishes. The high gonadal investment of male burbot suggests that sperm competition may be intense in this species. This study demonstrates that sexual dimorphism can be subtle and is present in a seemingly monomorphic species-the burbot.