Sexual Dimorphism in Swim Bladder Texture, Composition, and Muscle Nutrient Profile of Commercial-Sized Black-Spotted Croaker (Protonibea diacanthus)

The swim bladder (fish maw) of the blackspotted croaker (Protonibea diacanthus) commands a high price for males, yet the biological basis for this preference has remained unclear. We evaluated sexual dimorphism in commercially sized fish (n = 30 per sex) by comparing body weight, and assessed swim bladder texture and nutrient composition of both swim bladder and dorsal muscle in a subset of four fish per sex. Male swim bladders possess significantly superior textural properties than those of females, particularly in the key parameters of hardness (1804.33 ± 581.05 vs. 728.53 ± 46.81 gf), springiness (0.91 ± 0.03 vs. 0.73 ± 0.09), and chewiness (1161.18 ± 171.86 vs. 374.68 ± 97.72 gf) (all p < 0.05), whereas body weight, swim bladder weight, and total collagen content did not differ. Swim bladder amino acid and fatty acid profiles were largely similar between sexes, except for higher glutamic acid and leucine in males. In contrast, dorsal muscle proximate and nutrient profiles showed no significant differences between sexes, indicating nutritionally equivalent fish meat. These results demonstrate that sex critically affects swim bladder but not muscle quality, highlighting the need for further investigation into the mechanisms underlying these textural differences of swim bladder and explore potential dietary or genetic interventions to modulate swim bladder quality in aquaculture.