Directional asymmetry in gonad length indicates moray eels (Teleostei, Anguilliformes, Muraenidae) are "right-gonadal"

Abstract

Directional asymmetry indicates a unidirectional deviation from perfect bilateral symmetry, which was rarely examined in the inner organs of the teleost (Teleostei) compared to external traits. This study examines the directional asymmetry in the gonad length of 20 species of moray eels (Muraenidae) and two outgroup species with 2959 individuals. We tested three hypotheses: (1) moray eel species did not exhibit directional asymmetry in the gonad length; (2) the directional asymmetry pattern was the same for all selected species; (3) the directional asymmetry was not related to the major habitat types, depth and size classes, and taxonomic closeness of the species. Moray eels were generally "right-gonadal", the right gonad length being constantly and significantly longer than the left one in all studied Muraenidae species. The degree of asymmetry varied among species and was not significantly related to taxonomic closeness. The habitat types, depth, and size classes had intermingled effects on observed asymmetry without a clear correspondence. The directional asymmetry in the gonad length is a unique and widely occurring phenomenon in the Family Muraenidae, which was likely a by-product in the evolutionary history without significant disadvantage in survival.

Figure 1

(a) A female Gymnothorax minor, collection ID: TOU-AE-4676 and (b) a male collection ID: TOU-AE-4592, showing the measurement of gonad length at both sides. The ovary is enclosed in orange and testis in blue. The left side (L) and right side (R) are defined when the fish is head up and belly down, and therefore, left–right direction appears opposite when the belly is up as shown this figure. Scale bar = 1 mm. (Photo credit: Huang, L.Y.)

Figure 2

Biplot of the correlation-based principal component analysis on the standardized coefficients representing the directional symmetry in gonad length (log(GLR) is the natural log ratio of the gonad length, (a) and (b) is the intercept and slope of the relationship between the gonad length difference and the total length, respectively. Subscript F and M indicate females and males, respectively). The arrows indicate the correlation of variables to the first and second principal component axis (PCA1, 61.8%of total variance explained and PCA2, 23.8%of total variance explained). Numbers indicate the taxa examined (1: G. minor, 2: G. chilospilus, 3: G. eurostus, 4: G. shaoi, 5: G. prionodon, 6: G. fimbriatus, 7: G. hepaticus, 8: G. flavimarginatus, 9: G. kidako, 10: G. pseudothyrsoideus, 11: G. thyrsoideus, 12: G. rueppelliae, 13: G. neglectus, 14: G. margaritophorus, 15: G. pictus, 16: E. polyzona, 17: E. nebulosa, 18: U. macrocephalus, 19: U. micropterus, 20: S. sathete, 21: P. cancrivorus, and 22: D. anguillare). Numbers in blue belong to the Family Muraenidae and numbers in green represent the outgroup.

Figure 3

Heat-map of the standardized directional symmetry coefficients and habitat types (coral/rocky reefs, peddle beach, and seabed of soft materials), and maximum observed depth (shallow ≤ 50 m, medium: 50–100 m, and deep: > 100 m) and size categories (small: < 50 cm, medium (50–100 cm) and long (> 100 cm).