Biology of Finfish and Shellfish

4.2.1 Sexual characters in fishes

This is grouped into three categories.

1. Monomorphism

No external characters to distinguish the sexes even when they are sexually matured. This includes most of the pelagic fishes like sardine, seer fish, carangids, etc.

2. Sexual dimorphism

In many species, it is possible to determine the sex from their external body features. This phenomenon of differentiation of male and female sexes by external characters is called “sexual dimorphism”.

a. Permanent dimorphism

b. Temporary dimorphism

In permanent dimorphism, sexes can be distinguished after the onset of sexual maturity including the colour. e.g.: fighter fish (Betta splendeus).

In temporary dimorphism sexes can be distinguished only during spawning season. During other times, sexes cannot be distinguished. e.g.: common carp (Cyprinus carpio)

In respect of species which do not exhibit sexual dimorphism, the separation of sexes mostly rely on internal examination and observation of the gonads.

3. Sexual Polymorphism

In this, both the sexes can be distinguished by more than one character. e.g: Salmon.

Sexual differentiation can be made by observing the gonads only after attaining the maturity. There are two types of characteristic

1. Primary characters: the characters which are actually associated with reproductive process; in case of males – testis and ducts, in case of female – ovaries and ducts. This can be found out by dissecting the fish.
2. Secondary characters : These are more useful because the fish need not be sacrificed or killed. These characters occur in mature fishes. The secondary characteristics which have no relation with reproductive process but serve as additional structures for spawning. e.g: Claspers, Gonopodium, Papillae etc.

However in certain species of finfishes, variations occur in the morphology of fish.

i) Body shape :

Females are heavier and larger in size compared to the males because of the ovaries.

Genital papillae : It is a small tube in cloacal aperture and which distinguishes male from females e.g. darters, lampreys etc.

Pearl organ (Nuptial tubercles): These are horny short structures seen on the snout, cheek (head region) only in males. Once spawning is over nuptial tubercles will disappear. e.g.: common carp, minnows.

ii) Fins : Generally fins are larger in males than the females. In some fishes, pectoral fins can be used to distinguish between males and females. In males, they are rough and grainy in nature (Indian major carps). In some fishes, the caudal fin can be used to distinguish. e.g.: male sword tail has lower lobe much longer.

iii) Coloration: Most male fishes are brightly coloured and more intense when compared to females. This is common in most of the aquarium fishes. e.g. parrot fishes.

In Bow fish (Amia sp.), the juvenile develop a coloured circular spot in their caudal fin of both sexes but when they attain maturity, it disappears in females and it becomes very intense in males.

Accessory sexual characters: It Includes modification of anal fin to an organ called gonopodium (in males) which helps in the transfer of sperms during maturity. e.g.: mosquito fish, guppies.

Pelvic fins: The pelvic fins are modified into claspers in males and serve as claspers in many elasmobranches.

Female accessory sexual characteristic is seen in the form of egg laying tube or ovipositor. e.g.: Asiatic lump sucker.

iv) Head characters: In chimaerids, especially the males develop a spiny stout retractile knob like structures called frontal claspers. This sort of structure is also seen in forehead brooders. In salmons, males develop knob like hook and this is called as leype, seen at the tip of both the jaws.

v) Size : Deep sea male angler fish parasitic on the body of female. Fishes which have parental care, the secondary sexual characters are more pronounced. Sexual dimorphism is least pronounced in case of fishes which don’t exhibit parental care.