1. Sexual Characters in Fishes

2. Monomorphism
No external characters to distinguish the sexes
e.g. sardine, seer fish, carangids, etc.
Sexual dimorphism
It is possible to determine the sex from their external body features.
Permanent dimorphism
Distinguished after the onset of sexual maturity.
e.g.: fighter fish (Betta splendeus).
Temporary dimorphism
Distinguished only during the spawning season.
e.g. common carp (Cyprinus carpio)

3. Sexual Polymorphism Sexes distinguished by more than one character.
e.g: Salmon.
Primary characters
Males – testis and ducts
Female – ovaries and ducts - found out by dissecting the fish.
Secondary characters
It have no relation with reproductive process but serve as additional structures for spawning.
e.g: Claspers, Gonopodium, Papillae etc.

4. Body shape
Females heavier and larger - because of the ovaries.
Genital papillae
Small tube in cloacal aperture - distinguishes male from females.
e.g. darters, lampreys etc.
Pearl organ (Nuptial tubercles)
Horny short structures seen on the snout, cheek (head region) only in males.
e.g. common carp, minnows.
Fins
Fins are larger in males than the female and
In males, fins are rough and grainy in nature
e.g. Indian major carps.

5. Coloration
Most male fishes are brightly coloured.
e.g. Parrot fishes
Accessory sexual characters
Modification of anal fin to an organ called gonopodium in males.
e.g. Guppies
Pelvic fins
The pelvic fins are modified into claspers in males.
e.g. Sharks
Female accessory sexual characteristic is seen in the form of egg laying tube or ovipositor.
e.g. Asiatic lump sucker

6. Head characters
Males develop knob like hook and is called leype, seen at the tip of both the jaws.
e.g. Salmons
Size
Deep sea male angler fish parasitic on the body of female.
Sexual dimorphism is least pronounced in case of fishes which don’t exhibit parental care