Anatomy of finfish and shellfish

Unit 8 - Urino – Genital System

8.3.Sexual differences
The characteristics of sexual differences or sexual dimorphism that unable identification of the sexes is classified as primary and secondary. Primary sexual characters are those that are concerned actually with the reproductive processes. Testis and their ducts in the male and ovaries and their duct in the female constitute primary sexual characters.
The secondary sexual characters themselves are really of two kinds those which have no primary relationship with the reproductive act at all, and those which are definitely accessory to reproduction. A genital papilla is present in the male fishes of lampreys (Petromyzonidae) darters (Etheostoma nigrum) white bass (Morone chrysops).
“Tubercles” appear on various areas of the body surface in many males during sexual maturity. For example, smelts (Ormerus), some minnows (Cyprinidae). The “fins often” provide characteristic distinctive of males: on an average they are larger than in female. In some fishes, the “caudal fin” may show sexual dimorphism, for example the lower lobe as greatly extended in the males of the sword tail (Xiphopherus helleri) and somewhat enlarged in white sucker (Catastomus commersoni).
Obviously “colouration” in fishes often serves as a mark of sexual distinction and recognition; it is termed sexual dichromatism. In general, the males are brighter or more intense in colour than the females. For example, in orange spotted sunfish (Lepomis humilis) Bowfin (Amia calva), the male has a darker eye spot on the tail region. Other example are Wrasses (Labridae)and parrot fishes (Scaridae). Several different head characteristics also serve to distinguish the sexes among fishes for example, In Salmons and Trouts (Salmonidae) the breeding makes typically develop a knobby hook or kype near the tips of both the upper and lower jaw.

An accessory sexual characters marks the males of several species in which the anal fin becomes enlarged into an “intermittent copulatory organ”. This organ designated as the “gonopodium”, occurs in such fishes as the mosquito fish (Gambusia affinis) the guppy (Lebistes) and other males of live bearing top minnows (Poecilidae).
Pelvis fins are variously modified in the sharks and their relatives (Elasmobranchi) as intermittent structures, the “myxopterygia” (Claspers) that help to ensure internal fertilization, which is widespread in this group of fishes. A few accessory reproductive structures among females serve as sexual characteristics. An outstanding example is the egg laying tube or ovipositor in the female of the European betterlings – Rhodeus amarus.