Anatomy of finfish and shellfish

Unit 8 - Urino – Genital System

8.1. Introduction
The fish, like most animals, begins life as an egg and as in other invertebrates, the single cell egg can not develop unless it is fertilized by a sperm produced by a male. Fish sperm is most commonly referred to as milt.
Eggs may be fertilized either externally or internally. External fertilization takes place when the egg is penetrated by the sperm after the egg leaves the female’s body. Most fish are reproduced by this system. Internal fertilization occurs when the male introduces the sperm into the female’s body, where it makes contact with and fertilizes the egg. Some sharks are ovoviviparous; that is, the egg is fertilized internally and held within the female without attachment to her until it is ready to be extruded alive. In other species, such as some of the sharks and the sculpin, and the skate, the egg is penetrated by the sperm inside the female’s body, but it does not hatch until some time after being released from the female. Reproduction and associated activities in fish are generally referred to as spawning. The spawning season, or breeding period, is that time when the eggs of the female and the milt, or sperm, of the male are ripe. This period may last only a few days or it may extend into weeks and even months. Fish that live in tropical waters of fairly constant temperature may spawn year round.