Overview
Marine animals, and particularly bony fish, commonly reproduce by broadcast spawning. This is an external method of reproduction where the female releases many unfertilised eggs into the water. At the same time, a male or many males release a lot of sperm into the water which fertilises some of these eggs. The eggs contain a drop of nutrient oil to sustain the embryo as it develops inside the egg case. The oil also provides buoyancy, so the eggs float and drift with the current. The strategy for survival of broadcast spawning is to disperse the fertilised eggs, preferably away from the coast into the relative safety of the open ocean. There the larvae develop as they consume their fat stores, and eventually hatch from the egg capsule into miniature versions of their parents. To survive, they must then become miniature predators themselves, feeding on plankton. Fish eventually encounter others of their own kind (conspecifics), where they form aggregations and learn to school.
Internally, the sexes of most marine animals can be determined by looking at the gonads. For example, male testes of spawning fish are smooth and white and account for up to 12% of the mass of the fish, while female ovaries are granular and orange or yellow, accounting for up to 70% of the fish's mass. Male lampreys, hagfish and salmon discharge their sperm into the body cavity where it is expelled through pores in the abdomen. Male sharks and rays can pass sperm along a duct into a seminal vesicle, where they store it for a while before it is expelled, while teleosts usually employ separate sperm ducts.
Externally, many marine animals, even when spawning, show little sexual dimorphism (difference in body shape or size) or little difference in colouration. Where species are dimorphic, such as sharks or guppies, the males often have penis-like intromittent organs in the form of a modified fin.
A species is semelparous if its individuals spawn only once in their lifetime, and iteroparous if its individuals spawn more than once. The term semelparity comes from the Latin semel, once, and pario, to beget, while iteroparity comes from itero, to repeat, and pario, to beget.
Semelparity is sometimes called "big bang" reproduction, since the single reproductive event of semelparous organisms is usually large and fatal to the spawners. The classic example of a semelparous animal is the Pacific salmon, which lives for many years in the ocean before swimming to the freshwater stream of its birth, spawning, and then dying. Other spawning animals which are semelparous include mayflies, squid, octopus, smelt, capelin and some amphibians. Semelparity is often associated with r-strategists. However, most fish and other spawning animals are iteroparous.
When the internal ovaries or egg masses of fish and certain marine animals are ripe for spawning they are called roe. Roe from certain species, such as shrimp, scallop, crab and sea urchins, are sought as human delicacies in many parts of the world. Caviar is a name for the processed, salted roe of non-fertilized sturgeon. The term soft roe or white roe denotes fish milt. Lobster roe is called coral because it turns bright red when cooked. Roe (reproductive organs) are usually eaten either raw or briefly cooked.
"The reproductive behaviour of fishes is remarkably diversified: they may be oviparous (lay eggs), ovoviparous (retain the eggs in the body until they hatch), or viviparous (have a direct tissue connection with the developing embryos and give birth to live young). All cartilaginous fishes—the elasmobranches (e.g., sharks, rays, and skates)—employ internal fertilization and usually lay large, heavy-shelled eggs or give birth to live young. The most characteristic features of the more primitive bony fishes is the assemblage of polyandrous (many males) breeding aggregations in open water and the absence of parental care..."
There are two main reproduction methods in fish. The first method is by laying eggs and the second by live-bearing (producing their young alive).
- In the first method, the female fish lays eggs either on the sea floor or on the leaves of an aquatic plant. A male fish fertilizes the eggs, and both then work together to protect the eggs/babies from danger until they can defend themselves.
- In the second method, the male fish uses its anal fin to transmit sperm into the female fish and fertilize the fish eggs. Later, the female gives live birth to her fry.
Read more about this topic: Spawn (biology)