Aquatic Animal Reproductions

By Bryan Low

Reproductive behavior of fish is varied. Although in most cases, insemination takes place outside the body of the female fish where young emerge from the eggs as larvae, there are plenty of species in which fertilization is internal and females give birth to live young. Some fish spawn regularly, often once a year. However, others can reproduce only once in life and usually die shortly thereafter. The time of reproduction is controlled by external factors (including temperature changes, the level of light and day length) or internal cycles (such as changes in hormone levels).

Some fish prefer spawning and hatching eggs and releasing their young in places where their survival is the biggest. In order to come to such areas, some of them have to migrate thousands of kilometers.

Some species, both males and females, gather in shallow water to spawn without any form of courtship. However, some fish conduct complex courtship rituals to increase their chances of attracting a suitable partner. Before breeding the male fish changes color. Thus, the color of the male Callionymus lyra becomes more intense, which he shows to a possible partner in elaborate circular motion.

With most fish (as in many other aquatic animals) fertilization is external. The male sperm overflows (milt) over the eggs (spawn) which are released through the female's body. The milt is very thick, which prevents fast dispersing of sperm. The eggs of most sea fish floats thanks to the oil droplets and freely float in the water as part of the plankton. Eggs of freshwater fish are generally heavier and have a sticky surface, so they cling to objects in the water. Most freshwater fish build nests for their eggs, and some species protect and take care of their eggs. In almost all cases, the larvae emerges from the eggs which are not fully developed, and gradually develop a skeleton, fins and some organic systems.

External fertilization is a viable method of fish and other aquatic animal reproductions, partly because higher density makes the water a more suitable medium for the transfer of eggs and sperm then the air. In addition, the water provides nutrients and dissolves oxygen for the eggs that are developing. However, the odds of the embryo surviving to adulthood are quite small, especially in marine fish. In order to compensate, females often produce a large number of eggs (up to 5 million in some species). In a small number of species breeding begins with internal fertilization. For example, in sharks the ventral fins are changed into the copulatory male organ which the male uses to enter the females cloaca. The sea water washes the male sperm along the grooves on the body, and then the female's body. When fertilization is internal, the young that are released from the female's body are alive. In some species that give birth to alive young, the eggs develop easily within the mother's body. For others, including some species of sharks and rays, between the embryo that develops and the females there is a link through which nutrients are transferred to the fetus.

Females that carry alive young are investing considerable amounts of energy to carry and care for their young. However, compared with the fish that reproduce with external fertilization, at birth their young are more developed and have higher chances of survival. This means that the viviparous fish can produce and fertilize fewer eggs, and still maintain a stable population.

Compared with other groups of vertebrates, fish include a relatively large number of hermaphrodites, all among the scombriformes. Some fish, including certain species of carp, reproduce parthenogenetically, so that the eggs are fertilized without the males. Other types are reproducing with the so called ginogenesis. This includes types of Poecilia formosa, which is a hybrid of two other species, and its population consists only of females. Reproduction occurs when a male impregnates a female of any of the two parent species.


The male seahorse has an unusual role in caring the young. The female places her eggs in a bag in front of the male abdomen, where they are fertilized. The young are released when they hatch, 2-6 weeks later.

Preserving eggs

Several hundred cichlids species carries the eggs in cavities of the mouth and throat. When the young hatch, the parents also defend them from predators such as to allow them to hide in the shelter of their mouth.


Aquatic Animal Reproductions :: Aquaculture Probiotics for Fish :: Spawning Hormones :: Biosecurity Disinfectants :: Pond Aquaculture Water Management