Fish can be classified according to their reproductive strategies—those that lay eggs and those that bear living young.
The fish of this group are differentiated according to size, the type and quantity of their eggs, and the amount of care they provide for their brood. Useful knowledge about eggs. Fish eggs usually are small (between 1.5 and 3 mm on the average) and round. There are also species with egg-shaped, very large eggs (over 7 mm in length) as, for example, most of the mouth-breeding cichlids. Most fish eggs are heavier than water and sink to the bottom. Many are sticky and cling to plants or other structures. Others, such as the eggs of the rainbow fish (Melanotaeniidae) have sticky threads with which they attach themselves. The eggs of many armored catfish (Loricariidae) cling together in a thick clump, which the tending male carries around in his mouth. Many fish species, such as the kissing gourami (Helostoma temmincki) and spiny eels (Mastacemhelidae), lay eggs capable of floating; they contain oil drops that give them buoyancy in the water.
Schooling fish – Many fish, mostly smaller species (for example, many characins and carp) do not pair off in the wild but spawn in schools, often after very long spawning trips to a designated body of water. They lay large numbers of eggs, which either float and drift away, or they sink and attach themselves to plants or to the bottom. Since the female carries a large number of eggs in a limited space, each egg is small and contains little yolk. These fish do not care for their brood. The large number of eggs that they lay ensures that some of the eggs will survive loss by predators. Schooling fish often live in open waters without opportunity for cover (oceans, large freshwater seas, large rivers) where caring for the brood is not feasible.
Territorial fish – Fish species (such as cichlids and labyrinth fish) that live in waters with many hiding places occupy territories, prepare spawning areas, or build nests. In many of these species the males and females pair off for the rearing of a brood and even for the entire reproductive period. In other species one male mates with many females, forming a harem. In some species the males live together in colonies and mate with females that wander from one spawning place to another, having their eggs fertilized by different males. In general, territorial fish care for their brood. Among other things, the egg size depends on the availability of food in the water. Species from waters with abundant microflora and microfauna (algae, bacteria, infusorians, rotifers, and so on) generally have large spawns, in which the individual eggs are just as small and poor in yolk as in free-spawning fish. Other species—especially those from waters with a poor food supply—lay few eggs but these are large eggs that are rich in yolk.
Other Reproductive Strategies
- Some fish entrust the protection and the rearing of their young to other species. The cuckoo catfish (Synodontis petricola), for example, lets its eggs be cared for by mouthbreeding cichlids.
- Other fish can protect fertilized eggs in their bodies until the emergence of the young. Females of some catfish varieties (Aspredinidae) carry the spawn in their stomachs. Mouthbreeding cichlids and labyrinth fish protect the eggs in their mouths.
- The eggs of some egg-laying fish are fertilized internally. Females are first impregnated by the males and then lay their eggs alone. Examples are American Characins of the subfamily Glandulocaudinae and catfish of the family Auchenipteridae.