Egg-laying (Oviparous) Fish
Fish employ a great number of strategies to help ensure that their eggs hatch and the fry survive. Perhaps the simplest is the scattering of eggs and milt (sperm) into the water column, resulting in many eggs being simultaneously fertilized. Fish employing this method of reproduction, such as the freshwater white cloud, generally lay a great number of eggs, because fertilization of all is not guaranteed. Depending on the species, the eggs may float about in the current, or settle onto the substrate or among aquatic plants.
A great many of the eggs produced in this manner are consumed by other fish, and in some cases are consumed by the parents. In many environments, resident fish recognize the courtship behavior of egg-laying species and follow pregnant females in the hopes of acquiring a tasty meal. The enormous number of eggs produced by fish that reproduce in this manner is a strategy adopted to offset such predation.
Some fish take greater care in choosing an egg laying site than do the egg-scatterers. Such species may utilize surfaces such as plants or rocks, or caves and other protected sites. In many cases, one or both of the parents will remain to guard and aerate the eggs, and to keep the clutch free of debris.
One of the more unique strategies employed by egg laying fish is that developed by certain fresh and brackish water killiefish of the order Cyprinodontiformes. Loosely termed “annual killiefish” for their habit of laying eggs at the end of each rainy season, these fish inhabit water bodies that dry out each year. At the end of the rainy season the females deposit their eggs in the mud, after which they and the adult males die. The eggs are drought resistant and remain viable until the pools refill. The fry hatch nearly instantly upon the resumption of the rains and, in many species, are ready to reproduce within one week.
Killiefish, especially males in breeding condition, are quite beautifully colored creatures. Dealers take advantage of the eggs' tenacity by shipping them, in dried form, to hobbyists throughout the world. Killiefish interest groups, whose members exchange eggs by mail, are popular in several countries.
Egg-laying fish are, in some senses, more difficult to accommodate in captivity than are most live-bearers. The eggs are often extremely delicate and require special care. In many cases, the young hatch out at an extremely tiny size, making it difficult for hobbyists to provide them with suitable foods. However, these factors are somewhat offset by the fact that many egg-laying fish take very good care of their eggs, and, in some cases, of the young. In fact, given the proper environment, pairs of many egg-laying fish will do all the hard work for you. Freshwater discus, while by no means easy to maintain, go as far as feeding their young with specially produced mucus. Because the vast majority of fish are egg layers, it is very important that we learn as much as we can about them to encourage captive reproduction and discourage the taking of wild specimens.
A large number of freshwater and marine fish incubate fertilized eggs in their mouths or throats. It many cases the attentive parent, often the female, forgoes feeding until the eggs hatch. The fry of mouth-brooding fish remain in close proximity to the parent and quickly swim back into the mouth when danger threatens. One of the first mouth-brooding fish to be introduced into the aquarium trade was the Tilapia, an African species known to hobbyists in past years as the “Mozambique mouth breeder.” Interestingly, the Tilapia's highly effective reproductive strategy and its resistance to both high water temperatures and low dissolved oxygen levels have today rendered it an extremely important food fish.
For a variety of reasons, certain habitats seem to have given rise to a large number of fish species that utilize mouth-brooding as a reproductive strategy. Particularly well-known in this regard are many cichlids that are endemic to the various African Rift Lakes.
A great many fish are accomplished nest builders, and the resulting structures are a fascinating study in and of themselves. Well-known to European aquarists are the sticklebacks, tiny marine and freshwater relatives of the seahorse that construct intricate nests consisting of plant material.
To attract females, males of several cichlid species native to Africa's Lake Malawi construct sand “castles” greater than 3 feet in diameter at their base, while others evacuate pits up to 10 feet in width. Amazingly, the individual architects are only 6 inches in length. Breeding aggregations, or leks, may contain more than 50,000 nests.
Another well-known nest builder is the beta, or Siamese fighting fish, whose males build floating nests of air bubbles into which the females are induced to lay their eggs. In North America, many people are familiar with the circular pits evacuated in sandy substrates by various species of sunfish. Like many nest builders, male sunfish guard these nests jealously, aerate the eggs with fin movements, and periodically remove fungus and debris from among the clutch.