Reproduction in Osteichthyes

Abstract

Currently, >36,700 fish species are recognized. This is close to 50% of all known vertebrate species. Osteichthyes is a taxonomic group of fishes that includes the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii). Of these, the Teleostei, or the modern bony fishes in the class Actinopterygii are the majority and are represented by ∼27,000 species. Given that the teleosts are the largest group that includes many commercially important species, and that most of our information on fish reproduction comes from the study of these species, this article is focused on them. The teleosts exhibit the greatest diversity of reproductive patterns among the vertebrates. Most species are gonochoristic meaning that they have the familiar distinct female with ovaries and male with testes, forming pairs at spawning time. The vast array of colors and body forms, and reproductive strategies exhibited by teleosts attracts considerable attention from evolutionary biologists, zoologists, and those interested in fish culture (aquaculture and hobby aquaria). Sex determination, the process by which the sex of an individual is established early in development in teleosts is also diverse. It can be genetically or environmentally regulated, and these two mechanisms interact in different species. Many teleosts have genetic sex determination (GSD), meaning that ovarian or testicular development is determined by the action of one, or a complex series of genes. This is the typical condition in vertebrates, for example, the well-known X and Y chromosome system in most mammals, including humans. On the other hand, a good number of teleosts exhibit environmental sex determination (ESD), where factors such as temperature changes promote the development of one sex or another. However, GSD and ESD, once considered to be inherently incompatible, can coexist under environmentally relevant conditions. Some fishes develop as one sex and change to the other sex at some time in life. Sex change is documented in about 2% of teleost species spanning over 20 families. Such functional sex changes generally follow one of three patterns: protogynous (female-to-male), protandrous (male-to-female), and sequentially bi-directional. The common mode of fertilization is external, but some male fishes have a modified anal fin called a gonopodium used to inseminate the female for internal fertilization. Fish also exhibit diverse mating systems and promiscuity is common. In many fishes, the sexes differ in appearance. This can be permanent or a sex hormone-dependent, temporary differences only evident at the breeding season. Most Osteichthyes do not exhibit parental care, and abandon fertilized eggs once spawning is completed. Some fishes, however, do exhibit paternal and maternal care and in a few, even cooperativity. By way of introduction to reproduction in these fishes, the gender systems, modes of fertilization, diversity mating systems, secondary sexual characteristics, spawning site preparation and parental care will be covered. Examples of well-known species are presented to illustrate the diversity in this group of fishes.