A reproductive strategy is defined as the way in which a species allocates or budgets energy to produce viable offspring. Energy is diverted into actual offspring and into parental care. One usually distinguishes the R-strategy, in which energy is invested in a multitude of offspring that receive little or no parental care, and the K-strategy, wherein energy is invested in a few, large offspring that require considerable parental care. Teleost fish have developed a large variety of reproductive strategies and reproductive behaviours, ranging from mass spawning to parental care, from strict gonochorism (separate sexes) to simultaneous hermaphrodism, and from oviparity to vivparity.

Gonochorism and sex change

Most fish are gonochoric, that is their sexes are separate and determined once, for life (e.g. rainbow trout, Onchorynchus mykiss). However, many fish exhibit sex change during their lifetime. This is called successive hermaphrodism, which can be protandrous if the fish changes from a male to a female, such as the gilthead seabream (Sparus aurata). Other species are protogynous hermaphrodites and change from a female into a male (e.g. orange spotted grouper). Simultaneous hermaphrodites also exist, which behave almost simultaneously as males or females with cross-fecundation taking place; indeed individuals can change within minutes from displaying male sexual behaviour, with sperm release, to female sexual behaviour, with egg laying (e.g. western Atlantic serranid black hamlet).

Release of offspring: Oviparous or Viviparous

In oviparous fish, eggs are fertilised externally, after spawning. In viviparous fish, such as the guppy (Poecilia reticulata) or the mosquito fish (Gambusia affinis), the fertilised and developing eggs remain inside the body of the mother for a certain period of time and the offspring are released as free-swimming young. In fact, it is more appropriate to speak of ovoviviparity as there are no real exchanges between the mother and the embryos.

Ovarian development: Synchronous, Group synchronous, Asynchronous

Synchronous fish reproduce only once each year, or once in their lifetime. In such species, all oocytes develop simultaneously and are at the same developmental stage at a given time. This is the case for salmonids, for example. Group synchronous fish have two or more distinct populations of oocytes present at the same time and ovulate once in a season, or undergo multiple ovulations over a few days or weeks within the spawning season. Asynchronous fish, such as the zebrafish, are capable of ovulating on a regular basis, sometimes every day, over a prolonged period.

Spawning strategies

Most teleost fish are are either determinate of indeterminate batch spawners. In determinate spawners, the potential annual fecundity is fixed prior to the spawning period. The total number of eggs spawned per female in a year is called the realised annual fecundity. For indeterminate spawners, the potential annual fecundity is not fixed before the onset of spawning. In such species, eggs can develop at any time during the spawning season.

Some fish species are single or total spawners. Total spawners release their eggs as a unique event, or over a short period of time but as part of a unique event (as opposed to being spawned in several batches). Single spawners release all their eggs in a single lifetime event.

Spawning type

Pelagic spawners allow their eggs to be carried freely by the currents, however spawning is precisely timed and takes place in specific locations; this is either to minimise egg predation, to maximise dispersal, or to provide the best conditions for the pelagic larvae to survive upon hatching.

Some fish are nest spawners, for example, the male stickleback (Gasterosteus aculeatus) builds a nest and attracts the female to this nest to spawn. Others are demersal spawners, attaching their eggs to a substrate such as the vegetation or the stream bed (e.g. cyprinids and salmonids respectively).

Mouthbrooders care for their eggs and / or larvae and protect them from predation by taking them into their mouths (e.g. tilapia). Some fish breed inside invertebrates, for example the European bitterling deposits its eggs into a living mussel. Other forms of parental care include egg brooding in special pouches, such as in the seahorse male ("pregnant" males); and skin brooding, which involves the attachment of eggs and developing embryos to the skin of one of the parents (e.g. pipefish).




This factsheet was prepared with the expert assistance of scientist Alexis Fostier.