 Teleosts have four major life stages : the egg , the larva , the juvenile and the adult . Species may begin life in a pelagic environment or a demersal environment ( near the seabed ) . Most marine teleosts have pelagic eggs , which are light , transparent and buoyant with thin envelopes . Pelagic eggs rely on the ocean currents to disperse and receive no parental care . When they hatch , the larvae are planktonic and unable to swim . They have a yolk sac attached to them which provides nutrients . Most freshwater species produce demersal eggs which are thick , pigmented , relatively heavy and able to stick to substrates . Parental care is much more common among freshwater fish . Unlike their pelagic counterparts , demersal larvae are able to swim and feed as soon as they hatch . Larval teleosts often look very different from adults , particularly in marine species . Some larvae were even considered different species from the adults . Larvae have high mortality rates , most die from starvation or predation within their first week . As they grow , survival rates increase and there is greater physiological tolerance and sensitivity , ecological and behavioural competence . 
