Sex at Sea

Living organisms have evolved many different reproductive strategies. The strategies must be suitable for the environmental conditions in the habitats where the species live. In addition to egg and sperm production, there are other aspects to reproduction, such as displays by males to attract mates, size and number of eggs produced, care of eggs and offspring.

Broadcast Spawning: eggs floating freely Some fish gather and release eggs and sperm into the water where fertilisation will occur. This process is known as broadcast spawning. It usually occurs around areas of coral reefs affected by strong currents and when an outgoing tide is flowing. This carries the fertilised eggs quickly away from reef where predators live in the habitat. Eggs of this type float in the water column and are termed pelagic eggs. Mass spawning is when fish gather together in very large groups and release their eggs simultaneously. Broadcast spawners put all their energy into egg production and produce very large numbers of eggs.

Embryonic development takes place within the membranes that enclose the egg. The egg yolk provides energy and nutrients to the developing embryo. After several days the larva (immature fish) breaks free of the membrane. It still has some yolk for survival. The larva has very little locomotion and floats in the ocean as a temporary member of the zooplankton. Eventually it must find its own food

Demersal spawning Demersal eggs do not float. A female fish will lay the eggs in a nest in the sand or in a crevice in a reef or attach them to some part of the reef surface. After being laid, the eggs are fertilised by sperm released by a male. These fish typically give some care to their eggs by guarding them until they hatch or keeping them clear of debris. Read the anemone clownfish example on page 375