By: John Ballard
Many species of marine fish reproduce through broadcast spawning. Usually this involves one or more females releasing hundreds of eggs into the water. Closely following males release sperm simultaneously. The eggs are thus fertilised externally. The thousands of eggs receive no parental care, they develop in the water. With fish eggs, survival rates are extremely low. The few which reach maturity are enough to maintain the species, but the vast majority of eggs, sperm and larvae end up as food for planktivorous animals.
This is very different to most mammals and sharks, where fertilisation is internal. Few eggs are produced, but parental care ensures that survivor rates are high.
In our aquarium exhibits several species of fish spawn. The eggs are minute. Many are smaller than the head of a pin. We occasionally collect the eggs and attempt to raise them, but the process is difficult.
All eggs have a common feature known as an oil droplet. This orientates the egg, which floats with the oil drop upwards. After a few hours a developing embryo may be seen at the lower end of the egg. Later, usually after 24 hours, a little tail breaks free and the egg starts to swim. Over 2 days the larva absorbs the egg, during which time it develops eyes and a gut. Then it must find food.
To feed the larvae, we culture microscopic algae. This is fed to tiny organisms known as rotifers, which must be produced in great numbers. The fish larvae eat rotifers for nearly a week, after which they are fed newly hatched brine shrimp, a small crustacean.
So far, we have raised three of our exhibit species through the larval stage. Zebra, two bar sea bream and spadefish have been reared so far. We plan to continue with this important work as we increase our sustainability.