405_April_24

MARINE BIOLOGY WITH MIKE SCOTLAND - FISH IDENTIFICATION

species was seen to spawn on three consecutive days. As you can imagine, you cannot hover around to see if they spawn every day and night for the five month breeding season. However, they seem to spawn with a small number of eggs every few days over the breeding season. This is in contrast to some fish which release a lot of eggs at once in a single massive spawning event such as Groupers. In that case, there will be a higher level of loss to predation in this case of mass spawning aggregations for which they are famous. Mother Nature deals with this problem using mathematics. The Groupers produce so many eggs that they overwhelm the predators and enough eggs hatch into fish and grow to adulthood. This serial spawning has some advantages. It means that all the eggs will not be eaten as they can be spread around over time. The fish can continue to produce eggs and sperm over the five month breeding season and this gives them a great chance of creating offspring that will survive to adulthood. The peak in breeding was discovered to be in August and September. The reason is thought to be that there will be a greater food source for juveniles once hatched as summer approaches. The days are longer, more photosynthesis supports more marine life. The food chain swells with super abundance!

Above and Below: Pygmy leatherjackets, Brachaluteres jacksonianus , can change their colour to match their environment.

Courtship Behaviour

I have observed the courtship behaviour of Six Spined leatherjackets. The male sets up a territory with suitable nest sites. There is a brief period of courtship display. The female digs out a nest on the sand between some rocks whilst the male stands by and guards her. He nudges her belly area as the nest was being prepared. She lays a small number of eggs which the male fertilises and then departs. Males protect the nest, presumably from bottom feeding predators. This is similar to Triggerfish reproductive behaviour in the tropics. Every diver knows to keep clear of nesting Titan triggerfish as they are very aggressive and sometimes will attack divers ferociously. Another observation that Valerio confirmed was that Leatherjackets do not change sex like many fish do. This is called gonochorism. There is no sign of the common ovi-testis organ that occurs in many fish including Wrasse, Cod and Damselfish. That is, many fish have both gonads but one is dominant at one time. This enables them to change sex as they mature as the other gonad springs to life as a result of hormonal change.

Male Pygmy leatherjackets develop large ventral protruberances

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DIVE LOG Australasia #405 - April ‘24