Reef fish settle down as they grow up

Adult parrotfish (c) JP KrajewskiSexually active parrotfish stay at home while youngsters explore

Young parrotfish rapidly expand their coral reef home range but only until they reach adulthood, scientists have found.

Researchers in Australia mapped the movements of individuals from very young juveniles to mature adults.

They found that small parrotfish quickly expanded their range as soon as they settled on a reef.

This expansion stopped when the fish matured, and their final home range was unrelated to adult body size.

The results are published in the journal Proceedings of the Royal Society B.

Most animals move within a home range; the size of which is driven by avoiding predators, finding food and reproducing.

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“In many studies which look at coral reef fish movements, we see a significant relationship reported between body size and home range,” explained lead researcher Justin Welsh from James Cook University, Australia.

The team identified 75 individual fish by size, colour markings and scars from three species of parrotfish.

By observing their movements on a 80m (262 feet) section of the Great Barrier Reef, researchers were able to map the home ranges for very young juveniles new to a reef through to mature adults.

“What we found was that really small parrotfish expand their home range very quickly as soon as they settle on the reef,” Mr Welsh told BBC Nature.

These expansion rates were unaffected by the threat of predation, a change in sex or a dietary shift from carnivore to herbivore.

Mr Welsh explained that generally the diet of a species dramatically influences how rapidly an animal’s home range has to increase with its body size so that it can find enough food to meet metabolic demands.

Even though the juveniles have a change in their feeding habit as they grow, they found no evidence of a change in diet effecting the way their home range relates to their body size.

“In our study we found no effect of diet, which was quite striking, and highlights how very different coral reefs can be from the rest of the planet’s ecosystems,” Mr Welsh said

“Juvenile parrotfish behave quite similarly to reptiles which we believe occurs as both young parrotfish and reptiles share a common goal – which is to acquire as much food as possible in order to grow quickly and reduce the number of predators able to eat them.”

Juvenile parrotfish (c) JP KrajewskiParrotfish only display their spectacular colours when they are sexually mature

However, the team found that as the parrotfish matured and changed into their male and female colours they stopped expanding their home range.

It is this transition to sexual maturity, say researchers, that instigates a major behavioural change in parrotfish.

“Adult [parrotfish] on the other hand are very similar to mammals in which large social groups exist with complex courtship and mating behaviours and therefore it is beneficial to remain as close to a potential mate as possible,” Mr Welsh told BBC Nature.

“The fact that a fish’s social environment can have such a dramatic affect on their home range size opens up endless possibilities for new research.”

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