The only fish species on the Great Barrier Reef to care for their offspring as juveniles has found a win-win approach to protecting their young from parasites. As well as drastically reducing a major source of mortality in their children, the adult spiny chromis damselfish (Acanthochromis polyacanthus) gets to supplement their diet.

You may need to sit down for this, but Disney lied to you in Finding Nemo: clownfish do not care for their young after hatching, let alone show the overprotectiveness Marlin demonstrates in the film. Like many coral reef fish, clownfish males do guard the eggs against predators – but once they hatch, the larvae journey out into the open waters between reefs to feed on plankton until they are large enough to brave the concentrated dangers of the reef.

On the 2,300-kilometer (1,400-mile) length of the Great Barrier Reef, only one species behaves like that, and they’re not as pretty as clownfish. Juvenile spiny chromis damselfish remain on their parents’ reef after hatching, forming a roughly spherical school that is guarded by the parents as they feed on any plankton that enters their vicinity. Despite mostly keeping close to rock crevices in which they hide from predators, the males in particular sometimes guide the ball of young to the reef’s edge where plankton is most abundant.

Indeed, according to first author of the new study, Dr Lexa Grutter of the University of Queensland, the young spiny chromises sometimes “Become teenagers that refuse to leave home.” Millions of human parents can relate. 

This care is known to be vital to the young Acanthochromis polyacanthus’s success. When something happens to the adults, few juveniles survive. The young don’t need help to feed like a baby mammal or bird might, but fending off predators is a near full-time job. “Most carnivorous fish would enjoy snacking on the juveniles,” Grutter told IFLScience; “But we see the parents chase likely predators away.”

However, bloodsucking gnathiids are also a common threat to reef fish. The gnathiids – a type of crustacean that are like the vampires of the reef – go through three juvenile phases, each one of which requires sucking a fish’s blood to complete. Although they may only stay attached for a few minutes or hours, the parasites can lower their victim’s energy, and even their capacity to solve problems.

For any fish, losing too much blood to a parasite can be indirectly fatal, and Grutter noted in a statement sent to IFLScience; “From our previous research we know gnathiids are especially harmful to juvenile fishes by reducing their swimming, competitiveness, escape response, aerobic performance and overall survival.” She wondered if the parents might help with this as well.

In studies on captured Acanthochromis, Grutter and colleagues confirmed that parental fish lower the risk of mortality from gnathiids under conditions where predators are not a threat. Instead of scaring them away, the adult fish eat the gnathiids, something Grutter’s team confirmed occurs in the wild as well by studying the spiny chromis damselfishes’ feces.

Gnathiids and other external parasites are such a threat on coral reefs that cleaner fish have developed a niche eating the parasites of larger fish at cleaner stations. Much of Grutter’s research has focussed on the study of the distinctive relationships formed when carnivorous fish choose not to eat a cleaner fish so it can consume their parasites instead.

However, Grutter told IFLScience; “We did one study where bigger fish get cleaned more. Probably because they have more parasites.” Spiny chromis damselfish are not that large even as adults, and conditions would have to be bad for a cleaner to bother with their juveniles as clients.

Moreover, the juveniles are so small the parasites suck them dry in minutes, so there is little time for the adults to remove a parasite that has taken hold, as a cleaner would. Instead, the adult Acanthochromis eat the gnathiids out of the water column before they can get to their young.

“So, this is a fascinating example of great fish parenting,” Grutter said, supplementing the diet while keeping the young safe.

The team wants to know how other vulnerable young damselfish survive without parental support. Is it through spending the vulnerable years away from the reef where gnathiids are scarcer, or do they have some other defense?

Only two other coral reef fishes are known to provide post-hatching parental care, living in other parts of the world. Grutter is keen to learn whether other closely related Altrichthys species, resident of South-East Asian reefs, provide their young with similar anti-parasite protection.

The study is published open access in the journal Proceedings of the Royal Society B.