Don’t worry, cardinalfish, we didn’t want your eggs anyway


Bec Crew


Bec Crew

Bec Crew is a Sydney-based science communicator with a love for weird and wonderful animals. From strange behaviours and special adaptations to newly discovered species and the researchers who find them, her topics celebrate how alien yet relatable so many of the creatures that live amongst us can be.
By Bec Crew 26 April 2022
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Like an anatomical panic room, the mouth of a cardinalfish might just be the safest place in the ocean if you’re an unhatched fry.

With razor-sharp teeth framing its gaping maw, the male tiger cardinalfish (Cheilodipterus macrodon) makes a compelling case for any would-be predators to back off.

This proud papa probably has a mood to match his fearsome visage – while he’s brooding his clutch of eggs, he can’t actually eat anything. Incubating those eggs usually takes up to a month, which, to be honest, sounds pretty hellish.

Male tiger cardinalfish get lumped with this duty of care because they naturally develop longer heads and a larger jaw than the females – all the better to brood lots of babies with.

These eggs are fertilised in the water – the female will lay eggs near the male, who will fertilise them and then take them into his mouth.

Here’s a male yellow-striped cardinalfish (Apogon cyanosoma), photographed in the Solomon Islands, showing off a delicious mouthful:

Yellow-striped cardinalfish (Apogon cyanosoma) male with eggs in mouth – and yes, they are tiny eyeballs you spy!
Image credit: Minden Pictures / Alamy Stock Photo

It’s not a stress-free experience for the females, even if they don’t have brooding responsibilities. The females have to worry about the very real possibility that the male will accidentally (or on purpose) swallow the lot and be done with it. They may also swallow a few eggs, on occasion, just to get through the month.

Fortunately, female cardinalfish have evolved a nifty trick in response to this – they lay yolkless ‘dummy’ eggs alongside the real eggs in the hope that, if the male swallows some eggs, many of them will be dummy eggs.

Tiger and yellow-striped cardinalfish are among several cardinalfish species to display this mouth-brooding technique. Tiger cardinalfish are found throughout the tropical waters of the Indo-Pacific region, including all along the north-west, northern and north-eastern coast of Australia, from Western Australia to Queensland. Yellow-striped cardinalfish are native to the Indo-West Pacific, including around most of the mainland Australian coastline.

Cardinalfish are not the only animals to brood eggs in their mouths. Male jawfish from the family Opistognathidae, found throughout the warmer parts of the Atlantic, Pacific and Indian Oceans, also do it. And as you can see here, they do it with aplomb:

Male yellowhead jawfish (Opistognathus aurifrons) very briefly airing the eggs in his mouth. Image credit: Stock Photo

Darwin’s frog (Rhinoderma darwinii), native to the temperate forests of southern Chile and Argentina, is another mouthbrooding species. The male swallows its very young tadpoles and incubates them in his vocal sac until they hatch.

Watch here to see why its nickname is the vomit frog: