The female funnels her oil reserves from her food to produce, even by fish standards, a magnificent clump of large, nutrient-rich eggs. Then the male takes over, starving himself to offer one of the more impressive examples of dedicated protection of their offspring - in a unique way that starts a month before they are even 'born'.

The Sydney cardinalfish is endemic to Australia. It is particularly numerous around Sydney, hence its name but is mainly found from southern Queensland to southern NSW including Nelson Bay. It seems particularly fond of Fly Point.

It's a mouthbrooder.

From around November until March the female releases an egg mass close to a selected male, and a few seconds after he fertilises the eggs, he will slurp up the egg mass, and carry them in his mouth until they hatch. It's not hard to spot these males, the bottom of the chin and throat bulge with the mass of a few dozen eggs.

After spawning, his appetite is strongly suppressed for the month or more he will carry the eggs, and then the young fry, in his mouth.

It makes sense, of course, having a suppressed urge to eat your children, and it's made easier by the relatively sedentary nature of cardinalfishes, particularly when mouth-brooding.

They don't need a lot of calories to keep them going. During a three to five year lifespan, cardinals can have dozens of broods. The system works.

As Malcolm took these shots he could see that the male Cardinalfish would regularly open its mouth to rotate the egg mass, keeping them clean and aerated.

Occasionally it would partially expel the eggs before sucking them back in. Some mouthbrooder species seem to have the ability to dispose of eggs that are no longer viable, and they seem to eat a few - possibly the damaged or unfertilized ones.

The colour of the eggs will slowly change, as the young fish absorb the orange-red yolk, and build their silvery bodies. It seems that in some cardinalfish species around half of the eggs in a given egg-mass will survive to hatch. Upon hatching the youngsters of many cardinalfish species stay clustered together in dad's mouth for a few days.

We're not sure with the Sydney cardinal just how long this might be - one of those things that seems not to have been well studied for this species.

But for an animal this small to take a month or longer without food has to be a physiological stretch, and Cardinalfish fry are not so very different in form or composition from copepods, the small crustaceans that make up the bulk of cardinalfishes' diet.

So at some point it makes sense for the youngsters to swim away from the safety and danger of their father's mouth. Before he gets his appetite back.

Malcolm Nobbs is a recreational diver and photographer from Nelson Bay. Jamie Watts is a marine ecologist from the United Kingdom. The pair regularly collaborate on marine life articles.