When is a fish like a mammal?

Discus fish are hugely popular in the fishkeeping world, not just because of their Frisbee shape and their beauty, but also because of their unusual spawning behaviors. Both parents care for the eggs, and both continue to care for the fry after hatching. That part isn’t unique, but this is: both parents also produce a nutritious mucus on their skin, which the fry nibble on. This mucus is the sole food source for the fry until they reach a size at which they begin looking for live prey.

These facts are already well known to fish enthusiasts. But Jonathan Buckley of the University of Plymouth (UK) has just published a paper in the Journal of Experimental Biology which takes these cool factoids and makes them even cooler.

What Buckley and his team discovered is that 1) discus parents apparently wean their fry off their mucus diet, much the way a mammal weans its offspring off its milk diet, and 2) the mucus undergoes a “huge increase” in antibody and protein levels when the eggs are laid. The levels stay high during the fry-raising period, then return to pre-spawning levels once the fry are weaned. Thus the fry are nibbling on not just a protein source, but also a source of transferred immunity — just like nursing mammals.

Buckley is planning further research to determine just what triggers the changes in the mucus, and is postulating that it’s hormonally regulated. If so, then these fish will have yet another thing in common with mammals, for whom milk production is entirely regulated by hormones.

Check out the abstract of Buckley’s paper for the full coolness, including details on how the parents wean their fry. Hint: it involves running away.

This is such marvelous stuff that I’m feeling an impulse to set up an aquarium and get back into fishkeeping. (I had to give it up when I moved to Portugal; fish tanks don’t travel very well.) Let’s see, where in our apartment could I fit a tank…?