Discus mucus

Dis(cus)hing out some parenting

As mammals, we have an interesting relationship with what biologists refer to as ‘parental provisioning’, the act of providing your offspring with nourishment. On the one hand, lactation seems absolutely normal and we do not give two thoughts to the fact that women produce the food on which babies initially depend (ignoring the huge industry of mother’s milk replacements and supplements here for a moment). On the other hand, we pride ourselves very much on being the most ‘developed’ of animals, and being a mammal, and thus having mammary glands, is an important (albeit typically implicit) part of that self-aggrandizing. Oh yeah, and we make a very big fuss about breast feeding in public, either as people being outraged by it or as people outraged by the outrage. Millions of web bits and bytes have been filled with this topic.

But what if I told you that there is a fish that is ‘breast feeding’? And what if I told you that that fish has achieved real equality between the sexes, because both ‘lactate’? And what if I told you that that fish is even so civilised as to make an effort and look like a proper dish for its young[AJ1] ? And what if I told you that in that fish’s society, nobody raises an eyebrow over ‘breast feeding’ in public, regardless which sex is doing it? Well, I would hope that you would be amazed. Except, maybe, about the lack of eye-brow-raising. By now I expect you to know that fish do not have eyebrows. The fish I am talking about is a South American cichlid called the Discus fish. And yes, the cue is in the name. These fish are round and get their name from that ancient toy that Greek statues like to pose with. Now, their body shape certainly is fascinating. As is the history of their introduction into the aquarium trade and the long quest for a way to reliably breed them – to this day they belong to the more expensive fish in the standard stock of most aquarium shops due to all the effort that goes into that. But what I find most fascinating about them is their parental provisioning behaviour – which is truly remarkable.

When discus babies hatch, they have but one desire: find their parents and eat their mucus. Yes, that’s right, that slimy substance all fish secrete to cover and protect their bodies – mucus. That’s what baby discus are after. And in anticipation, their parents start overproducing mucus in the runup to the hatching date. Because those little swimmers come with a great appetite. So great, in fact, that they eventually start harming their parents if the generations are not separated. Under natural conditions, this is easily achieved: the parents just swim away. In a tank, parents are more confined. And so baby discus can actually kill their parents, if left unchecked. That all too familiar thought ‘my children are killing me’ – it is quite literally true for discus[AJ1] . But if separated too early, the babies do not get important proteins and immune-system-boosting substances that the parents’ mucus provides and the little ones may fall sick and die themselves. And this is partly why it proved so difficult to successfully raise Discus in the aquarium trade for a long time; traditional knowledge was to separate parents and offspring as soon as possible… because many fish parents tend to eat their babies. Quite to the opposite effect, if you keep Discus babies from eating their parents, they will not develop properly. So there you have it: lactation isn’t all that special. In fact, mucus feeding is common in many other fish species. And yes, I likened the scraping off of a slimy substance of a parent’s body to that sacred act of lactation; after all, mammary glands are just modified sweat glands! So next time you have a glass of milk, maybe you will have to suppress the thought ‘hmmmmm, cow sweat’. You are welcome.