We often tend to think of fish as cold creatures, devoid of feelings, which is why it is difficult for us to project human emotions and behaviours onto them. And yet science, albeit only recently, has begun to reassess the intelligence and social behaviours of these animals; the latest discovery concerns the clownfish (Amphiprion ocellaris).

A loving father

According to a study conducted by researchers at the University of Illinois, published in the journal Hormones and Behavior, male clownfish are characterised by a strong paternal instinct. The parental care displayed by this species — that is, the set of behaviours parents undertake to promote the growth of their offspring — is so strong that if a male with no offspring were entrusted with eggs to which he has no relation, he would care for and protect them as if they were his own, clearing them of fungi, removing debris and fanning them with oxygen-rich water. "All other fish would simply eat them," the authors of the study explained.

The love hormone

This behaviour is rather unusual in fish and, according to scientists, it is regulated by a particular molecule called isotocin. This molecule is very similar to oxytocin, also known as the love hormone. Oxytocin is the molecule that drives contractions during childbirth and strengthens the bond between mother and newborn, but it is also the hormone of falling in love and of affection. In the same way, isotocin "creates" a bond between the male clownfish and the eggs, prompting him to care for them. Confirmation of the important role of this molecule was provided to scientists by an experiment which revealed that, by "blocking" isotocin, the animal stopped tending to the eggs. "This study shows first and foremost that, regardless of the species or gender of the parents, oxytocin generally appears to be linked to high levels of parental care," said Justin Rhode, a professor at the University of Illinois and co-author of the study.

Just like Nemo

The study inevitably called to mind the Disney Pixar animated film Finding Nemo. The film tells the story of Marlin's desperate search for his son Nemo, who has been captured by a diver. Just as in the fiction, reality too has demonstrated the paternal instinct of these colourful fish. Parental care on the part of male parents is uncommon among vertebrates; mammal fathers tend as a rule to be indifferent to the survival of their offspring. According to the researchers, male clownfish make excellent fathers because they share a confined habitat with their partner and young — namely the venomous anemones that provide shelter to these fish.

When we were fish

"Vertebrates evolved more than 400 million years ago and 300 million years ago they were fish," said Dr Rhodes. "Paternal care has very deep and ancient origins, and the molecular mechanisms or genetics that drive it are probably similar across all species. I think we can learn a great deal from our fish ancestors."