By Victoria Gill
Science and nature reporter, BBC News
Discus fish nurture and feed their offspring for weeks
Discus fish are surprisingly attentive parents, scientists have found.
The colourful little creatures are known to feed their offspring with a nutritious mucus on their skin.
Now a study has suggested that the tropical fish actually wean their fry, "encouraging" them to forage for themselves, and that when it comes to looking after their young, discus fish have more in common with mammals.
Researchers describe their behaviour in the Journal of Experimental Biology.
This nurturing in discus fish is a well recognised behaviour, but this is the first time it has been studied in detail.
Mr Jonathan Buckley from the University of Plymouth, UK, was a member of the team that carried out the study.
Along with his supervisor, Dr Katherine Sloman, and colleagues in Brazil, he found that, when it comes to looking after their young, discus fish have more in common with mammals than with most other fish.
The fish are native to the Amazon
Because of their beauty and popularity with tropical fish enthusiasts, they are often referred to as the "King of the aquarium"
Each parenting pair will co-ordinate their own individual "flicking" action to transfer their offspring to the other parent
He explained: "For the first couple of weeks - when the fry first hatch - the parents take amazingly good care of them."
Both parents' skin is covered in the mucus; the offspring surround the parent and constantly nibble on it.
At this stage, the tiny, vulnerable fry are never on their own. The male and female even share parental responsibility - "flicking" the young from one parent to the other when they need a break from feeding them.
Mr Buckley likened this to mammals suckling their young.
He and his team have now documented some even more striking similarities between the way these fish take care of their fry and the way mammals nurture and feed their babies.
He told BBC News that, after the first two weeks, the parents appeared to deliberately wean their young.
"In week three there's a change - the parents are constantly swimming away," he explained.
"We think this is the beginning of the weaning period - they're trying to make it more energetically efficient for the fry to forage rather than feed."
When the researchers studied the mucus itself, they found that it contained antibodies - immune system-bolstering substances.
"This transfer of antibodies to offspring is primarily a staple of mammalian parental care and [previously] unseen in fish," he said.
Parents share responsibility for their fry
These findings show, Mr Buckley added, just how much fish are underestimated in terms of the complexity of their behaviour.
The work could also show how the fish might be affected by pollution in their Amazon habitat.
Pollutants, particularly from mining, run into the waters and the scientists think that these could be absorbed into the mucus and, subsequently fed to the fry.
"The capture and sale of this species generates a significant amount of money for the people who live in that part of Brazil," Mr Buckley said.
"So it's important to understand how the fish are affected by changes in their environment."
Dr Sloman collaborated with Adalberto Val from the Laboratory of Ecophysiology and Molecular Evolution in Manaus, Brazil, to set up the colony of breeding discus fish that were used in this study.