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The cannibal rays that eat fellow fish
CT scans have revealed that the species of ray called giant guitarfish is cannibalistic. Until now, scientists never knew its true eating habits.
Museum scientists made an unexpected discovery when examining the jaws of a giant guitarfish, which lives in shallow coastal waters. Until now it was thought to have a diet of shellfish.
But when experts looked at the jaw closely under the Museum's micro-CT scanner, the fish's mouth showed evidence of a far broader diet: other rays.
A wedgefish specimen similar to the one which researchers worked on © Frederik H. Mollen (Elasmobranch Research, Belgium)
The tissue around the jaw bones was full of 25 to 30 embedded stingray spines, showing for the first time that these large rays regularly eat their smaller counterparts.
The findings are published in a paper in Royal Society Open Science.
The embedded spines were particularly surprising for experts, because giant guitarfish have flat teeth perfect for crushing prey with hard outer shells, but not suited to slicing through flesh.
The micro-CT scans of the fish jaw show the stuck stringray spines (coloured red) © Mason Dean
Zerina Johanson, a Museum expert on shark evolution and a co-author on the paper, says, 'These teeth are clearly flat, so we would normally associate them with animals that crushed their prey between their jaws.
'We would expect them to eat crustaceans like a crab or lobster, or bivalves such as clams.
'But it's clear that this ray was managing to eat a much broader diet. It is estimated that this ray measured 2.6 metres, so perhaps it had grown so large it could swallow smaller rays in chunks.'
Poisonous prey
Even more surprising is that the poisonous stingray spines did not seem to stop the fish eating its smaller counterparts. Some of the spines became lodged in the jaw's cartilage, which then grew over the spines, perhaps to try to heal the damage.
Sharks and rays are closely related, so this finding adds to the scientific community's knowledge about the bodies and behaviour of both of these animals.
Zerina says, 'This is the one of the first times that we have seen that kind of reaction in shark cartilage. Moreover, the poisonous spines didn't seem to deter the larger ray, and it is unlikely that the poisons were the cause of death.
'Some of the spines look like they were stuck in the jaw for a long period of time, so the fish was probably eating rays regularly.'
A scan showing a cross-section of the ray jaw, including a series of flat teeth on the top right © Mason Dean/Zerina Johanson
New discoveries with new technologies
In 2015 scientists at the Museum bought the fish jaw dry and without any flesh on it. The specimen was collected in the Sula Sea in the Philippines.
It is thanks to a micro-CT scanner that the inside of the specimen could be analysed in such detail.
Zerina explains, 'This work could not have been done without the use of a CT scanner. It is such an important piece of technology because it allowed us to look inside the jaws without destroying or dissecting them.
'If we had not had the scanner, we would have had to cut through the tissue and dissect the specimen.'
After seeing this evidence of cannibalistic behaviour in the guitarfish jaw, Zerina and her colleagues investigated other fish jaws in the Museum's collections. She found that many spines were embedded into other ray jaws.
Zerina says, 'These findings show that more work needs to be done on existing fish collections. This is a previously unknown food source for this species, so I hope this research will motivate others to look at their own collections with fresh eyes.
'The way curators and researchers study their collections changes constantly as technology is developed and new discoveries emerge.'
