Matt Kolmann

Matt Kolmann

Nov 03, 2016

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Our first paper on chewing in floppy-jawed critters is out!

So our first paper on insect-feeding in stingrays was released in Proceedings of the Royal Society: Part B and it got a lot of press. 

Science: http://www.sciencemag.org/news...

National Geographic: http://news.nationalgeographic...

LiveScience: http://www.livescience.com/560...

MotherBoard: http://motherboard.vice.com/re...

ScienceDaily: https://www.sciencedaily.com/r...

CBCNews: http://www.cbc.ca/news/technol...

GizModo: http://gizmodo.com/stingrays-c...

the University of Toronto Scarborough website: https://www.utoronto.ca/news/u...

...hell we even got on Reddit for a little while!

Here's the gist of what we found:

Freshwater stingrays are found in South American river basins and feed on a diverse array of prey.  Many of these species specialize on a single kind of prey, be they fish, crustaceans, snails, or even aquatic insect larvae.  But not all of these prey are created equal – some prey are harder, softer, or tougher than others.  Insectivorous freshwater stingrays are the only elasmobranchs (sharks and rays) to feed on insects – which are difficult to eat and digest due to high amounts of chitin in their exoskeletons, a remarkably complex and tough material.  Other vertebrates, namely mammals like shrews, bats, and tenrecs also eat insects and they use complex jaw motions – chewing – to shred chitin to allow digestive juices to breakdown prey.  Stingrays can protrude their jaws away from their skull as well as protrude these jaws laterally, to the left or right.  Using high-speed videography we determined that stingrays do actually chew their food – just like mammals.  We also found that these stingrays lift their disk to suck prey underneath the body – thereby capturing food with their pectoral fin ‘limbs.’  This decoupling of behaviors, prey capture and prey processing, is reminiscent of what is seen in several major radiations of bony fishes where the oral jaws suck prey and jaws in the throat (pharyngeal jaws) crush prey.


 Q. There are freshwater stingrays?

A: There are many!  You can find freshwater stingrays in many tropical rivers – in North Australia, New Guinea, Thailand, and even West Africa, to name a few places.  But the largest concentration of freshwater rays are found in South America – from Guyana and Venezuela in the north, through Brazil and Peru, and even down into Argentina and Uruguay.  These rays (Potamotrygon) were originally marine, but invaded South America millions of years ago, back when the Andes hadn’t risen and the middle of the continent was filled by a shallow, slightly salty lagoon, called the Pebas Mega-Wetland (the best name for a big salty swamp that I can think of).  They must have been successful, because now there’s over 30 species of freshwater rays in South America – all inhabiting freshwater.

 Q. Why is chewing in rays important?

A: Some of these freshwater rays (potamotryonids: literally – ‘river rays’ in Greek) are unique in that they eat aquatic insect larvae – the juvenile phase of insects like dragonflies, beetles, and caddisflies, which live along the bottoms of rivers and ponds.  Insect larvae are made of chitin – a tough substance like plastic that has to be sheared or shredded apart – that’s why stingrays chew, to tear chitin into bits.  Aquatic insects are quite plentiful, but they are tough to eat – so stingrays that chew have found a means of eating a plentiful food option that they don’t have to compete with other animals to eat!

 Q. I don’t understand why you think the comparison to goats is interesting.  Why goats?

A: For a very long time, scientists thought that only mammals, like goats, could chew.  Some of the earliest mammals fed on insects, and these sorts of critters were the land animals that ‘inherited the earth’ after the dinosaurs went extinct.  Chewing is widely believed to have been a real important adaptation that helped mammals take advantage of new diets when they diversified some 60-70 million years ago.  What we’re finding more recently is that other organisms that we wouldn’t have expected to be able to chew, are arguably doing the same thing as mammals – and they’re chewing because this behavior seems to be the best solution for eating tough substances like insects, grasses, and even bone.  Now we know there are carp that chew.  Lizards that chew, even some dinosaurs are hypothesized to have been chewers.  But chewing has never been observed in sharks and stingrays, and we’re the first to find it.

 Q. Why don’t other stingrays eat insects?

A: Well, we think that to chew on insects you need really flexible jaws.  Most stingrays have these, although some of the rays I’ve studied previously, ones that eat mollusks have very rigid jaws.  So those clam-eating stingrays probably can’t chew, just crush.  Freshwater rays have particularly kinetic, or mobile jaws – the jaws of all rays are extended away from the skull during feeding by two cartilages, the hyomandibulae.  But what makes freshwater rays so unique is that they have an extra joint that runs between the hyomandibulae and the jaws – making their jaws extra flexible.  But maybe the answer to why more stingrays don’t eat insects in that because there just aren’t that many aquatic insects in the oceans – so only freshwater rays have had the opportunity!

Q. Why would anything eat an insect anyway?

A: Insect larvae are packed with nutritious fats that they’re saving prior to metamorphosing – when these insects change from generally flightless juveniles to volant (flying) adults.  All these fat stores are very nutritious and freshwater stingrays sure seem to love them!  When I was conducting my experiments, the rays would ignore other foods if I dropped insects into the tanks – even though they took more of an effort to eat!

1 comments

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  • Christina Tran
    Christina TranBacker
    Very cool! I feel like I'm never going to get tired of watching them chew. :) Is there a homologue to the extra joint (that runs between the hyomandibulae and the jaws) in mammals? Are our jaws structurally similar or are the freshwater rays' totally unique?
    Nov 03, 2016
  • Matt Kolmann
    Matt KolmannResearcher
    There is absolutely no homologous structure in our jaws. Or the jaws of anything else we're aware of (stay tuned with regards to that sentiment!) - although the ligament in which the angular cartilage develops is present in all sharks and rays. Our lower jaw is composed mostly of a single bone, the dentary (mandible), which forms (ossifies) around the 'original' lower jaw skeleton - the Meckel's cartilage. The Meckel's cartilage is what forms the lower jaw in sharks, rays, and ratfishes, and in bony fishes is replaced by dermal bone - essentially big bony scales which make up the majority of the skull as we think of it commonly.
    Nov 03, 2016

About This Project

I'm interested in how stingrays, with jaws made of cartilage, consume tough or stiff prey like insects, crabs, and mollusks. I use high-speed videography and measure bite forces to analyze how rays use their jaws to eat tough prey. These freshwater rays invaded South America 30+ million years ago and diversified to feed on a variety of prey. How does feeding specialization evolve and what does it look like?

Blast off!

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