Chow Down like an Amphibian

The way larval water-dwelling amphibians eat is quite different from that of their land-dwelling well aged counterparts.

Larval Salamander Feeding

Larval aquatic amphibians suck in their prey much like a vacuum, this is called negative pressure (1).  Think about when you were a kid (or possibly even now) and you lined up M&Ms on the table and then used your mouth to directly suck them up from the table… I’m definitely not doing that right now as I write this article… definitely.  Anyway negative pressure basically means that the pressure inside of the amphibian’s mouth is lower than that of the surrounding water it lives in.  When the amphibian opens its mouth wide, the pressure between its mouth and surrounding water needs to balance out so there is a quick rush of water into the amphibian’s mouth to do so.  If there is anything in the water near the amphibians mouth, say a scrumptious worm, it will be carried into the mouth (1-3).  What happens next depends on the age of the larval amphibian!  After the amphibian has sucked the food into its mouth it has to close its mouth and expel all that extra water it took in while its mouth was wide open.  Amphibians that are just about to finish metamorphosis and begin their land-life have likely lost their gills and only have lungs to breath through like us.  This means, the extra water has to be pushed out the same way it came in while not losing its food too.  This is called bidirectional flow because the water comes in one way, then it has to change directions and go out the same way, like doing a u-turn on a two-way road.  If the amphibian still has gills then when the amphibian closes its mouth the water is pushed further back into the mouth and out the gills.  This is called unidirectional flow because the water moves in only one direction: in through the mouth and out through the gills, like a one-way road (2).

Adult Salamander Feeding

Although some adult amphibians, particularly salamanders and caecilians, live in the water as adults and feed by the same suction feeding means (1-3), many adult amphibians live on land and feed much differently.  Many adult frogs and salamanders will project their tongue to grab a prey, or if they are close enough the frog or salamander will just move its head to grab its prey (4).  After the food is in its mouth it has to be swallowed, since frogs and salamanders don’t really chew they can have a fairly large food item (potentially still squirming) to swallow.  To help with this the frog or salamander will close its eyes and pull its eyeballs inward, they likely make contact with the food in its mouth and help push the food down toward the esophagus and into its stomach (4,5).

Wood Frog Tadpoles Eating

Bonus video of Wood Frog (Lithobates sylvaticus) tadpoles eating spinach. It isn’t as closely related to the rest of the article but it is tadpole and food related and relaxing to watch… in my opinion. Enjoy.

Citations:

1. Pough, F.H., R.M. Andrews, M.L. Crump, A.H. Savitzky, K.D. Wells, and M.C. Brandley (2016). Chapter 11: Feeding. Herpetology: 4th edition, pp 379-405. Sunderland, MA: Sinauer Association Inc.
2. Gans, C. and G. De Gueldre (1992). Striated Muscles: Physiology and Functional Morphology. In Environmental Physiology of Amphibians (ed. Feder, M.E. and W.W. Burggren), pp 277-313. Chicago, IL: University of Chicago Press.
3. Gillis, G.B. and G.V. Lauder (1994). Aquatic Prey Transport and the Comparative Kinematics of Ambystoma tigrinum Feeding behaviors. Journal of Experimental Biology 187: 159-179.
4. Levine, R.P., J.A. Monroy, and E.L. Brainerd (2004). Contribution of Eye Retraction to Swallowing Performance in the Northern Leopard Frog, Rana pipiens. Journal of Experimental Biology 207: 1361-1368.
5. Larsen, L.O. (1992). Feeding and Digestion. In Environmental Physiology of Amphibians (ed. Feder, M.E. and W.W. Burggren), pp 378-394. Chicago, IL: University of Chicago Press.