By Diane Pitassy
The Division of Fishes collection staff sees a lot of fishes in jars in the course of a day, so it takes a REALLY unique specimen to bring work to a screeching halt. This specimen of Antennatus coccineus, the Scarlet frogfish,did just that. Who hasn’t had a zipper busting meal at one time or another? Sneaking that second office donut while no one is looking is one thing, but eating a meal equal to your own body mass, as did this individual, is quite another. So after gawking at this frogfish for a bit, we hit the library to seek out how it is even possible for this fish to consume such a large meal.
First, the frogfish has to catch its prey. Frogfishes, having the shape of a semi-deflated basketball, are not fast swimmers, but they can mimic their environment precisely and so use more cunning to catch a meal. Frogfishes employ several strategies to catch other fishes and invertebrates: some lie in wait, attacking prey as it swims by, while others very slowly stalk their prey or even attract their prey via an elongate dorsal spine with a bulb of luminous bacteria at the tip that acts as a fishing lure. Once the prey is within close range, the frogfish rapidly expands the size of its mouth, increasing the internal volume of the mouth by up to TWELVE times its size when closed. This creates a sudden massive suction that pulls the prey into its mouth before its jaws snap shut. Also, the jaw anatomy of the frogfish permits it to close its mouth while maintaining the oral cavity at almost its fully expanded capacity. Many other species of fishes employ this behavior but can only expand their mouths up to a mere six times the resting size of their oral cavities. The frogfish can also expand its mouth much more rapidly than other species. For example, the frogfish takes only 6-7 milliseconds, compared to the 15 milliseconds required by the stonefish, another lie-in-wait predator. What does this all mean? The frogfish is able to take in a fish as large itself very rapidly and then can quickly close its mouth to seal the meal inside.
The next step is managing to swallow the supersized meal. Ever take an extra big bite and feel it painfully scrape down your esophagus? Well, now imagine that piece of steak is alive and has rigid spines and is struggling mightily NOT to be forced down the chute. To overcome this, the frogfish uses water pressure. Instead of immediately forcing out the water sucked in with the prey through its gills, the frogfish closes off its gill openings, contracts its closed mouth, and forces the water and prey quickly into its stomach--like being flushed through a pipe. Once the prey has been firmly secured in the stomach, the frogfish reopens its gill openings and mouth, contracts its body, and pushes the water back out, just like squeezing a water balloon. If the prey is very large, the fish may not be able to push the prey directly into the stomach by using water pressure. Instead the frogfish uses a secondary set of teeth, called pharyngeal or gill arch teeth, which line its “throat”, to move the prey down its digestive tract. Using a raking motion, the recurved upper and lower pharyngeal teeth alternately grasp the prey and move it down the throat, while also preventing the prey from escaping back up to the mouth.
It is well known that the stomachs of frogfishes are highly expandable, yet scientists still do not know what makes the stomachs so elastic, or how frogfishes are able to eat prey up to 1.5 times their size without damaging other internal organs. Scientists also don’t know how long it takes for frogfishes to digest such colossal meals. These mysteries are yet to be solved, but frogfish specimens, such as this one housed in the National Fish Collection, remain invaluable in that they inform scientists about what species these cryptic fishes are eating and how frogfishes fit into coral reef food webs.
So enjoy your Thanksgiving meal, but know that the special pair of stretchy “eatin” pants you own is nothing compared to a frogfish’s stomach!
Answers to our burning frogfish questions were provided by Dr. Ted Pietsch and Dr. David Grobecker from their comprehensive work, Frogfishes of the World: Systematics, Zoogeography, and Behavioral Ecology. For more information about all things anglerfish visit Ted’s page on the Tree of Life web project.
And if you STILL cannot get enough on anglerfish, go to this great Natural History Museum, London article on reconstructing the dinner of a deep sea anglergish by micro CT scan.