A friend and I were looking at pictures of traditional Native halibut hooks and discussing how they work. That led to thinking about how halibut, which are mighty predators, capture their prey. Do they chase after a suitable prey and snap their jaws shut on an unwary fish? No. They slowly approach and then suddenly open a huge mouth, expand their throat and gill chamber, and set up such a low-pressure cavity in their mouth and throat that the prey is sucked in. Suction feeding like this only works in water, relying on the viscosity of water to help move the prey into the waiting maw.
High-powered suction feeding is very common among bony fishes. It may have been the original feeding method used by the several kinds of fishes that evolved from the earliest fishes with jaws. Since then, during their evolutionary diversifications, fishes developed many different morphological modifications to accomplish suction feeding, but all rely on the same fundamental principle of making a big vacuum-like cavity that produces an inrush of water along with the prey (if the predator’s aim is good!).
Having a jaw that opens and shuts (as those early fishes did) was the beginning point; just opening a jaw in water creates a little suction. Since those early days, a few hundred million years ago, the skulls of fishes have become complicated systems with several moving parts. For simplicity, imagine a bony rectangle with loose joints; it can shift to become a parallelogram. Any smaller bones hinged to that rectangle also shift when the rectangle does. That’s just a start on cranial flexibility, known formally as cranial kinesis. (For comparison, humans and other mammals typically have skulls with no moving parts, with a lower jaw attached). How the different cranial parts move varies among species; for example, in some fishes, the upper jaw, along with the lower one, is protruded far in front of the face, making a long tube. Or the whole cranium (or just the upper jaw) can be tilted far back while the lower jaw drops open, making a very wide opening. The lower jaw also may be loosely attached to the cranium, allowing it to open very wide.
The small muscles of the head are inadequate to account for the sudden powerful creation of the huge oral cavity; it can be very sudden indeed. Sometimes the whole operation is completed in less than a hundred milliseconds — in some cases in about ten milliseconds! That’s faster than the blink of a human eye. Such speed takes big, strong muscles. It turns out that the big axial muscles of the fish’s body are typically involved in motions such as tipping the skull sharply upward. Those big axial muscles are also the ones used for swimming, so presumably there are some trade-offs and compromises made.
Closely related species have sometimes evolved to use suction feeding for quite different diets. Here’s an example from two species of freshwater sunfish. Bluegills and pumpkinseed sunfish belong to the same genus (Lepomis) and often live in the same lakes, but they feed in different ways. Bluegills slurp in plankton, while pumpkinseeds forage on snails. Pumpkinseeds have a unique pharyngeal muscle that gives them enough bite force to crush snail shells; bluegills can’t do that. It happens that both of these little sunfishes are preyed upon by their much bigger relative, the large-mouth bass, another suction feeder. The bass have big mouths and are less accurate in capturing prey that bluegills are; the smaller mouths of bluegills create a higher flow velocity and a steeper pressure gradient for the intake of prey.
High-powered suction feeding is used as the principal prey-collecting method by predators that do not actively chase their prey. In addition to many bony fishes, it is reported from many other aquatic vertebrates. Walruses, for example, after locating their benthic prey, can pick up a clam and suck the meaty prey right out of its shell. There’s even one little shark that widens its shoulder girdle to help create suction. However, aquatic predators often combine some suction feeding with their own rapid movement toward the prey. This is usually called ram feeding or lunge feeding. Humpback whales are a good example: opening their huge mouths and throats at the same time that they surge quickly through the water toward a shoal of herring. Herring themselves are reported to capture their invertebrate prey by moving quickly toward it and then expanding the mouth and throat to suck in the prey.
When vertebrates evolved to live on land, suction feeding was lost; air is not sufficiently viscous to help move prey toward the predator’s mouth. But some vertebrates with terrestrial ancestors have moved back into the water and regained some ability to suction feed. This group includes some turtles and amphibians; flamingoes and certain ducks combine suction feeding with filter feeding on small particles.
To put all this in some perspective, consider how other vertebrates get solid food to their mouths. After moving to where the food is, primates, including humans, typically use their hands to grasp food and put it in their mouths. So do squirrels and many other rodents. Elephants use their long trunks. In contrast, most other terrestrial mammals, as well as most birds, bring the mouth to the food (although parrots can use their feet to bring food to the beak). Some fishes also bring the mouth to the food, biting off bits (e.g., parrotfishes that nip corals, and some sharks that chomp out chunks of vertebrate prey). In fact, among the many species of fishes, variation in the details of feeding method is sure to be enormous.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Friday. Her essays can be found online at onthetrailsjuneau.wordpress.com.
