Fish feed processing (II)

Author: Susanne Driessen, EZNC
Here we present another article in the series on the functional morphology of fish. The first part of this article on feed processing appeared in the most recent newsletter.
The size of a fish’s mouth determines the size of the morsels it can take in. The mouth is measured at ‘gape’(open to maximum), so mouth size can be limiting. The sudden opening of the mouth can also generate a strong vacuum for drawing in food, a strategy used by ‘filter feeders’ (for plankton, detritus, etc.) and ‘ambush predators’ (fish that strike from their hiding places to catch small animals). The water that rushes in can be drained out through the gills, which are often larger for this purpose. A carp can suck in a standard fish pellet (fish food), at a distance of one head length, at a speed of 0,6m/sec. The bigger the difference between the size of the mouth when closed and open, the greater the suction.

Another variant also exists: the fish has no large gaping mouth, but rather a large cavity behind its mouth. This results in not only greater suction but also better aim, effective for fish that eat selectively (insects, crustaceans, small groups of plankton or detritus) or for sucking food out of small spaces (holes, cracks). The internal mouth size is also dependent on the size and the mobility of the cheekbone (hyoid). A large, free-hanging cheekbone, unlike a fixed cheekbone, enlarges the space of the mouth.

A plankton eater or detritus eater can feed not only by suction, but also by swimming for a long time with its mouth open to bring in greater amounts. The gills are then constantly open to drain the water taken in. The gill lamellae (size, spacing and total gill area) also determine the filtering capacity. Structures that are closer together are better at catching small particles.

An intermediate form is ‘gulp feeding’: intermittently taking big gulps. This works well when the feeding areas aren’t too large.

One aid to eating plankton and detritus is a sticky palate, tongue and/or saliva to hold on to small particles. Fish can also taste what they eat with the taste buds on their tongue and palate. Some fish first take small particles (plankton and detritus) into their mouth, taste, select, and then spit out what they don’t want. In these fish, the internal mouth is long rather than large, to give the food a chance to flow along the taste buds. Some fish species even have barbels (mouth filaments) with taste buds; they select their food by tasting the water and the bottom they swim over.