Do Fish Sleep?: Fascinating Answers to Questions about Fishes

Four: Fish Lives

FOUR Fish Lives Question 1: What and how do fishes eat? Answer: Eating is one of the most important activities for a fish. Smaller fish eat proportionately more than larger individuals because their metabolism is faster. Different fish species have very different kinds of diets—some have narrow food preferences while others require a more varied diet. Some fish are vegetarians; for example, redband and princess parrotfish eat seaweeds on coral reefs, and the Chinese grass carp eats aquatic vegetation in lakes. Herbivorous fishes must tear or grind the plant material extensively before they can digest it. They also have longer digestive tracts because it is harder to digest plant material. While some parrotfish eat algae, others actually bite off chunks of coral to eat the living polyps. These parrotfish poop out ground-up corals as a fine white powder that becomes the beautiful sand we see on tropical beaches. Menhaden, Brevoortia tyrannus (fig. 12), filter phytoplankton (microscopic plants) from the water through their gills. Others in the herring family also filter feed but eat mostly zooplankton (tiny animals). Filter feeders have structures called gill rakers for separating plankton from the water, hard projections that point inward from the gill arches. Their shape and number are a good indication of the fish’s diet. Fishes that eat large prey have short, widely spaced gill rakers that keep prey from escaping. Those that eat smaller prey have longer and more numerous gill rakers , and plankton-feeders have the longest, thinnest and most R5367.indb 49 R5367.indb 49 10/26/10 7:14:23 AM 10/26/10 7:14:23 AM 50 DO FISH SLEEP? numerous rakers. The largest fish in the world, the 15-ton whale shark, Rhincodon typus (see color plate A), is a filter feeder. It swims slowly through the water with its huge mouth open, filtering out plankton and small fishes. Herbivorous fish play important roles in their ecosystems. Plankton eaters can control populations of phytoplankton that might otherwise bloom to excess. Grazers are essential to the health of coral reefs; they crop the algae that would tend to grow over and smother corals. Without substantial populations of herbivorous fish like surgeonfish, princess parrotfish, and red band parrotfish (see color plate E) to keep algae under control, a reef can be taken over by the algae. Different fish specialize in different types of algae and the right combination of herbivores is important to coral recovery, as found by Deron Burkepile and Mark Hay of Georgia Tech, in a study showing that biodiversity is important in maintaining healthy ecosystems. Millions of algae-eating carp are being employed in China to clean up a lake that has been suffering from algae blooms. Tahu Lake had been massively polluted with sewage and agricultural waste that stimulated blooms of microscopic blue-green algae, forcing authorities to cut off water to over two million people. They hope that the carp, which consume huge amounts of algae, will clean up the lake. Some fish feed on benthic (bottom-dwelling) animals on or in the mud or sand. Marine benthic feeders include flatfishes and cods, and freshwater examples include carp and bullheads. Figure 12. Atlantic menhaden, Brevoortia tyrannus. (Courtesy of NOAA.) R5367.indb 50 R5367.indb 50 10/26/10 7:14:23 AM 10/26/10 7:14:23 AM FISH LIVES 51 Some marine species prefer mollusks, while others prefer small crustaceans; in fresh water, insect larvae are most desirable. However, many species will eat whatever is abundant. While feeding, most of them swallow some sediment and detritus (decaying plant material). Some obtain nutrition from detritus, but for others it is “junk food.” Feeding styles vary from suction feeding for plankton and benthic invertebrates to taking bites out of larger prey. The structure of the mouth reflects how the fish feeds. Some fish that capture large prey can protrude their jaws so that the whole jaw apparatus moves forward. Once the food is in their mouth, teeth tear or break the prey’s skin. Additional teeth in the pharynx (throat) are also used to grind or mash food. Archerfish, Toxotes chataraeus (see fig. 8, in chapter 3), in mangrove swamps of the Indo-Pacific specialize in terrestrial food (ranging from flies to small lizards sitting on branches that hang over the water) that Impaired Feeding and “Junk Food” The killifish, Fundulus heteroclitus, a common resident of estuaries , can survive...