Fish Feeding
Fish can be classified broadly on the basis
Of their feeding habits as detritivores, herbivores, carnivores, and omnivores. Within these categories fish cane characterized further as (1) euryphagous, having a mixed diet; (2) ste-nophagous, eating a limited assortment of food types; and (3) monopha-gous, consuming only one sort of food. A majority of fishes, however, are euryphagous carnivores. Often the feeding mode and food types are associated with the body form and digestive apparatus. For example, longer guts with greater surface areas typify species that feed on detritus and algae and take in a high percentage of indigestible material such as sand, mud, or cellulose. In contrast, carnivorous species tend to have shorter gut lengths. Among carnivorous fish, however, gut lengths are often greater in those fish that prey on small organisms (relative to their own size) than those that prey on large organisms. Thus, the herbivorous, euryphagous Sacramento blackfish (Orthodox microlepidotus) has vastly longer gut than the carnivorous Sacramento squawfish (Ptycho-cheilus grandees), which feeds largely on other fish. The Sacramento hitch (Lavonia exilicauda) has an intermediate gut length correspond-in to its diet of small zooplankter’s (Kline 1978). Digestive area can also be increased through the use of spiral valve intestines, found in the Chon-drichthyes and in “ancestral” bony fishes, such as sturgeons and lung a spiral valve is a longitudinal fold which spirals down the length
The intestine, much like a spiral staircase down a lighthouse.
Structures in the buckle-pharyngeal cavity often correlate with food and feeding habits. For example, the pharyngeal pad (or palatal organ) situated dorsally at the entrance to the esophagus has been implicated in removing excess water from the ingested food of carp (Cyprians carpio) (Jar 1957). In contrast, the pharyngeal valve hanging from the roof of the pharynx of scared fishes probably assists in the placement of pieces of coral for grinding by the pharyngeal teeth and
Frication from epithelial mucus cells (Kapok et al. 1975). Likewise, the bony or cartilaginous gill racers which protrude from each gill arch
Be specialized for modes of feeding.
Many of these gill racer specializations can be found in the sunfishmily (Centrarchidae). In piscivorous members of the family, such as largemouth bass (Micropterus psalmodies), the gill racers are short, out, widely spaced, and pointed. They function mainly in preventing prey from escaping through (and damaging) the gills but may also partially rescale fish as they pass by the racers. The shortest and stubbiest gill racers in the family are possessed by members, such as the rehear sunfish (Loomis microlophus), that are specialized for feed-in on snails. Gill racers of intermediate length, thickness, and spacing are found in euryphagous species, such as bluegill (L. machrochirus).The longest and finest gill racers in the family are those of the crappies(Pomoxis spp.), reflecting the importance of zooplankton in their diets, which they “pick” individually from the water column. Among fishes in general, the finest, most closely spaced gill racers are possessed by species which strain plankton out of the water, such as the Atlantic menhaden (Brevoortia tyrannous). Large schools of this abundant clupeid decrease the phytoplankton as well as the dissolved oxygen concentrations of the waters they swim through (Obviate et al. 1972). Dense con-cent rations of the food organisms in the water stimulate faster swimming by the menhaden (to 2.5 body lengths per sec). Durbin and Durbin (1975) showed that the feeding response of menhaden is linked to the presence of comparatively large zooplankton or the larger phytoplankton rather than to much greater densities of the small phytoplankters, which are filtered less efficiently.
Mouth structure is also related to the feeding modes and habits offices. Mouth structure is highly variable, and this variability explains in part the evolutionary success of both teleports and elasmobranches. The “ancestral” mouth consists of firm jaws lined with sharp teeth for grasping active prey. Such jaws are still possessed by many piscivorous fishes (such as barracuda and pike). More common among modern fishes are jaws modified for suction feeding. In suction-feeding fishes the jaw is shortened to limit the gape while the expansibility of the orobranchial (mouth and gill) cavity is maintained, resulting in increased water velocity through the smaller mouth when the cavity is expanded or contracted. In elasmobranches, such as skates (Raided), rays (e.g., Myliobatidae), and nurse sharks (Orectolobidae), the strong oral suction created allows them to feed effectively on benthic invertebrates. In teleports anterior extension of the hyomandibular bones and lateral expansion of the opercula apparatus allows the orobranchialcavity to enlarge quickly and create strong suction. Alexander (1970) trained fish to take rings of earthworm off the free end of a nylon tube connected to a pressure transducer and recorder. Negative pressures from -80 cm water (18°C) in the temperate black bullhead (Ictalurusmelas) to -400 cm water (27°C) in the tropical butterfly fish (Pterophyl-lum scalar) were measured.
The feeding of many sharks represents an unusual type of predation that of taking bites from prey larger than the predator. The saw-tooth edges on the awl-shaped teeth of the lower jaw and the bladelike teeth of the upper jaw teeth, coupled with a head-shaking action, pro-vide for efficient cutting through flesh of larger, slow-moving or disabled animals. The well-developed jaw musculature and the unique position of the hyomandibular cartilages provides support and positioning forth jaws when the mouth is open to effect a deep, gouge like bite (Moss1977). In contrast, filter-feeding elasmobranches such as the basking shark (Cetorhinus), whale shark (Rhino o don), and manta rays (Mobulidae) have comparatively weak jaw musculature and reduced dentition. In-stead, numerous, elaborate gill racers function to strain small organisms from immense ventilator water flows.