Lower metazoans demonstrate a remarkable variety of feeding mechanisms. Most sponges are suspension feeders that subsist on such fine particles as bacterioplankton and dissolved organic matter. Sponges acquire food and oxygen from water that flows through them; this flow is actively generated by sponges beating their flagella (microscopic whiplike structures). This process also acts as a means of waste removal for sponges. The movement of water through sponges is aided by ambient currents passing over raised excurrent (providing outward passage) openings, which creates an area of low pressure above these openings. Sponges are also capable of regulating the amount of flow through their bodies by narrowing or partly closing off various openings. The volume of water passing through a sponge can be enormous—as much as 20,000 times its volume over a 24-hour period.
Sponges are size-selective particle feeders. Their aquiferous systems create a series of "sieves" of varying mesh size. The largest diameter of incurrent openings is usually around 0.002 in (50 pm), which keeps larger particles from entering the aquiferous system. A few species have larger incurrent pores, reaching diameters of 0.006-0.0069 in (150-175 pm). Some sponges trap roughly 90% of all bacteria in the water they filter. Other sponges also take significant amounts of dissolved organic matter into their aquiferous systems. In some demosponges, 80% of the organic matter that is filtered through their aquiferous system is too small to be seen by light microscopy. The other 20% is composed primarily of bacteria and dinoflagellates. Other sponges harbor symbionts such as green algae, dinoflagellates (zooxanthellae), or cyanobacteria, which also provide them with nutrients.
Many invertebrates are predators that feed on protozoans, other invertebrates, and fishes. The discovery of several Mediterranean species of sponges that capture and digest entire animals came as a surprise to marine biologists. These species of the family Cladorhizidae have no choanocytes or aquiferous systems, but anatomic and biological analysis revealed the presence of spiky filaments with raised hook-shaped spicules. These carnivorous sponges capture and hold small crustaceans with their spicules, which act like Velcro® tape when they come in contact with the crustaceans' ex-oskeletons. Once captive, the crustaceans cannot free themselves. They struggle for several hours, which indicates that the spicules do not produce any paralyzing or toxic secretions. Cells then migrate around the helpless prey, and digestion takes place outside the cell walls.
Most cnidarians are carnivorous, using cnidocytes on their tentacles to capture prey. Polyps, the sessile stage of cnidar-ians, are generally believed to be passive predators, feeding on animals that blunder into their tentacles. Some cnidarian medusae possess sensory structures resembling primitive eyes; they are active predators. Many corals and anemones feed by suspending thin strands or sheets of mucus over the surface of their colony. The sticky mucus collects fine particles of nourishment from the water; cilia present on the organisms drive the food-laden mucus into the mouths of coral or anemones. Many species have developed complementary adaptations such as ectodermal ciliary currents on their tentacles, oral discs or columns, or the ability to position themselves strategically within the water's flow pattern.
In sea anemones, the presence of nearby food evokes behavior that has two phases: a prefeeding, and a feeding response that leads to the ingestion of prey. The prefeeding response consists of the expansion of the oral disc, the movement of its tentacles, and both the extension and swaying of the column. This prefeeding behavior increases the chances of catching nearby food. The feeding response, which takes place after the prey has made contact with the anemone's tentacles or oral disc, includes the discharge of nematocysts and ingesting movements. Sea anemones are able to detect prey from the prey's emission of small dissolved molecules of
amino acids, tripeptides, and vitamins. The cnidae, which ensure the capture of the prey, are spirocysts and nematocysts. The numerous spirocysts on the tentacles of sea anemones appear to have an adhesive function. The contact of solid food with the tentacles leads to a massive discharge of spirocysts, which hold the prey while the nematocysts inject their toxin. Ingestion is directed by the chemical and mechanical stimuli produced by the immobilized prey. After ingestion, the prey is enclosed by filaments whose cnidoglandular tracts contain nematocysts but no spirocysts. The penetrating filaments of these nematocysts inject more toxin. The prey is then subject to the action of secretory cells that ensure its extracellular digestion.
The prey of sedentary cnidarians is composed of small motile animals such as zooplanktonic larvae, isopods, am-phipods, and polychaetes. Sea anemones found closer to shore may complete their diet with larger sessile prey dislodged by wave action or foraging predators. The size of the prey is generally small considering the diameter of the sea anemone, and many species may be considered microphagous. Of the nine common species of Caribbean reef sea anemones, seven are planktivores. Condylactis gigantea and Stoichactis helianthus, which eat macroscopic prey such as gastropods and echinoids, probably depend on heavy wave action in the reef to supply them with prey.
Comb jellies are entirely predatory in their habitats. The long tentacles of ctenophores have muscular cores with an epidermal cover that contains colloblasts, or adhesive cells. The tentacles trail passively through the water or are twirled about by various circular movements of the body. Upon contact with the prey, the colloblasts burst and discharge a strong sticky material. In ctenophores, which bear very short tentacles (orders Lobata and Cestida), small zooplankton are trapped in mucus on the body surface and then carried to the mouth by ciliary currents (along ciliated auricular grooves in ctenophore lobates and ciliated oral grooves in cestids). Under conditions of starvation in an aquarium, lobate adults often swim vertically through the water only to descend with their lobes extended; in this position the width of the lobes may be as much as 116% of the animal's length. The lobe width decreases to about 79% of the body length after food is placed in the aquarium and the animal begins to feed. Once the digestive tract is full, these adults still continue to feed by entangling their prey in mucus, which produces a bolus or clump near the mouth. Quite frequently they will either spit out this ball of food or completely empty their digestive tract and continue to feed. This behavior pattern can continue for several hours until the concentration of food is reduced to the point at which all the prey that have been captured can be ingested.
Most pelagic ctenophores and cnidarians feed primarily on copepods. There are a few intriguing examples of pelagic cnidarians that feed exclusively on one type of prey. The siphonophore Hippopodius hippopus feeds only on ostracods; the hydromedusae Bougainvillia principis feeds mainly on barnacle nauplii; and Proboscidactyla flavicirrata eats only veligers (mollusk larvae). Some cnidarians and ctenophores feed specifically on gelatinous prey or fishes. Ctenophores of the genus Beroe offer well-known examples of selective feeding on gelatinous prey: B. cucumis feeds exclusively on the ctenophore Bolinopsis vitrea, and B. gracilis feeds only on Pleurobrachia pileus. When beroids prey on animals larger than themselves, they appear to attach themselves to the prey and suck its tissues into their mouths. Beroids lack tentacles; however, they do possess some 3,000 macrocilia that are hexagonally arranged and form a ciliary band around the inside of the mouth that beats inward, and forcing tissue from the prey into the beroid's pharynx. Gelatinous species that include high proportions of soft-bodied prey in their diets often eat fish eggs and larvae when they are available.
The diets of gelatinous predators generally show some selectivity, and are dependent on factors including the prey's size; the width and spacing of the predator's tentacles; the predator's swimming behavior and speed; water flow; and the prey's ability to escape. In general, species that catch large prey have few and widely spaced tentacles, while those that feed on small prey have numerous closely spaced tentacles. Most gelatinous predators move while feeding, which allows them to make use of water currents that will bring prey toward their tentacles. Cnidarians that are ambush predators are able to catch large, fast-moving prey; cruising predators prefer small, slow-moving prey. For example, siphonophores, which are ambush predators, tend to select large and relatively swift prey, while Aurelia aurita, which is a cruising predator, selects slow-moving organisms. Swimming offers the advantage of allowing predators to scan larger volumes of water; however, it also has the disadvantage of alerting the prey to the predator's presence. Some predators deal with this disadvantage by remaining stationary while they are "fishing."
Swimming is not the only way for prey to escape. Bivalve veligers will close themselves off when they are disturbed by the medusae of Chrysaora quinquecirrha; 99% of captured veligers are ingested alive.
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