Unlike the megamouth and whale sharks, the basking shark does not appear to actively seek its quarry but it does possess large olfactory bulbs that may guide it in the right direction. The basking shark is a passive filter feeder, filtering zooplankton, small fish, and invertebrates from up to 2,000 tons of water per hour. It is believed they may exist to lure plankton or small fish into its mouth. The megamouth shark has luminous organs called photophores around its mouth. Whale sharks have been observed "coughing" and it is presumed that this is a method of clearing a build up of food particles in the gill rakers. Any material caught in the filter between the gill bars is swallowed. This fine sieve-like apparatus, which is a unique modification of the gill rakers, prevents the passage of anything but fluid out through the gills (anything above 2 to 3 mm in diameter is trapped). During the slight delay between closing the mouth and opening the gill flaps, plankton is trapped against the dermal denticles which line its gill plates and pharynx. The whale shark sucks in a mouthful of water, closes its mouth and expels the water through its gills. In addition to these bony fish, four types of cartilaginous fishes are also filter feeders. They are also a natural check to the deadly red tide. Adult menhaden can filter up to four gallons of water a minute and play an important role in clarifying ocean water. For example, the Atlantic menhaden, a type of herring, lives on plankton caught in midwater.
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