ASK A SCIENTIST
Question: How come only some sharks are able to lie at the bottom of the ocean and not swim, but other sharks like the great white shark or the tiger shark have to swim at all times?
Good observation, Morgan. One of the pictures many of us have in our minds is that of a "shark walker" gently guiding a shark around in a shallow pool to revive it after the shark was held captive for a short period. As you noted, there are many shark species, all requiring the flow of water across their gills to obtain oxygen. What is considerably different among sharks, skates and rays - those fish with skeletons of cartilage instead of bone - is the way they accomplish the act of respiration. The nose openings, by the way, are for smelling and not for breathing.
For open ocean sharks and rays constantly on the move straining out smaller food items from water or searching for larger prey, simply keeping the mouth open a little is sufficient to drive oxygenated water into the mouth, across the gills, and out the gill openings in a process called ram ventilation. Restraining such species that have no other way than moving to produce a flow of water across the gills will gradually suffocate them. And this is what occurs in such open water species as the great white shark and manta ray.
What about bottom-dwelling skates, rays and sharks that spend most of their time resting on the bottom, conserving energy until a time when food is more accessible or when it is safe to hunt? There is no water being forced into their mouths, and once more, even if they could suction water into their mouths like the rest of the fish species (with bony skeletons), their mouths on the underside of their bodies would take in a lot of sand or other debris off the sea bottom. This ventral mouth under and behind the snout tip distinguishes all sharks, skates and rays from the bony fishes, who instead have a mouth at the tip of their snout. So how has nature solved the problem of respiration for these bottom-resting species? The answer is that they have water-intake openings topside just behind each eye, looking like ear openings, but having an entirely different function. The name of these paired openings is the spiracle. The spiracle is reduced in size or absent in open water sharks and rays. It really is a modified opening of the forward most gill element that is converted in development from an outlet for respired water to an intake port. And its location migrates from the ventral surface where the gill openings are to the upper surface behind the eye to draw in clean water for respiration. Muscles surrounding the gill cavity gently contract and relax to generate a flow of water into the spiracle and out the gill openings, the mouth being kept closed throughout this process. Surprisingly, some bony fishes like the open-water tuna also have ram ventilation, but if tuna are restrained, their fish-like mouth pump jumps into action to keep sea water passing into the mouth and out across the gills; no "tuna walkers" are necessary.
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