Large Plankton-eating Sharks
Huge Fish Filter Small Animals from Surface Waters
There are only three species of shark that filter-feed. They are all very large and swim slowly through the water using their modified gills to sieve out the animal plankton.
The Basking Shark
Cetorhinus maximus, which is confined to cooler waters, is enormous, and it can reach almost 40 feet. Water is filtered passively – the Basking Shark simply swims slowly with its mouth wide open. Water passes over the gills and the food is filtered out by the gills and swallowed. Plankton in the cooler water favoured by Basking Sharks tends to ‘bloom’ seasonally. The mixing of water during winter storms provides nutrients, and the returning sunlight of spring leads to a massive increase. Summer is usually less productive, but there is another blooming before the winter sets in. With less plankton available at the surface in winter the Basking Shark goes deeper and is thought to feed on whatever plankton can be found.
It was once thought that Basking Sharks might hibernate in winter, but recent research using tags tracked by satellite has shown that they can go remarkably deep (more than half a mile down) and travel long distances. One was tagged in British waters and swam to Canada, well over five thousand miles, before dropping its tag 100 days later off the coast of Newfoundland.
The world population of Basking Sharks is thought to be somewhere in the region of 8,000. Their habit of swimming slowly at the surface, and their tolerance of approaching boats, has made them an easy target and they continue to be heavily overfished in parts of their range. The most valuable part is the fin, for shark fin soup. Collecting shark fins accounts for millions of sharks each year and is considered one of the major reasons for the decline in shark numbers worldwide.
The Whale Shark
Rhincodon typus lives in warmer waters than the Basking Shark, and it is the largest fish in the world at over 40 feet long and weighing as much as 20 tonnes. They use their sense of smell to help locate their prey, then the huge mouth opens and the water is actively pumped over the gills. Whatever gets caught on the gills (mostly plankton, but often including small fish that could not swim out of the way) is then swallowed.
The Megamouth Shark
The third shark that feeds on plankton is the Megamouth, Megachasma pelagios. It is very rare and was only discovered in 1967. Very few have been seen, caught or filmed – so very little is known about it. Unlike the other two filter-feeders the Megamouth prefers deeper water. There are special light emitting organs inside the mouth, and it is thought that these might help lure small fish and plankton. Like the Whale Shark the Megamouth actively seeks its prey and filters the water by pumping.
