Hot! Whale Shark

Coral Reefs: Diversity on Display Whale Shark

The gigantic, polka-dotted Whale Shark (Rhincodon typus) is the largest living fish. Exactly how large Whale Sharks can grow is a subject of intense fascination and debate. Reports of individuals estimated to be as long as 60 feet (18 metres) persist, although the largest actually measured was about 45 feet (13.7 metres) long and the largest recently measured and reported in the scientific literature was 39 feet 8.5 inches (12.1 metres) long.

Even a mere 45-foot (12-metre) Whale Shark is an awfully BIG fish such a shark has a mouth almost 6 feet (2 metres) across, gill slits over 5 feet (1.5 metres) in length, pectoral fins 8 feet (2.4 metres) long, a maximum girth of about 22 feet (6.7 metres), a first dorsal fin about four feet (1.2 metres) tall, a caudal fin spanning some 16 feet (4.8 metres) and a total mass of about 13 tons (12 tonnes). But these mere numbers cannot convey the sheer enormity of these animals. Until one experiences being in the water next to a free-swimming Whale Shark, such figures are meaningless and without implication.

Despite their great size, Whale Sharks have tiny teeth and feed largely on plankton. It may seem strange that the largest shark in the sea feeds on some of its smallest creatures. But feeding on tiny, super-abundant prey is highly advantageous from an energetic standpoint. Such planktonic animals termed zooplankton occupy a trophic (feeding) position near the base of the food web, where most of the sun s radiant energy remains trapped within their tiny bodies. Because animals are inefficient at converting calories into tissues, roughly 90% of the energy available at a given trophic level is lost to each subsequent level. An upper-level carnivore such as the Shortfin Mako (Isurus oxyrinchus) is removed from the tiny, planktonic photosynthesizers at the base of the epipelagic food web by four or five trophic levels. In comparison, the Whale Shark feeds primarily one or two trophic levels above the phytoplankton. Thus, only one thousandth or ten thousandth as much energy is available at the trophic level of the Shortfin Mako as is available to the Whale Shark at its trophic level. It is no coincidence that the very largest whales feed almost exclusively on zooplankton, occupying a trophic level that broadly overlaps that of the Whale Shark.

Unlike filter-feeding whales, however, the Whale Shark is not dependent upon zooplankton as food. In addition to tiny planktonic crustaceans such as krill, crab larvae, and copepods, the Whale Shark s diet includes larger, nektonic prey, such as small schooling fishes, squids, mackerels, and even small tunas. There are also reports of phytoplankton, algae, and kelps among Whale Shark stomach contents. Thus, it is a gross oversimplification to categorize Whale Sharks as planktivores , for they feed on a wide variety of organisms from plants and tiny drifters to surprisingly large, powerful swimmers.