Very little information is known about the natural ecology and behaviour of the basking shark. It receives its common name from its feeding behaviour, when individuals appear to be 'basking' on the water's surface, swimming very slowly with their entire dorsal fin out of the water (2). These sharks feed passively (unlike the also plankton-feeding whale shark and megamouth shark (Megachasma pelagios) which can use its head muscles to suck water into the mouth), merely by swimming through the water with their mouths gaping (2). As water passes over the gills, plankton are retained; a fairly large shark can filter roughly 1,500 cubic metres of water an hour (6). These giant fish have occasionally been observed leaping out of the water (2), which is probably related to social behaviour (12). Basking sharks are usually solitary, although pairs and groups of up to 100 individuals have been seen (2). This species mysteriously disappears from coastal waters in the winter months and it was recently suggested that they 'hibernate' in the deep water. It is also thought that during this time of low food availability basking sharks shed and then replace the gill rakers (11). This suggestion has been refuted by scientific satellite tracking of sharks, revealing extensive migrations throughout all seasons (13). The only pregnant female ever caught gave birth to six live young; the prevailing view is that that these sharks are ovoviviparous (8), and it is likely that they only give birth every two to four years (9).
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