Fish o' the Day - thresher shark

Thought it was time for another shark. In a couple of weeks, most of marine biology types will heading off the Sea Lab in a few weeks to start their Sharks and Rays short course - here's a little something to whet their appetities.

The thresher sharks are group of several species (three or four, the geneticists are working it out) in the genus Alopias. They're primarily open water animals, seldom seen in inshore waters, and are cosmopolitan in tropical and temperate oceans. The common thresher (A. vulpinus) is most common in shelf waters. Threshers can be big - the common thresher A. vulpinus can reach lengths of about 20 feet and weigh more than a half ton. The pelagic thresher (A. pelagicus) is the smallest, topping out at about 10 feet. That's a pelagic at left. Of course, a significant part of the length is made of the long tail which may be the same length as the rest of the body. They are ovoviviparous, with eggs hatching internally and the young being delivered at sizes in excess of a meter in length. Like the sand tiger, they're known for intrauterine cannibalization among embryos.

Threshers feed primarily on schooling fish, and have been observed to use their huge caudal fin to stun baitfish prior to feeding on them. Threshers are acrobatic, and are one of only a few shark species known to leap clear of the water surface.

There are no verified acounts of thresher attacks on humans, although there's a really dramatic ocean legend of one decapitating a fisherman with its tail.

All three of the recognized species are listed by the IUCN as vulnerable; if we decide that there's a fourth species, it will be too.