Sunday, March 17, 2002		Nature

They ride on sharks for survival
Nutan Shukla

IT takes a brave fish to approach and attach itself to one of the most feared killers of the deep sea. But convenience, more than bravery, is the key when a remora fish hitches a ride on a shark. Whales, manta rays, other large fish and turtles also often have one or two free-riding remora fish firmly attached to their bodies.

Remoras, which may be up to 3 ft long, fix themselves to the underside or topside of their large, slow-swimming hosts by means of a flat, oval sucker on the top of their head, so they are also known as suckerfish or sharksuckers. Suckers are in fact modified dorsal fins and the flat fin rays inside it resemble the ridged sole of a shoe. Whenever the fish feels it presses its head suckers onto a host before pulling the centre of the cup away to create a vacuum, thus it attaches itself to its cruising companion, no matter how quickly the host is travelling.

This method of travelling has many benefits for remoras. The host’s body becomes a platform for a free ride. It is also an effective shield from predators and a hiding place from which the remora can dart out after prey.

The host transports it to new feeding grounds, as well as provides it with scraps of food that fall from its mouth. Sharks are particularly messy eaters, so there are always plenty of tidbits for their travelling companions. Some of these fish swim close to the host without actually attaching themselves to its body, but they follow the creature’s every twist and turn. Some remoras have been seen in mouths and gill chambers of their hosts, behaving like cleaner fish. Certainly, the stomach contents of remoras include a high proportion of parasitic copepods and isopods.

In the Yucatan caves, where large sharks go to bathe in fresh-water currents, remoras have been seen to eat the parasites that have been dislodged. In the Bimini lagoon, remoras dart in to eat the afterbirth debris following lemon shark births.

If no host is available, remoras sometimes swim round in circles, stacked like dinner plates one above the other — the smallest at the bottom and largest at the top. They swim round in this way until a suitable host comes along.

Remoras have appeared in ancient tales. They were thought to slow down ships; indeed, the Greeks called it the ‘ship-holder’. Emperor Caligula was delayed on his voyage to Antium by remoras, as was Mark Antony’s ship at Antium. Mark Antony lost a battle and Caligula lost his life. Remoras have also been ground up and used in potions to ‘delay’ childbirth and extend lovemaking.

In Madagascar, dried remora was placed around the neck of an unfaithful spouse in order that he or she would return to the partner and ‘stick’. And there were stories told by Christopher Columbus of natives in the Caribbean tethering remoras on lines and getting them to attach to sea turtles that could then be hauled in.

Some hangers-on, like black-and white, zebra-striped pilot fish, do not actually ride, but are ready to grab a titbit from their outsize partners, usually a shark, ray or giant grouper. At one time, it was thought that they guided the larger swimming companion, but it is more likely that they are seeking a safe refuge; and what better place than near the business end of a shark? Pilot fish are careful not to get too close to the jaws, but copy every move the larger fish makes, perhaps trying to ride on the pressure wave ahead of its snout.