GUEST COLUMNIST: TONY PORCO

Q: What is that striped fish that is upside-down on the Nurse shark?


Remora on back of nurse shark

A: That fish is called a remora. Remoras, also known as sucker fish or shark suckers, are usually found hitching rides from sharks, sea turtles, rays, whales, dolphins, and other large sea animals. They also attach themselves to the hulls of boats and ships. There is even one recorded instance of a remora latching onto a human being, a National Geographic writer who was researching a story about the Great Barrier Reef near Australia. (The diver was unharmed, and figured that it was a sign he should be coming out of the water!)

The remora's "sucker" is actually a modification of the first dorsal fin that is located on the backs of many fishes, including sharks. The disk-shaped surface has small barbs on its edges that allow it to hook onto an animal to which it wants to attach itself. When these hooks are in place, powerful muscles then raise the rim of the disk to create a small vacuum between it and the skin of the host animal. This makes the sucker similar to a human-made suction cup. The sucker's grip is quite strong. When the New York Aquarium conducted an experiment to test its strength, an aquarist was able to pick up a bucket holding twenty-four pounds of seawater by picking up the tail of a remora attached to it. The remora can dislodge itself at will from the host animal or object. Remoras usually attach themselves to the outside skin of their hosts (the remora in the aquarium collection normally rides on the larger of the two nurse sharks, attaching itself to the shark's body just behind the head). However, young remoras sometimes inhabit the mouths of large sharks and manta rays! These young acquire their sucking disk when they are about 3/4 inch long, and it becomes usable when they are about twice that size. (Full-grown remoras can reach a length of three feet.) While they can swim by themselves, remoras seem to prefer to spend as much time as possible hitching rides. Patient visitors to the aquarium will sometimes see the remora in the shark tank swim away from the large nurse shark, often for several minutes.

Remoras are not parasites of their hosts, and do not do them any harm. Instead, when their host eats, they feed on leftover scraps of whatever their host is eating. This kind of biological relationship, in which one organism benefits while the other is neither helped nor harmed, is known as commensalism. (Some remora species, however, seem to act as "cleaner fish," eating some small parasites and dead skin from the host's body. It is unclear if this has significant benefits to the host. The major benefit that the remora receives is protection from predators through association with a larger animal.)

Humans and remoras have had an interesting relationship themselves over the years. In his book The Shark: Splendid Savage of the Sea, Jacques-Yves Cousteau related this remora story from Madagascar, an island nation off the east coast of Africa:
 

"On a sand beach off a tiny island in the Mozambique Channel, northwest of Madagascar,
there exists the only shark fishery of the entire island continent. Here, an old Arab who
does not share the beliefs of the Malagasy natives [that sharks are the reincarnations of
human ancestors and should therefore not be fished] stretches out lines from the edge of
the shore and catches an appreciable number of sharks every night. As recently as a few
years ago, he also gathered in the remoras-the extremely powerful sucking fish that
attach themselves to sharks-and sold them, alive, to the tribes of fishermen in the other
islands in the channel. The new owners of these fish attached them by the
tail to a solid length of fishing cord and then set them free in the waters along the
barrier of the reefs. Once liberated in this fashion, the remoras often attached
themselves to others of the large fish or to the turtles in which those waters abound,
and the hosts they had selected were then hauled in and sold. This delightful custom
has now practically disappeared, and with it has gone a tidy source of income for
the shark fisherman."
Similar uses of the remora for fishing sharks or turtles have been reported in Australia, Japan, and China. Remoras also figured in legend many years ago. The Roman leader Mark Antony's loss at the naval battle of Actium, in 32 BC, was blamed on remoras that kept his flagship from moving as fast as it could have. This blame was, of course, unfair; when ships were in the doldrums, a sailor was usually sent overboard to examine the hull to look for problems. Since there were almost always remoras on the underside of the hull, the fish were probably blamed.

Like most fish (and unlike sharks and rays, which have skeletons made of flexible cartilage), remoras have hard bones in their bodies. Their biological family classification (Echeneidae) is one of the many belonging to the order Perciformes, or perch-like fish. This order includes many well-known species of fish, such as mackerel, marlin, tuna, groupers, and, of course, perch themselves. The remora is actually more closely related to these fish than to the shark, the animal with which it will always be associated in the human mind. TP

BIBLIOGRAPHY:

Allen, Thomas. The Shark Almanac. New York: Lyons Press, 1999.

Banister, Dr. Keith, and Dr. Andrew Campbell, eds. The Encyclopedia of Aquatic Life. New York: Facts on File, 1986.

Burton, Maurice, and Robert Burton, eds. The Marshall Cavendish International Wildlife Encyclopedia. New York: Marshall Cavendish, 1991.

Chadwick, Douglas. "Great Barrier Reef." In National Geographic, Volume 199, No. 1, January 2001.

Cousteau, Jacques-Yves. The Shark: Splendid Savage of the Sea. New York: A & W Visual Library/Doubleday, 1970.

Moyle, Peter. Fish: An Enthusiast's Guide. Berkeley: University of California Press, 1993.

Porco, Tony. Personal observation of remora (Echeneis naucrates) in the collection of the National Aquarium, Washington, DC, 2001.

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