Why Do Sharks Expose Their Dorsal

Fins Above the Surface?

Since sharks have gills, they do not need to go to the surface to breathe. Why do they go to the surface and expose their dorsal fins outside of the water?

Nicolas

Rimouski, Quebec

Great question, Nicolas!

In movies, sharks are often portrayed with their dorsal fins 'knifing' through the water. It makes for a dramatic image  the fins look sharp and kind of dangerous  and one that is easy to fake  it is far cheaper to build a convincing-looking fin than a realistic-looking whole shark (largely because  thanks do wonderful underwater documentaries like those on Discovery's "Shark Week"  most of us know how real sharks look and move). So, whenever movie-makers want to add a little 'danger' to an ocean-based story, all they have to do was show one or more phony shark fins above the surface.

But  in truth  sharks in the wild very, very rarely swim with their dorsal fins exposed above the surface. Most shark species swim fairly close to the bottom or in mid-water (the large expanse of ocean between the bottom and the surface). Left to its own devices, however, most sharks will not approach the surface at all. Sharks can be lured to the surface with floating bait and, in investigating such hand-outs, sometimes their dorsal fins break the surface of the water. Sometimes sharks enter water so shallow that they can barely swim, and  as a result  their dorsal fins sometimes poke through the surface. Many species of sharks, for instance, use shallow-water coastal areas as nurseries for their young (called "pups"); such places offer the newborn sharks plenty of food but relative safety from fish that are big enough to eat the pups. On the Queensland coast of Australia, I have seen groups of up to 48 newborn Blackfin Reef Sharks (Carcharhinus melanopterus) swimming in mangrove swamp waters so shallow that  from a distance  their fins sticking up through the water looked like a flotilla of tiny sailboats!

It is also not uncommon for largish sharks  such as the Tiger Shark (Galeocerdo cuvier)  to pursue prey such as schooling fishes and sea turtles into water so shallow that their dorsal fins, and sometimes their entire backs, are exposed above the water. Once, on a shallow part of the Great Barrier Reef of Australia, I saw a small (1-metre or 3-foot long) shark actually 'walk' completely out of the water. The shark was an Epaulette Shark (Hemiscyllium ocellatum)  a species named for the large, dark spot on its 'shoulder'. These prettily marked, slender sharks have highly mobile paired fins  pectorals behind the gills and pelvics behind the belly  which they use to clamber over the bottom, looking like a salamander with ping pong paddles for feet! Anyway, on one particular morning, I saw a little Epaulette Shark actually crawl out of the ocean, clamber clumsily over some 15 metres (50 feet) of exposed coral rock, and plunk into a pool of water that had been trapped by the out-going tide. Unfortunately for the little shark, a Spotted Snake Eel (Myrichthys maculosus  a small type of eel closely related to the undeservedly dreaded moray eel) was trapped inside the same tide pool, and wasn't at all happy about the little shark showing up uninvited. The snake eel wrapped its body around the Epaulette Shark, and the two writhed and spun in the shallow pool like play-fighting puppies. After a few seconds of this rough treatment, the little Epaulette Shark managed to disentangle itself from the eel and clambered out of the pool and back over the coral rock to the sea.

There may be a very good reason why sharks usually do not expose their dorsal fins or backs above the water. An American shark biologist named Chris Lowe and his wife, Gwen Goodman-Lowe, discovered recently that at least some sharks  like people  actually sun-tan. A part of the sun's light called 'ultraviolet radiation' can actually penetrate the water up to a distance of about a metre (three feet) with enough energy that it can cause skin damage to a human or shark; in response, the skin produces a dark pigment called melanin. Chris and Gwen demonstrated that young Scalloped Hammerheads (Sphyrna lewini) held in shallow-water outdoor pools actually tan in a few days. When Chris and Gwen covered part of the little hammerheads' skin with an opaque (non-see-through) patch, the only part of the shark that did not tan was that under the patch! I have seen a similar phenomenon with Nurse Sharks (Ginglymostoma cirratum) in the Florida Keys: when the sharks first move into shallow water in preparation for mating, they are a reddish or tan color; but after a few days, many of them have become a rich chocolate brown. At the time, I thought that perhaps the sharks were tanning, but no one had ever reported tanning in shark. Now, thanks to the work of Chris and Gwen Lowe, we know that it is possible.

Thus, if sharks can sun tan, it seems likely that they  like humans  can also sun burn. A dorsal fin poking out of the water for extended periods would receive much harsher ultraviolet radiation than one that is submerged. Sun burning could take place in a matter of hours, possibly resulting in a very cranky shark. So maybe it's a good thing for us that sharks usually don't go swimming about with their dorsal fins sticking out of the water!