The Press Reads: Sharks

Helfman_SharksOur occasional summer Friday series on the blog, The Press Reads, features short excerpts from recent JHUP books to whet your appetite and inspire timely additions to your summer reading list.  In advance of Shark Week, we dive into Gene Helfman and George H. Burgess’s Sharks: The Animal Answer Guide. Gene Helfman is a professor emeritus at the Odum School of Ecology’s Program in Conservation Ecology and Sustainable Development, University of Georgia and is co-author of Fishes: The Animal Answer Guide, also published by Johns Hopkins. George H. Burgess is coordinator of museum operations and director of the Florida Program for Shark Research at the Florida Museum of Natural History. To read the authors’ website for Sharks: The Animal Answer Guide click here.

 

What is the largest shark that ever lived?

If we disregard the speculated lengths of the living Whale Shark, the largest true shark in terms of weight to swim the seas (or lakes) occurred around much of the world during the mid-Miocene through the Late Pliocene, 16 to 1.6 million years ago. This creature was Carcharocles megalodon, the Megatooth Shark. Recent fossil evidence has confirmed the need to change the Megatooth’s genus name from Carcharodon to Carcharocles because we now know that modern White Sharks—Carcharodon—evolved from mako shark ancestors, not Megatooth ancestors. Researchers previously thought a direct connection existed between the Megatooth and White Sharks because of tooth similarities, but new fossil findings confirm separate ancestries and therefore identities (although not all experts agree on this change).

Giant teeth from the Megatooth Shark are found in fossil deposits on land and in the sea in Europe, Africa, Australia, India, Japan, North and South America, and some western Pacific islands. The largest teeth ever found came from South Carolina and Peru and measured 17.6 to 17.9 cm (7.25–7.37 in) from the tip to the end of the root, or base, with an enamel height of 16.8 cm (6.9 in). Paleontologists use a formula for calculating shark length based on tooth size, but they use enamel height rather than total tooth size because teeth vary greatly in root shape. They also make the reasonable assumption that the proportions of the Megatooth Shark are similar to those of the White Shark. A Megatooth Shark with an enamel height of 168 mm would be about 16 m (52 ft) long and weigh approximately 48,000 kg (105,000 lb). The jaws of such a monstrous shark would have been more than 1 m (3.25 ft) wide, the dorsal fin would be 1.4 m (4 ft) high, and the tail would be 1.75 m (6 ft) tall. Several people have reconstructed the jaws using entire sets of fossilized Megatooth teeth.

The Megatooth Shark, although probably the largest predatory shark that ever lived, occurred with other relatively gigantic 5- to 6-m long (16–20 ft) predators during the same time, including the Speartooth Mako (Isurus hastalis) and a hemigaleid, Hemipristis serra, a relative of the modern Snaggletooth Shark, Hemipristis elongatus. Megatooth Sharks also swam the seas with their smaller cousin, the White Shark.

Why do sharks swim at the surface with their dorsal fins sticking out?

Assuming we are talking about sharks in water deep enough to submerge their entire body, no one really knows the answer. This behavior certainly makes life easier for Hollywood filmmakers during dramatic, suspenseful moments, but that doesn’t help the sharks much. The key probably lies in thinking more in terms of why sharks swim near the surface, which they certainly do more than many other large fishes. That their dorsal fins stick out is an artifact of having a stiff fin protruding from their back more than a result of the shark actually trying to stick it out of the water.

Do sharks play?

Play must have evolved after sharks came on the scene. With a few possible exceptions, we are unaware of convincing examples of playlike behavior in sharks. But it’s a big ocean.

Some behavior by White Sharks has been interpreted as object play. Boat crews in South Africa have suspended cloth bags with bait. Known individual White Sharks grabbed hold of the bag while the crew pulled on the other end in a sort of tug-of-war, like a dog pulling on the other end of a sock. The sharks showed no indication that they were trying to eat the bag or its contents. These sharks also pushed floats around at the surface, with or without bait present. Is this play or something else? You be the judge.

Another, more intriguing possibility comes from the Porbeagle, a relative of the White Shark. Several websites describe apparent play behavior observed in groups of Porbeagles off the Cornish coast of England. They are said to roll over and wrap themselves in kelp, to chase other Porbeagles that have kelp draped over them and bite at the kelp, and to “play” with objects at the surface such as pieces of driftwood or fishing floats, including “passing objects from one individual to another and repeatedly tossing the objects clear of the water.” We have been unable to find the original reports of this behavior, and the web reports, all of which are secondhand, appear to repeat one another.

Do sharks talk?

Sharks, unlike bony fishes, are not known to make sounds. Bony fishes rub bones together, grind throat teeth, or vibrate their gas-filled swim bladders. Sharks have neither bones nor gas-filled bladders; grinding teeth would be possible only in rays or maybe horn sharks. Other than incidental sounds associated with crushing prey, sharks maintain silence (despite the roaring sharks that seem to populate many of the grade B shark attack movies). However, sharks do locate bony fishes by eavesdropping on their prey’s sounds.

How do sharks avoid predators?

Sharks avoid or deter predators in four major ways: (1) by running away; (2) by hiding, either in a hole or in plain view but with camouflage coloration; (3) by defending themselves with teeth, spines, toxins, or other structures and chemicals; or (4) by being hard to chew or swallow. Chondrichthyan fishes do all four.

What is a baby shark called?

The young of many bony fishes are called fry, and some have special names—alevins, parr, and smolt in salmon; elvers in eels; and ammocoetes in lampreys. In addition, many bony fishes have very small larvae that are very different from the adults, and these larvae have their own names. But shark babies are miniature adults, not distinct larvae. Baby sharks are usually called pups.

What can an ordinary citizen do to help sharks?

A lot! Some actions are quite simple:

■ Refuse to eat sharks, and make your refusal known at restaurants and seafood counters. Encourage your friends and family to also refuse to eat sharks. Some shark populations can sustain well-managed fisheries, but most can’t, and seafood sellers seldom know where their sharks come from and whether they were caught sustainably. There are many other tasty kinds of fish on the menu. To know which ones to avoid or consume, consult the readily available Seafood Watch guides such as the ones produced by the Monterey Bay Aquarium.

■ Support anti-finning campaigns. Write letters to politicians and government agencies supporting all bans on shark finning.

■ Discourage shark fishing “kill” derbies by writing letters to local papers pointing out that many of the sharks caught in these “contests” are declining in numbers. Such derbies are commercial enterprises organized to promote tourism and sport fishing. Businesses hate the negative publicity created by a well-written letter to the editor of a local newspaper.

■ Most importantly, stay informed. The avalanche of information on sharks and shark conservation grows annually. It may appear hard to keep up, but a few sites troll the scientific literature and media sources and summarize current information. If you want access to the best science on shark conservation, check out the websites of the IUCN Shark Specialist Group and the Florida Museum of Natural History, the latter maintained by the second author of this book (GHB) and his dedicated staff. Other useful sites include the Global Shark Conservation Campaign of the Pew Charitable Trusts and Shark Year Magazine.

 

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