Secrets of the Ocean Realm |

Instructional Objectives
Background Information
Activity
Procedures
Elementary and Secondary Extensions

Topic: Scavenging Stomachs

Instructional Objectives:

Students will:

Learn about the role played by sharks in marine food chains.

Conduct a scavenger hunt to collect items similar to those recovered from shark stomachs.

Create an informative and entertaining natural history exhibit for their school.

Background Information:

Sharks are among the most successful and ancient groups of fishes. Fossil evidence indicates they have existed for more than 400 million years. Presently, there are more than 370 species of sharks known to science. Existing sharks range in size from the whale shark-which, at up to 60 feet, is the largest fish-to tiny sharks that are mere inches in length at maturity. Small sharks are not unusual; in fact, many reach maximum lengths of just a few feet, and most sharks are no larger than an average adult human being. Only a handful of sharks, including the blue, white, hammerhead, and reef shark, are considered dangerous to humans, and the chances of being attacked by even one of these is extremely remote.

Contrary to popular belief, not all sharks must keep swimming in order to breathe. The horn shark, swell shark and sand tiger shark are examples of sharks that can breathe while motionless. Sharks are also widely adapted to an amazing range of habitats-from the deep sea to fresh water. They hunt using special senses, detecting electrical fields emitted by other animals and feeling vibrations given off by movements in the water. Some sharks can smell a blood concentration in the water as small as one part in one million! In spite of their reputation as awesome and invincible predators, sharks perform a variety of essential roles in the balance of nature. They cull out old, injured and diseased animals, leaving healthier populations to survive and reproduce. They are also very effective scavengers, consuming even garbage and other human debris.

Activity: Shark Stomach Scavenger Hunt

Time Needed For Activity: One to two school weeks

Target Grade Level: Middle School

Materials:

The following is a partial list of items that have been removed from the stomachs of sharks. Have students conduct a scavenger hunt in their homes, neighborhoods and around businesses for as many of these items as they can collect. Be sure they explain that they are collecting for a school science display on sharks-most people find this very inspiring!

automobile license plates
shoes
clothing
musical instruments
hubcaps
bones
buoys
fishing tackle
tarpaper
necklaces
gunny sacks
human limbs (use mannequin parts)
fishes and crustaceans (use replicas or taxidermy specimens)
animal furs and hides

Procedures:

Caution: students conducting door-to-door and/or business-to-business scavenger hunts should be chaperoned. Secure the cooperation of parents or guardians. Results are best when students work in teams.

Allow a number of days (up to two weeks if necessary) for the scavenger hunt to yield a variety of items.

Secure the cooperation of the school administration to display the accumulated booty and signage in a prominent display case, or on a table in the school cafeteria (the latter is surprisingly effective!). Add newspaper clippings about shark-related incidents and magazine photos of sharks in action.

Extension for Elementary:

Discuss the role of sharks as the ocean's great scavengers. What would happen to other marine animal populations if sharks were eliminated? (This is a very real possibility in some fisheries.) Consider the loss of sharks in terms of human activity. Would the ocean be cleaner, healthier and safer without them? How much safer would humans really be if all dangerous sharks were exterminated? Fact to consider: the chances of being attacked by a shark are one in 100 million; on the other hand, commercial fishermen catch 100 million sharks every year! Who should fear whom?

Extension for Secondary:

Remember that during the program, mating hammerheads sank to the seafloor when they embraced and stopped swimming. Also according to the program, sand tiger sharks gulp air at the surface in order to hang motionless in the water column. It is widely known that most sharks must keep swimming in order to breathe; they must also do so in order not to sink. Why is this? Unlike most fishes, sharks lack gas bladders with which to maintain neutral buoyancy in the water. Instead, they possess large livers; the oil within the livers provides them with much of their needed buoyancy.

Conduct an experiment on the physics of buoyancy. Using an aquarium or another large, clear water vessel, test and measure the buoyancy of a gas-filled balloon and an oil-filled balloon, each attached to a small balance weight. Have students observe and record the results of each immersion test.

Summing up the experiment's results, point out how the gas-filled balloon will behave much as a fish's swim bladder does, providing positive buoyancy when fully inflated, neutral buoyancy when partially inflated, and negative buoyancy when under inflated. The oil-filled balloon will behave much as a shark's liver does, providing the same range of buoyancy-positive when full, negative when empty, and neutral when partially filled, though without the more dramatic results of the water-filled balloon. Conclusion: the fish's gas bladder is the more effective organ.

Students may note that the air-filled balloon will "want" to completely emerge from the water whereas the oil-filled one may not. Point out how this is an indication of the much lower density of water compared with oil-the water is effectively squeezing the air-filled balloon out and onto its surface.

What happens when a water-filled balloon is immersed to different levels without an attached weight? Point out how the density of water supports itself equally, and therefore a water-filled balloon with no additional weight will remain more or less motionless at whatever level it is immersed. This balloon represents those animals without buoyancy organs of any kind-they are the drifters that tend to hang suspended in the water column regardless of their depth.

Sea Dwellers

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