An incredible diversity of marine life lives in Western Australia's Shark Bay, an ecosystem of great interest to marine scientists. The scientists have studied the differences between green sea turtles and loggerhead sea turtles in this area.

These turtles have quite different physiques, diets, and diving and breathing patterns. This last factor in particular accounts for the green sea turtle being ten times less likely to be eaten by a tiger shark than the loggerhead turtle.

In this lesson, students will consider the reasons for the significant difference in vulnerability between the turtle species. They will be asked to think critically about hints they'll be given concerning Shark Bay's sea turtles and their ability to evade tiger sharks.

Students will conduct an investigation to "discover" information that will help them answer questions.

Geography, life sciences

Standard 8: "The characteristics and spatial distribution of ecosystems on Earth's surface"

One to two hours

• Computer with Internet access

Students will

• discuss ways that animals avoid predators;
• look at pictures of green sea turtles and loggerhead sea turtles, and answer questions about their appearances and physiques;
• discuss which turtle species seems most likely to be eaten by tiger sharks;
• brainstorm reasons that may account for the green sea turtle's reduced vulnerability to tiger sharks;
• compare their predictions to actual Crittercam findings; and
• perform puppet shows or demonstrations showing the differences between the species' diving and breathing strategies.

Ask students to name some ways that animals avoid predators. What strategies and special adaptations do animals use to avoid being someone else's lunch? List their ideas on the board, and feel free to add some of your own. Some examples include speed, agility, camouflage, fighting or defensive behaviors, and good hearing, sight, or smell.

Have students look at this map to see where Shark Bay is located, or point out its location on a wall map.

Tell the class that Shark Bay is an area that's teeming with all sorts of marine life, from sea grasses to dolphins to tiger sharks. It is also home to green sea turtles and loggerhead sea turtles, which are both eaten by tiger sharks.

Have students visit the following Web sites to see pictures of both turtle species and the tiger shark. As they look at the pictures, ask them to pay careful attention to the turtles' body shapes and structures and to compare the two types of turtle. This will be challenging, but they should do the best they can. Ask them to write answers to the questions below and then discuss them as a class.

National Geographic: Field Tales—Getting the Shot (Scroll down to see the photograph of a loggerhead sea turtle.)
National Geographic: Field Tales—Filming Tiger Sharks
Loggerhead Turtle
Sea Turtles (The top photograph is of a green sea turtle.)
Tropical Oceans Animals: Green Turtle

• Which turtle has the longest fins? What might this tell you about the turtle's speed in the water?
• Which turtle has the sleekest body? What might this tell you about its ability to escape tiger sharks?
• Which turtle do you think seems most likely to be eaten by a tiger shark, based on what you've learned so far?

Students should be encouraged to make informed guesses to answer the above questions. After discussing their ideas, inform them of these turtle facts:

• Green sea turtles have longer fins, sleeker bodies, and smoother shells than loggerheads. Loggerheads have rough shells with barnacles attached to them. These features make the green sea turtle better able to swim fast and maneuver quickly in the water.
  
  
• Loggerheads have stronger shells than green sea turtles. Many loggerheads have wounds on their shells from tiger shark bites, showing that they've been bitten but have been able to escape.

Ask students again which species they think is more likely to be eaten by a tiger shark. After they've considered this question for a minute, explain that, despite the loggerhead's strong shell, it is ten times more likely to be eaten than the green sea turtle.

Inform the class that even though we've known for some time that loggerheads are more likely to be eaten by tiger sharks, scientists were not certain that it was due to the loggerheads' slower speed. So researchers used Crittercam on both species of turtle to see what else they could learn.

Divide the class into small groups, and ask them to brainstorm other reasons that might explain why green sea turtles are less likely to be killed by tiger sharks than loggerhead turtles. In other words, what might the Crittercam team have found? Provide students with this information for a hint:

• Tiger sharks hunt at the ocean floor in shallow waters and look upwards to locate their prey.
  
  
• Both species of sea turtle dive and find their food at the ocean floor in shallow waters, but they then surface to breathe.

Share these results of the Crittercam turtle study with the class:

• Crittercam found that loggerheads spend up to two hours diving and foraging at the seafloor for food. They then come to the surface for about two minutes to breathe.
  
  
• Crittercam found that green sea turtles spend a considerable amount of time at the seafloor feeding on sea grass and jellyfish. They then come to the surface to take one quick breath before diving back down.

Ask students if they now understand why loggerheads are in considerably more danger from tiger sharks than are green sea turtles.

Discuss how these results compare with students' predictions. Did anyone predict the results of the Crittercam investigations?

Divide the class into small groups and have each group make puppets or models representing a green sea turtle, a loggerhead sea turtle, and a tiger shark. They can use socks, construction paper, or other craft materials.

Ask each group to practice and perform a scenario involving the three animals. They should show the turtles diving and surfacing and the shark looking for prey. Encourage students to be creative but require them to be accurate in their depictions and comparisons of the two species of turtle.

• Have students research the diets of green sea turtles and loggerhead sea turtles. They will discover that these two species eat very different types of food. The green turtles eat mainly sea grass and jellyfish, while the loggerheads eat shellfish. A good Web site to begin with is Sea Turtle Species of the World.
  
  Discuss why students think two species of sea turtles would have such different diets and feeding habits. Bring up the concept of an ecological niche, which is defined as "all of the conditions in an ecosystem that an animal requires for its survival." Food is among the most important. If both species occupied the exact same niche and ate the same food, would it be easier or more difficult for them to both live in Shark Bay?
  
  
• Have students conduct further research on sea turtles to find out about the threats these animals face throughout the world. Every species of sea turtle is endangered; ask students to find out why and to learn about the efforts being undertaken to help these animals. Have the students compile their findings into oral presentations or posters.

This material is based on work supported by the National Science Foundation under Grant No. 0229817.

Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Loggerhead Turtle
National Geographic: Crittercam Chronicles
National Geographic: Field Tales—Filming Tiger Sharks
National Geographic: Field Tales—Getting the Shot
Tropical Oceans Animals: Green Turtle