An incredible diversity of marine life lives in Western Australia's Shark Bay, an ecosystem of great interest to marine scientists. 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.

Students will consider the reasons for this significant difference in vulnerability between the green turtle and the loggerhead turtle. 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.

The Suggested Assessment in this lesson asks students to draw time lines showing the differences between the turtle species' breathing times. The students will then write paragraphs describing their time lines and explaining how the Crittercam study might help sea turtle conservation efforts.

If you have access to the Crittercam video, have students view the video after doing this lesson.

Geography, life sciences

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

Two hours

• Computer with Internet access

Students will

• hypothesize the reasons that green sea turtles are less likely to be eaten by tiger sharks;
• discuss how scientists might investigate whether the green sea turtle's speed is the only reason this species is less likely to be eaten;
• discuss the results of the Crittercam study;
• draw time lines showing the differences between the turtles' breathing habits; and
• write paragraphs describing what their time lines show and how the Crittercam study might help sea turtle conservation efforts.

Have students go to the following Web pages to see pictures of green sea turtles, loggerhead sea turtles, and tiger sharks:

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

Have students look at a map of Shark Bay, 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's also home to green sea turtles and loggerhead sea turtles, which are both eaten by tiger sharks.

Divide the class into small groups.

Tell students that green sea turtles are ten times less likely than loggerheads to be killed by tiger sharks. Have students work in their groups to hypothesize the reasons why this might be the case. They should list as many possible reasons as they can.

Provide this information to the class, and have them take notes. Or copy these facts and give them to students as a handout:

• Green sea turtles have longer fins, sleeker bodies, and smoother shells than loggerheads. Loggerheads have rough shells with barnacles. These features make the green sea turtles 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.
• Both species of sea turtle dive and find their food at the ocean floor in shallow waters, but they then surface to breathe.
• Tiger sharks hunt at the ocean floor in shallow waters; they look upwards to locate their prey.

Ask groups to look at the facts you've given them and compare these facts to their hypotheses about why green turtles are less vulnerable to tiger sharks. Can they add anything to their list of hypotheses? Do the facts you've given them make complete sense, or are there any contradictions?

Discuss groups' ideas as a class, and explain that before the Crittercam studies were conducted, it was thought that the green sea turtle's speed and agility were the main reasons it was less likely to be captured by a tiger shark.

If students haven't yet been introduced to Crittercam, allow them to get an overview of this technology at National Geographic's Crittercam Chronicles.

Ask students how they think scientists might conduct an investigation to determine whether speed is the only reason that green sea turtles are more likely to avoid and evade tiger sharks. What questions would the scientists try to answer? What steps might they take in their investigation? How could Crittercam help? Discuss these questions as a class.

Share with the class these results of the Crittercam study on green sea turtles and loggerhead sea turtles:

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

Ask students to explain what the Crittercam findings suggest about the sea turtles' susceptibility to tiger shark attacks. What do these diving and breathing patterns have to do with the turtles' vulnerability to tiger sharks?

Discuss how these results compare to students' predictions about why green sea turtles are in less danger than loggerheads. Did anyone predict the results of the Crittercam investigations?

Ask students to follow these instructions to draw time lines that show the diving and breathing patterns of green sea turtles and loggerhead sea turtles.

• Hold a page of paper horizontally (the long way), and write the word "ocean surface" at the top and "ocean floor" at the bottom.
• Draw a line across the page, about an inch (2.5 centimeters) from the top.
• Draw 15 marks across the line, evenly spaced from one another. Label these marks 1 to 15; the time line now represents a time period of 15 minutes.
• Draw a line from the ocean floor to the ocean surface showing how the green sea turtle might swim up to the surface.
• Continue the green sea turtle's line along the time line to show how long it would spend breathing at the surface before diving down again.
• Draw the line showing the green turtle descending once again to the ocean floor to feed.
• Repeat the above steps for the loggerhead sea turtle, drawing a line to show it surfacing, breathing over a period of time, and descending.
• Draw and label the turtles at the surface.
• Draw a tiger shark below the surface.

Have students write paragraphs describing what their time lines show, stating which turtle the tiger shark will probably be most likely to eat, and explaining how they think this Crittercam finding might be helpful in turtle conservation efforts in Shark Bay.

Explain that scientists have learned in their studies at Shark Bay that male loggerheads are three times more likely to be attacked by tiger sharks than female loggerheads. Scientists still don't know why, but they're planning to study this question in the future. Ask students to hypothesize why this difference between male and female loggerheads might exist and to describe their ideas in a paragraph or two.

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.

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