The ocean is divided into three light zones: the photic zone (less than 220 yards or 200 meters from the surface of the ocean), the "twilight" zone (between 220 to 1,100 yards or 200 to 1,000 meters from the surface), and the aphotic zone (more than 1,100 yards or 1,000 meters from the surface). In the "twilight" zone, it becomes increasingly difficult to see, and colors become obscured. The aphotic zone receives no light at all, and this region's inhabitants live in complete darkness…with the exception of the lights they emit from their own bodies. This phenomenon is known as "bioluminescence." While bioluminescent organisms can be found at all levels of the sea, their light-emitting capabilities come in particularly handy at the deeper levels.

Students will explore the reasons for bioluminescence by conducting a simulation and viewing pictures of bioluminescent marine animals on the Web. They will conclude by pretending to be deep sea divers and writing journal entries about their impressions of a bioluminescent animal they have encountered.

Geography, life science

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

Two hours

• Computer with Internet access
• Yardstick or meterstick
• Flashlights—enough that an even number of small groups (two to four students each) can each have one
• Index cards/small pieces of paper—the same number as you will have groups [Note: On each card, draw a short series of dots and dashes (or the words "short" and "long") to represent a light pattern that a group will emit with its flashlight. Every group will be paired with another group that has an identical pattern, so draw the same pattern on two cards before going on to the next card (Example: Two cards might have the pattern "short, short, long, short, long.")]

Students will

• view pictures of camouflaged animals, and discuss other types of animal adaptations;
• hypothesize what it's like in the deep ocean;
• look at pictures of bioluminescent marine animals;
• do a simulation with flashlights, trying to identify other bioluminescent animals of the same species;
• identify and list the reasons that many marine animals are bioluminescent; and
• write journal entries pretending to be divers encountering bioluminescent animals for the first time.

Write the word "adaptation" on the board, and ask students if they know what this word means. Provide a definition from your classroom dictionary, or use this one: "an adjustment to conditions in the environment." For example, human adaptations to cold environments include wearing warm clothes and installing central heating in their homes, while a Siberian husky's adaptation to its cold environment includes a very heavy coat of fur.

As an option, have students look at the insect pictures at Insect Camouflage and Mimicry, and ask them to explain these animals' adaptations.

Ask students to think of other examples of animal adaptations. How are wild animals in their area adapted to the local climate or other conditions? What about wild animals in other places, including the ocean?

Have a student hold up a yardstick or meterstick. Tell the class that the very deepest place in the world's oceans (the Mariana Trench in the Pacific) is 11,933 of these yardsticks (or 11,000 of these metersticks) deep. That's about seven miles (11.3 kilometers)!

Ask the class to hypothesize what it's like in the deep ocean. After they have provided some of their initial ideas, ask them to consider how much sunlight they think there would be in the deeper parts of the ocean. They should realize that the deeper areas are very dark. At approximately 220 yards (200 meters), it becomes more and more difficult to see. Below about 1,100 yards (1,000 meters), there's no sunlight at all.

Have students speculate about how deep sea animals might be well-adapted to their dark environment. Discuss their ideas.

Direct students to photographs of luminous organisms. They should click on the individual photos in the left frame to see larger pictures of the animals in the right frame. The text is very advanced, so just ask them to look at the pictures.

Point out to students that the bright parts of these animals are actually lights on their bodies (except for the last two animals, which don't have lights, but look like they do). Tell the class that the word we use to describe animals that emit light is "bioluminescent" ("bio" for life," and "lum" for light), and write this word on the board.

Divide the class into small groups of two to four students each. Make sure there is an even number of groups in the room, and ask groups to spread out around the room. Explain that each group represents an animal that lives in the deep ocean and that there are two of each kind of animal in the classroom. Their goal will be to figure out where the other animal of their type is.

Give each group a flashlight and a card with a light pattern. Explain that a dot (or the word "short") means they'll put the flashlight on for just a split second, while a dash (or the word "long") means they'll hold it on for about one to two seconds. You might want to demonstrate this concept to the class with your own flashlight first.

When all groups are ready, turn off the lights and ask them to begin emitting their patterns with their flashlights. While one student in each group beams the light, the others in the group should look carefully around the room for the same signal.

Once every group has found its "match," have students return to their own seats. Discuss how the activity they have just done relates to bioluminescent deep sea animals. Students should realize that, just as they used their lights to find other animals of the same species, deep sea animals use lights to locate others that are like them. Why would they want to do this? Generally, to find a mate!

Have students, either in groups or individually, go to the Harbor Branch Oceanographic Institution's Bioluminescence site to learn about some of the other marine animals that have bioluminescent capabilities. They should select "Why they make light" from the far left and go through each of the sections in this category ("finding food," "finding a mate," "defense"). Ask them to take notes as they look at the pictures and read the text. They may need help with a few vocabulary words, but the text should not be too challenging for this grade level.

Discuss the reasons why many marine organisms are bioluminescent, and list these reasons on the board.

Choose one photograph of bioluminescent animals from either UC Santa Barbara's Bioluminescence Web Page or the Harbor Branch Oceanographic Institution's Bioluminescence site. Ask students to imagine that they are deep sea divers seeing one of these animals for the first time. Have them look carefully at the picture, and then ask them to write a one-page journal entry describing what the animal looks like and explaining why they think it might be bioluminescent. They should consider things they have discussed and learned in the previous parts of this lesson.

• Divide the class into small groups, and have each group choose one of the bioluminescent behaviors they have learned about (e.g., finding food or finding a mate). Have each group perform a skit featuring one of these behaviors. After a group has completed its performance, ask the rest of the class to guess which behavior the group was acting out. Invite students to bring in props, and let them use the flashlights you used earlier in the lesson (or glow sticks—see below).
  
  
• To add a festive flavor to the lesson, bring in some glow sticks, glow straws, or glow bracelets (available at many party supply stores). Give one to each student, and explain that these accessories give off light in a way that's very similar to how many marine animals emit light. There's a chemical reaction inside these glow sticks that produces light, much like there's a chemical reaction inside the animals' organs that creates the light. [Note: More information on this chemical process is available for you at the Harbor Branch Oceanographic Institution's Bioluminescence site; select "How they make light." This reading is too advanced for grades 3-5, however.]

This lesson is made possible by a generous grant from the National Oceanic and Atmospheric Administration National Marine Sanctuary Program.

Harbor Branch Oceanographic Institution: Bioluminescence
NOAA: Sustainable Seas Expeditions
National Geographic: Blue Frontier
Sea and Sky: Monsters of the Deep
UC Santa Barbara: Bioluminescence Web Page