At the start of this lesson, I ask students this question: What do you know about animal life cycles? What do animals look like when they are born? How do they grow and change?
Then the students briefly share their ideas with neighbors and I walk around and listen to what they have to say.
Next I tell them that today we will look at the life cycle of a lesser-known animal group. At the end of the lesson they will be able to state what the life cycles of these animals have in common with all living things.
I show them this short video, Lesser Known Animal, to get them interested in these unusual ocean creatures.
I show students a short visual presentation I created, The Life Cycle of a Sea Squirt. We briefly discuss the different stages. For students that are curious about the similarity between tadpoles and the larval form of sea squirts, you can point out that in their larval form, sea squirts have the most basic parts of a backbone system, though this quickly goes away. So, while the adult form is very clearly a boneless animal, technically, they are not invertebrates!
The presentation, The Life Cycle of a Sea Squirt, contains 9 slides that present this information in a manner accessible to third graders.
I ask students to write down at least 2 observations about the sea squirt life cycle. If they need focus, I ask them, "Are they born? Do they grow and change? Do they have an adult form? Do they die?"
Next, I project this excellent jellyfish life cycle animation from the New England Aquarium in Boston, Massachusetts. We read through it together and only after we have clearly read the information do I allow a student to come up and run the mouse for the class. After she has correctly chosen the first stage of the jellyfish life cycle, students discuss their observations with a partner for 1 minute before I call on a second student to come up. An alternative is to run the animation myself the first time through. I have found that with a text rich animation such as this it is best not to let students proceed through it independently until I have clearly set the expectation that they should read and ask themselves questions or make observations about each stage. Otherwise they will just "click through". After the jellyfish animation is complete, I again ask students to make a few observations/notes/questions about what they observed.
Sea squirts and other tunicates have a very unusual and complicated lifestyle. If you'd like to read a bit more about the sea squirt life cycle, this is a fun and informative site.
The sea squirts are asexual and actually expel both sperm and eggs at the same time, which are then get fertilized and grow.
At the end of this lesson, I ask students to form an inside-outside line and share their observations with 3 different partners.
In the first rotation, they answer this question:
What did the two life cycles have in common? Be as specific as possible. (In other words, it's not enough to say that both jellyfish and sea squirts are animals, or that they both live in the ocean).
On the second move, students answer this question:
What was different between the life cycle of these animals?
On the third move, students respond to this prompt:
What connections can you make between the life cycles of these animals and more well-known animals, such as, for example, cats, elephants, fish, or frogs?
If the students are engaged, I use this question for a fourth rotation:
What did you find the most interesting/surprising? Why?