Tide Pool Babies: Sea Stars, Sea Slugs and Crabs

Compare larvae and early embryos of tide pool animals to find relatives that adults hide.

Part 1: Larvae in the plankton

Read the passage and the table, then answer questions 1 to 4.

Small Swimmers at Kelp Point

1At low tide in May, Mr. Benedikt Oyelaran's eighth-grade marine biology class explored the rocky tide pools at Kelp Point on the Pacific coast. Marisol Takahashi and her group counted ochre sea stars clinging to the rocks, purple sea urchins tucked into holes, periwinkle snails, a bright orange sea slug, shore crabs hiding under stones, a few small shrimp and a tidepool sculpin, a small fish that rests on the bottom. The students looked and photographed, then left every animal where they found it.

2In the afternoon the class visited the marine lab next to the beach. Dr. Ansel Brightwater showed them plankton, the tiny living things that drift in seawater, under a microscope. Many tide pool animals start life as larvae, small swimming young that often look nothing like their parents. The lab also raises embryos of a few local animals from eggs so that students can watch them develop.

3Marisol was surprised by the sea star larva. An adult sea star has five arms arranged around a center, but its larva has a left side and a right side that mirror each other, and it swims with bands of tiny beating hairs. The sea slug larva swam inside a tiny coiled shell, much like the larva of the periwinkle snail. The periwinkle keeps and enlarges its shell, but the sea slug sheds its shell when it settles on a rock. The young crab had a long, jointed tail held out behind its body. In an adult crab, the tail is short and folded tightly under the body.

4Dr. Brightwater then described an early embryo stage. A few days after an egg begins to divide, the ball of cells forms a dent, which becomes the first opening into the gut. In some animals this first opening becomes the mouth. In others it becomes the anus, and the mouth forms later at the other end. The lab has checked this for five of the animals in the table. Fish, humans and other animals with backbones form the anus first.

5"Adults are shaped by the way they live," Dr. Brightwater said. "A sea star creeps over rocks, and a crab scuttles sideways. Embryos and larvae change less in some ways, so they can keep clues to who is related to whom."

Table: Adults, larvae and early embryos at Kelp Point (— = not checked)
Animal	Adult body	Larva body	Shell in larva?	First opening becomes
Ochre sea star	5 arms around a center	Left and right sides mirror	No	Anus
Purple sea urchin	Round, 5-part pattern	Left and right sides mirror	No	Anus
Sea slug	Soft body, no shell	Inside a coiled shell	Yes, shed at settling	Mouth
Periwinkle snail	Coiled shell	Inside a coiled shell	Yes, kept for life	Mouth
Shore crab	Short tail folded under	Long jointed tail behind	No	—
Coonstripe shrimp	Long jointed tail behind	Long jointed tail behind	No	—
Tidepool sculpin	Fish with a backbone	Small fish with a tail	No	Anus

An adult sea slug has no shell, but its larva swims in a coiled shell like a periwinkle larva. What does this suggest?

A. Sea slugs are likely related to snails and come from ancestors that had shells.
B. Sea slug larvae borrow empty snail shells, so the two animals often live together.
C. Sea slugs are more closely related to sea stars, since both adults creep over rocks.
D. Sea slugs lose their shells because they stop needing them, so they are unrelated to snails.

Use the table. Circle every animal whose larva has a feature that its adult does not have.

Ochre sea starPurple sea urchinSea slugPeriwinkle snailShore crabCoonstripe shrimp

An adult crab looks quite different from an adult shrimp. Which inference does the crab larva support best?

A. Crab larvae are young shrimp that turn into crabs after they settle on the rocks.
B. Crabs are likely related to shrimp-like animals and come from ancestors with long tails.
C. Crabs grow a long tail while young because they swim in open water before settling.
D. Crabs are more closely related to sea urchins, since both adults hide under rocks.

Use the last column of the table and paragraph 4.

of the animals in the table are shown to form the anus first. Of those animals, has a backbone as an adult.

Part 2: Who is related to whom?

Use the table and the passage to answer questions 5 to 9.

Use the last column of the table and paragraph 4. Based on this embryo evidence, which animal are sea stars more closely related to?

A. The shore crab, since both have hard outer parts and live among the same stones
B. The sea slug, since both larvae swim in the plankton before they settle down
C. The tidepool sculpin, since both embryos form the anus first and the mouth later
D. The periwinkle snail, since both are slow animals without a backbone that cling to rocks

Fill in each blank with an animal from the word bank. Two words are left over.

Word bank: periwinkle · shrimp · sculpin · crab · sea slug

The coiled shell of its larva links the sea slug to the . The long jointed tail of its larva links the crab to the . The fate of its first opening links the sea star to the .

A brittle star is a tide pool animal with five thin arms around a small center disc. Based on the pattern in the table, what would you predict about its early stages?

A. A larva with mirror-image sides, and a first opening that becomes the anus
B. A larva inside a coiled shell, and a first opening that becomes the mouth
C. A larva with a long jointed tail, and a first opening that becomes the anus
D. A larva with five arms like the adult, and a first opening that becomes the mouth

Marisol writes, "Sea star embryos form the anus first, like fish embryos, so sea stars are a kind of fish." Which revision is the most careful?

A. Sea stars turn into fish for a short time, since their embryos develop like fish embryos.
B. Sea stars are closer relatives of snails, since an adult body matters more than an embryo feature.
C. Sea stars are fish that lost their fins and backbones when they started creeping over the rocks.
D. Sea stars are likely more closely related to fish than to snails, but they are not fish.

A classmate says, "Sea stars must be closer relatives of snails than of fish, because sea stars and snails are both slow animals with no backbone that cling to rocks." Write a claim that answers the classmate. Give at least two pieces of evidence from the table or passage, and explain why embryos and larvae can show relationships that adults hide.