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Shells, Sea Stars and Green Anemones: Interactions at Gull Ledge

Grade 7 life science: analyze hermit crab shell counts, a sea star removal study and an anemone light test to explain patterns of interactions.

Grade 7ScienceInteractive · Printable

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1 NGSS standard
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12 questions
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Format: Interactive · Printable
Grade: Grade 7
Subject: Science
Question types: Fill in the blank, Multiple choice, Circle the answer, Short answer

About this worksheet

What students practice

Students use survey and tank data to explain competition for a limited resource.
Students use a removal study to explain how a predator affects its prey and other species.
Students use test data to identify a mutually beneficial relationship and find the same pattern in other ecosystems.

How to use

Read paragraphs 1–2 and Table 1 together, then let students work through each study. Before the last section, ask pairs to name one example of each kind of interaction from their own town.

Curriculum

NGSS standard covered

MS-LS2-2

Explain patterns of interactions among organisms in ecosystems: competition, predation and mutually beneficial relationships.

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Shells, Sea Stars and Green Anemones: Interactions at Gull Ledge

Use tide pool counts and marine lab tests to explain competition, predation and mutualism.

Hermit crabs and empty shells

Read the passage, then use Tables 1 and 2 to answer the questions.

Low Tide at Gull Ledge

1Every April, Ms. Noelani Kealoha takes her seventh graders from Seabright Middle School on a field trip to the Gull Ledge Marine Lab. At low tide, the class explores the rocky shore with the lab's biologist, Dr. Tomás Arriaga, who has studied the same stretch of shore for years. He lets the students compare their counts with the lab's records.

2Unlike most crabs, a hermit crab has a soft, curled abdomen with no hard covering. It protects its abdomen by living in an empty snail shell, and it moves into a bigger shell as it grows. Hermit crabs cannot make shells, so new empty shells appear only when snails die. A crab in a shell that is too small cannot pull its whole body inside, which makes it easier for gulls and fish to grab. The class counted crabs and empty shells in three pools and watched each pool for 10 minutes, counting shell fights: times when one crab grabbed another crab's shell and tried to pull it out (Table 1).

3Back at the lab, Dr. Arriaga showed the class a tank test. Two tanks each held 12 hermit crabs collected from the South pool, with the same water, food and temperature. Tank A got no extra shells. Tank B got 12 extra empty snail shells of many sizes. After 3 days, the class watched each tank for 10 minutes (Table 2).

4Ochre sea stars live on the same shore. A sea star wraps its arms around a mussel, pulls the shell open a little and digests the soft body. Mussels filter tiny bits of food from the water. Mussels, barnacles and many seaweeds all need a spot of bare rock to attach to. In 2021 the lab marked off two areas of rock, each 10 m by 10 m, with about 6 sea stars in each. In Area A, nothing was changed. In Area B, workers moved every sea star they found to a distant part of the shore each month. Each year, they measured how much of the rock mussels covered and counted the kinds of seaweeds and attached animals (Table 3).

5In the sunny pools, green anemones get much of their green color from tiny single-celled algae that live inside the cells of their tentacles. Inside the anemone, the algae are sheltered from grazing snails and take up nitrogen from the anemone's wastes. In light, the algae make sugar by photosynthesis. To test whether the algae help the anemones, the lab kept anemones without feeding them for 6 weeks: one tank had light for 12 hours a day, and the other was kept dark at the same temperature (Table 4).

Table 1. Three Pools at Gull Ledge (one 10-minute watch each)
Pool	Hermit crabs	Empty shells found	In too-small shells (%)	Shell fights
North	40	22	10	1
Middle	45	6	35	4
South	50	1	60	9

Table 2. Tank Test After 3 Days (12 crabs per tank; one 10-minute watch)
Tank	Extra empty shells	Shell fights	In too-small shells
A	0	8	6
B	12	1	1

Use Table 1. How many hermit crabs in the South pool were living in shells that were too small? crabs

Which pattern does Table 1 show?

A. Shell fights were most common in the pool with the most empty shells.
B. Fewer empty shells went with more fights and more too-small shells.
C. The share of crabs in too-small shells was close to 35% in each pool.
D. Pools with more hermit crabs also had more empty shells lying around.

Why does the tank test add strong evidence that the pool survey alone does not give?

A. The tank crabs were larger than the pool crabs, so their fights were easier to see.
B. The tanks differed only in extra shells, so fewer fights can be linked to shells.
C. In the tanks, gulls could not reach the crabs, so the crabs stopped fighting.
D. The tank crabs were fed, which shows the pool crabs were fighting over food.

Which kind of interaction among the hermit crabs do Tables 1 and 2 show?

A. Competition: crabs in one pool struggle over a scarce supply of shells.
B. Mutualism: crabs trade shells so that each crab ends up in a better fit.
C. Predation: the larger crabs hunt the smaller crabs and eat them for food.
D. Predation: the snails catch the crabs that live in too-small shells.

Suppose a storm washes about 30 empty snail shells of many sizes into the South pool. Which change over the next few weeks is most likely?

A. Fewer fights and fewer crabs in too-small shells, like the North pool
B. More shell fights, because the crabs will fight over the new shells
C. More crabs in too-small shells, because the storm will crack many shells
D. Fewer hermit crabs, because the extra shells will crowd them out of the pool

Sea stars, mussels and green anemones

Use paragraphs 4 and 5 and Tables 3 and 4 to answer the questions.

Table 3. Sea Star Removal Study (A: sea stars left; B: sea stars removed)
Year	Mussel cover, A (%)	Mussel cover, B (%)	Kinds of species, A	Kinds of species, B
2021	30	30	15	15
2022	31	45	15	13
2023	29	60	14	10
2024	30	75	15	8
2025	30	85	15	6

Table 4. Unfed Green Anemones After 6 Weeks (5 anemones per tank)
Tank	Algae left (% of start)	Color at the end	Mass change (%)
Light 12 h a day	95	green	−6
Dark	15	pale white	−24

In 2025, the mussel cover in Area B was how many percentage points higher than in Area A? percentage points

Circle the two results from Table 3 that, together, best show that the sea stars keep the mussel population down.

Area B dropped from 15 to 6 kinds of species.With sea stars left in place, mussel cover in Area A stayed near 30%.Area A had 14 or 15 kinds of species each year.With sea stars removed, mussel cover in Area B rose from 30% to 85%.

The kinds of species in Area B fell from 15 to 6. Which explanation fits paragraph 4 and Table 3 best?

A. The barnacles and seaweeds fed on sea stars, so they starved once the stars were gone.
B. Area A lost about as many kinds, so a change in the weather caused the drop.
C. The mussels in Area B ate the barnacles and seaweeds that grew near them.
D. Without sea stars eating them, mussels spread over the rock that other species needed.

Use Table 4. The anemones in the dark tank lost how many more percentage points of their mass than the anemones in the light tank? percentage points

10.

Both tanks were unfed and kept at the same temperature. Which explanation of Table 4 fits the passage best?

A. The anemones eat their algae, so the dark-tank anemones used up their algae faster.
B. The dark tank was colder at night, so its anemones burned more of their own mass.
C. In the dark, the algae could not make sugar to share, and most of them were lost.
D. The anemones need light to see their prey, so the dark-tank anemones caught fewer fish.

11.

The green anemones and their algae both benefit from living together. Circle the two examples from other ecosystems that show the same kind of interaction.

An owl swoops down and catches a mouse in a hayfield.Bees drink nectar from apple blossoms and carry pollen from flower to flower.Two kinds of songbirds fight over the same tree holes for nests.Fungi on tree roots take sugar from a tree and bring it water and minerals.Coyotes and foxes hunt the same rabbits in a meadow.

12.

A visitor at Gull Ledge says, “Sea stars are bad for the shore. They kill so many mussels that the shore would have more life without them.” Do you agree? Write a claim, use evidence from Table 3, and explain your reasoning about how predation and competition affect the populations on the rock.