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Eggs in the Pond Mud: Rotifers and Road Salt

Grade 8 natural selection math: use salt tests on rotifers hatched from dated pond mud and a daughter-count model to explain a rise in salt tolerance.

Grade 8ScienceInteractive · Printable

Aligned toMS-LS4-6

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What’s included

Single worksheet
Auto-graded online
1 NGSS standard
Printable answer key
10 questions
Classroom-ready

Details

Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Sorting, Short answer

About this worksheet

What students practice

Students compute the percent of a population with a trait from survival counts in dated mud layers.
Students use a model of daughters per female to predict how a trait's share changes over generations.
Students explain with evidence how natural selection made salt tolerance more common in a pond.

How to use

Read the passage together and point out how deeper mud holds older eggs. Students can work the model in Part 2 with a calculator, then write the explanation on their own.

Curriculum

NGSS standard covered

MS-LS4-6

Use math and data to explain how natural selection changes the share of a trait in a population over time.

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Eggs in the Pond Mud: Rotifers and Road Salt

Use hatched eggs from old mud layers and a daughter-count model to explain a change in a pond population.

Part 1: Eggs from the mud

Read the passage and Table 1, then answer questions 1–4.

Birch Hollow Pond

1Birch Hollow Pond lies beside a busy state highway. Since the winter of 1998, road crews have spread salt on the highway after every snowstorm, and melting snow carries some of that salt into the pond. In spring the pond water is now many times saltier than it was in the early 1990s. On a pond study day, Mr. Caspian Odhiambo's eighth grade class looked at drops of pond water under microscopes. Linnea Haukeland spotted dozens of rotifers, tiny animals about 0.2 mm long that sweep food into their mouths with rings of beating hairs. Many of the females carried eggs stuck to their bodies.

2Dr. Paloma Restrepo, a lake scientist from the nearby university, explained how these rotifers live. In the warm months a female makes eggs that hatch into daughters without any mating, and each daughter gets a copy of her mother's genes. A new generation can hatch every few days, so the pond goes through many generations each summer. In the fall, the rotifers also make tough resting eggs that sink into the mud. Mud builds up on the pond floor year after year, so deeper mud holds older eggs. Resting eggs can survive in the mud for decades and still hatch.

3In 2020 Dr. Restrepo's lab pushed a long tube into the pond floor and pulled up a core of mud. They cut the core into slices and used the depth of each slice to find about when it formed. They hatched resting eggs from each slice. Each hatchling's family was raised for two generations in the same clean lab water, with the same food, at 20 °C. Then young females from each slice were placed for 48 hours in water as salty as the pond during spring snowmelt, and the lab counted the survivors. The lab ran the same test on eggs from Fern Lake, a similar pond 12 km away with no road nearby. Table 1 shows the results.

4Next, the lab compared daughters. A rotifer whose family survived the salt test is called tolerant, and one whose family died is called sensitive. The lab raised 10 tolerant and 10 sensitive females for 10 days in two kinds of water: clean water, and water as salty as the pond in early summer, about half as salty as the test water. Every female survived the 10 days. The graph shows the average number of daughters each female had.