Interpreting and evaluating experimental results with graphing

Water fleas and predator cues

Units 5 and 7
9 points
About 25 minutes

You can use a calculator on this question, just like on exam day.

A long question with an experiment and a data table that you turn into a graph. On this site you describe the graph you'd draw (type, axes, units, scale, points and error bars), then analyze the data and make and justify a prediction. On the exam: Question 2 of 6 in Section II (90 minutes, 50% of the exam score); about 25 minutes suggested.

The question and its sources

Water fleas (Daphnia) are small freshwater animals that can reproduce asexually, so a single female can produce a clone of genetically identical offspring. Some Daphnia grow a tall crest on the head, which makes them harder for certain predatory insect larvae to catch and eat. Researchers investigated whether chemicals released by these predators affect crest growth.

Experiment

The researchers used newborn Daphnia from a single clone. They raised groups of 20 newborns in containers of lake water at 20°C with the same food supply. Each container also held a mesh cage with 0, 5, 10, 20 or 40 predatory larvae per liter. The cage let chemicals from the larvae pass into the water but kept the larvae from touching or eating the Daphnia.

After 6 days, the researchers measured the height of each Daphnia's head crest.

Source: Hypothetical study

Table 1. Head crest height after 6 days

Predator larvae per liter	Mean crest height (μm)	±2SE (μm)
0	42	6
5	61	7
10	88	8
20	121	9
40	126	10

Source: Hypothetical data

Suggested time: 25 minutes

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Part (a)

1 point

Describe what is meant by phenotypic plasticity.

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Part (b)

4 points

(i) Describe the graph you would construct to represent the data in Table 1. In your description, name the type of graph, give the label and units for each axis, describe the scale for each axis, list the values you would plot, and describe the error bars. (ii) Based on the data, determine whether the crest height at 5 larvae per liter is significantly different from the crest height at 0 larvae per liter.

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Part (c)

2 points

(i) Calculate the percent increase in mean crest height at 20 larvae per liter compared with 0 larvae per liter. (ii) Justify the researchers' use of Daphnia from a single clone in this experiment.

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Part (d)

2 points

Researchers find that, when no predators are present, Daphnia with tall crests produce fewer offspring than Daphnia without crests. Clone P grows a tall crest only when predator chemicals are present. Clone T always grows a tall crest. (i) In a lake where the predatory larvae are absent for most of the year, predict which clone will have the higher fitness. (ii) Justify your prediction.

Your answer to part (d)

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