Scientific Explanation Assignment - Scenario Weight: 3% of your final Biology 140 grade Word count: typed, size 12 font, max. one page double-sided,...
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Scientific Explanation Assignment - Scenario
Weight: 3% of your final Biology 140 grade
Word count: typed, size 12 font, max. one page double-sided, double-spaced
Due date: By the start of your Lab 4, either by email or give to your instructor
Sea urchins (Strongylocentrotus spp.) are round, spiny marine animals that are closely related
to sea stars. They use their spines and tube feet to move about on rocky shorelines along the
Pacific Coast. They typically move about at night, feeding on macroalgae, especially large
brown kelp (e.g. bull kelp (Nereocystis luetkeana), and giant kelp (Macrocystis spp.)). When
abiotic and biotic conditions are unfavourable, sea urchins hide in crevices or depressions in
rocks. Here they are protected from strong water movement (e.g. currents and waves), which
can dislodge the urchins, and from predators, such as sea stars, sea otters and fish. They are
also protected from moving debris that can damage their spines.
While in the refuge site (crevice or depression), sea urchins have limited access to food and
must rely on bits of unattached macroalgae that drift by their location. If the density of sea
urchins in the refuge site is high, competition for this drifting macroalgae can be intense and
the sea urchins must leave their refuge to find food.
Table 1. The impact of abiotic conditions on sea urchins.
Abiotic conditions
Impact of conditions
Current speed
>5 m/s - 50% chance of dislodgement
Wave velocity
>0.25 m/s - debris in the water can damage the spines of urchins
Table 2. The abiotic and biotic conditions at two intertidal sites.
Intertidal Site 1
Intertidal Site 2
Current speed
0.3 - 1.7 m/s
1.0 -7.3 m/s
Wave velocity
0.0 - 0.15 m/sec
0.0 - 0.35 m/sec
Light Intensity
<1-95,000 lux
<1 - 80,000 lux
Density of sea stars
2.4 individuals/m2
2.1 individuals/m2
Density of urchins
35 individuals/m2
10 individuals/m2
Based only on the information provided above, answer the question:
At which intertidal site would you expect sea urchins to spend more time hiding in refuges?

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Student name:
Lab section:
Scientific explanation assignment
Present your answer in the format of a scientific explanation in the boxes below. Type and use
full sentences. Attach the marking rubric to the front of your assignment. Either use the pages
from your workbook or download the pages from Canvas.
Evidence and reasoning/integration (max. one page double-sided, double-spaced)

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