Virtual Urchin - Our Acidifying Ocean

In Part 2, you and your lab assistant have set up six slides of urchin larvae:

1. three replicates from pH 8.1

2. three replicates from pH 7.7

Now you need to measure the length of the arms in the larvae.

In this section, you will be measuring the length of the upper arms to determine what effect the pH level may have had on the larvae. All of the larvae will be oriented as seen here, and the arms are measured from the tip to the end, as shown by the blue arrow:

This is a six day old sea urchin (Paracentrotus lividus) larva at the 4-arm stage.

To measure, you will use an interactive ruler.

The interactive ruler can be moved in two ways: it can be dragged into position, and rotated to fit the angle of the larva’s arm. A properly positioned ruler has its left edge aligned with the tip of the skeleton, and parallels the upper arm.

Grab the ruler anywhere in the gray area (with numbers) to drag the ruler into place.
Use the brown tip to rotate the ruler into the correct position to align it with the larva’s upper arm. You want the ruler to line up exactly parallel to the arm you are measuring.

Try it now:

Be accurate in your measurements!

You can view and measure your larvae either using the Normal or the Polarized light setting by clicking the buttons below the slide image.
Note that it is typically easier to see the skeleton using Polarized light!

Then, for each sample, you record your measurement. Note that the scale on the ruler is in micrometers (µm), and that each mark is 20 µm (1000 µm = 1 mm).

The arm shown here is approximately 420 µm long. Do you see why?

Your measurements need to be accurate to within a range of ±10 µm from the actual measurement. If your measurement is accurate enough, you will see a green OK symbol. A red X means try again.

How to Measure Larvae