Investigation 3:

What factors influence populations of settling organisms?

Does sunlight or shade affect where they grow? Is water depth an important factor? Is their position affected by strong currents? Are they influenced by the presence of other organisms, such as Ciona savignyi?

You can begin to answer these and other questions with the use of settling plates. You'll find directions for making them below.

First, some thoughts about experimental design-

To find out whether an organism is found more often in one place than another, you need a way of counting or measuring how abundant they are in each place. How can you do that? Counting organisms on a rope is not easy. They grow out in all directions, some covering others.

Using settling plates instead of ropes makes measuring easier. Organisms that attach to a plate can be identified and counted. The percentage of the plate covered by a species can be estimated. You can even use photography to record growth patterns. By placing growth plates in places with different environmental conditions, the importance of factors such as light, depth, current flow, and salinity can be measured.

Ideally, settling plates are made from a rigid, slightly rough material like ceramic tile. A lower cost alternative is plastic, cut from lids of containers such as coffee cans, bulk yogurt, etc. There is one problem--it is slippery and hard for settlers to stick to. Keeping in mind that plates used to compare two or more factors must be identical to one another, you might use sandpaper to buff their surfaces briefly. Use a power sander or other device that will give a uniform treatment to all plates.

With the design below, you can hang as many plates as you choose in series. You can unclip them and measure or photograph the animals growing on the tops and bottoms, without the rope getting in the way.

Materials You'll Need-

• Plastic lids (of uniform material, preferably without printed images)
• Scissors
• Auger for making holes in plastic lids
• Brass S hooks, 3 for each plate
• Needle nose pliers
• 3/16 inch polypropylene or nylon rope, for hanging plates
• Scrap iron nuts, washers or other convenient weights for rope ends

Construction of settling plates-

1. Cut identical sized squares or rectangles from plastic lids.
2. Make holes in centers of plates, and thread open S hooks through the holes. Pinch ends of S hooks with needle nose pliers to close them.
3. Cut rope to desired intervals between plates, adding about 5 inches to either end for tying knots. Keep some extra rope for tying apparatus to the dock.

4. Tie S hooks onto ropes above and below each plate. Close rope end of these hooks tightly. You can choose to leave free ends of hooks slightly open so that loops from plate will just pass through them.

5. Burn rope ends with a match so they won't fray. Assemble several plates in a series, fasten a weight to the bottom. Now it's ready to go in the water!

If you plan to detach ropes from plates when measuring growth of organisms, you will need a system for labeling tops and bottoms of plates, and numbering each plate in a series. How could you do this?

Experiments to try with settling plates-

Hang settling plates from floating docks. In choosing where to place them, consider how you would investigate differences between:

• settlement on the upper sides and bottom sides of plates
• plates hung at different depths
• plates with different conditions of lighting
• plates in strong currents vs. weak currents
• plates hanging where salinity or temperature are different
• plates hung near or far from objects that might be sources of planktonic larvae

When settling organisms appear how will you measure and compare population sizes? For several ideas, go to Measuring and Recording.