— Written by William Dejesus

I am measuring consumption of giant kelp by purple sea urchins, Strongylocentrotus purpuratus, living in experimental temperature waters. Twelve tanks were evenly divided into two different temperature seawater tables. The ambient temperature water table will stay consistent with current water temperatures outside, usually around 16-17°C. The warm water table is kept between 21-22°C. Drippers are used at the end of the inflow tubing to ensure consistent flow rate across tanks. The warm water table system contains a reservoir bucket that is used to pump the water to each tank ensuring consistent temperatures across the tanks.

Warm water table with inflow tubing from reservoir

Reservoir allowing incoming seawater to warm up

Urchins in this experiment were collected from various areas around the Channel Islands. The size of the urchins was measured upon entry to the lab, where almost all fell between 4 and 6 centimeters in diameter. All urchins were given a week of acclimation to the storage table. All relevant urchins are given a 3-day temperature and starvation acclimation period before being distributed to their individual tanks for trials. Using an acclimation period minimizes the affects of prior food availability and behavioral responses to increased temperatures. The twelve urchins subject to feeding trials are given 1 pre-weighed blade of giant kelp each per trial. Only blades of kelp are being used for feeding because of urchins’ prior known preference for this part of the plant. Each trial will last approximately 3 days, with deviations accounted for in rate calculations. The trial period was determined to be long enough for significant consumption but short enough that kelp degradation was minimal. There will be two trials each week with a 24 hour “No Food” break once a week. This allows for the trials to remain structured on a weekly basis. Statistically relevant deviations between trials with and without the “No Food” break before feeding will be acknowledged if found. Any experimental urchins that yield little to no kelp consumption (>1g) will be removed from experimentation after two consecutive trials with minimal consumption. These tanks will be replaced with acclimated urchins when the next trial begins.  Consumption of kelp is measured using wet weights of the kelp before and after each trial. Each blade is spun in a salad spinner before being weighed to reduce extra water weight in a consistent fashion.