Feeding Rate Variation in Purple Urchins

— Written by William Dejesus

My results showed feeding rate variation between trials with and without a 24-hour starvation period. A decrease in consumption rate was consistently observed in trials without the prior starvation period in the warm treatment. The ambient treatment did not show a significant pattern, which could have resulted from a less intense environmental constraint than what the warm treatment experienced.

It appears that consistent food availability could lead to decreased feeding rates. This can be applied to the kelp forest community in that a healthy abundance of giant kelp will prevent large spikes in feeding rate over time. The kelp forest community, like all ecological communities, requires a stable balance of trophic level interaction in order to function properly.

My investigation of the purple urchin’s feeding behavior has lead to the conclusion that the kelp forest community will need more urchin predators, such as the California sheephead in order to maintain the high abundance of biodiversity that it is famous for. Without these key predators, it is likely that urchins will consume kelp at a faster rate, inevitably leading to monoculture urchin barrens. These barrens are unsustainable for almost all species that call the kelp forest home, as well as the purple urchins, who will have eaten themselves out of house and home. Although urchins can survive for long periods of time without abundant food, previous studies have found that urchin health in barrens is extremely low with high potential for widespread disease.

Operation PURFECT (Purple Urchins Routinely Feeding in Extreme Climate Temperatures) Methodology

— 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.

Will one of Japan’s most famous delicacies survive a changing climate?

— Written by Will Dejesus


Purple urchin feeding on giant kelp

Sea urchin is a famous Japanese delicacy used in sushi, called uni. The gonads, or sex organs, is the part of the urchin that is actually eaten. It is high in protein and contains omega-3 fatty acids, which can help lower blood pressure. Sea Urchins can be found along the West coast from Baja to Seattle. Sea Urchin harvesting along the coast of California has become one of the highest valued fisheries in the area. Almost 75% of the urchins caught along the California coast are sent to Japan, where they import over 250 million dollars worth every year.

Not only are sea urchins a relevant part of the food and fishing industry, but they also play a critical role in the kelp forest communities, as their preferred food source is giant kelp. With few native predators, sea urchins have the power to decimate kelp forests, which are already struggling with changes in ocean chemistry. Areas where sea urchin population has sky rocketed, kelp density has been nearly wiped out in a short period of time. From ocean to table, sea urchins play an important role in coastal communities on land and below the surface.

I am interested in taking a closer look at the feeding habits of these invertebrates given future predicted changes in ocean temperature. Their reactions to temperature changes in general and especially in feeding behavior is all of interest in this study. This will begin to paint a clearer picture of what we can expect our marine ecosystems and seafood industry to look like in the future.