Award: OCE-2023693

Purple sea urchins are vital to the California coast. Their populations fluctuate widely from year to year with large population booms some years and busts in others. In the ocean, many animals, including sea urchins, start life as tiny, microscopic larvae.  Larvae are first released into the ocean from adult animal, called spawning.  Then the larvae float and swim around in the ocean until they finds a permanent home, such as a rocky reef, and attach there to grow into an adult, called recruitment. 

To understand why urchin populations fluctuate, we studied how changes in ocean currents, water temperature, and food supply are connected to urchin recruitment.  We did this by using a high-tech computer model to simulate spawning, drifting, and recruitment of urchin larvae in the Southern California Bight (the curved coastline from Santa Barbara to San Diego).  Our project evaluated how the following four factors influenced larval recruitment:

- Temperature: How hot or cold the water was for both the adults and the larvae.
- Food: How much plankton was available for the larvae to eat
- Currents: How far the larvae traveled to get to their final destination or recruitment site
- Swimming Behavior: Whether larvae just drift or actively move up and down in the water.

Compared to food, currents, and behavior, the temperature that the adults experience while making larvae before they spawn was found to be the most important factor influencing recruitment. If the water is too warm when adults are reproducing, the number of baby urchins that year is very low. This explains why "El Nino" years, which bring warm water to the California ocean, produce fewer baby urchins. We also found that larvae recruit in higher numbers when they stay closer to home or their spawning location. If the larvae use specific behaviors, like moving to deeper water during the day, they are more likely to stay near the reefs and find a suitable home. By understanding these connections in the ocean, scientists can better predict when urchin populations and populations of other marine animals might spike or collapse. This helps us protect and manage the delicate balance of California's underwater ecosystems.

Last Modified: 02/24/2026
Modified by: Rachel D Simons