Factors influencing juvenile sheltering in sea urchins (Strongylocentrotus Franciscanus and S. Droebachiensis)

Juvenile red sea urchins (Strongylocentrotus franciscan us) are often found under the spines of adults, but those of the green urchin (S. droebachiensis) do not shelter to the same extent. As broadcast-spawners with free-swimming larvae, two questions arise: (1) why and; (2) how does sheltering occur? Juvenile S. franciscanus increased sheltering in response to increased predation risk and hydrodynamic factors, whereas juvenile S. droebachiensis did not. Sheltering afforded protection from a predatory starfish (i.e. Pycnopodia helianthoides) in S. franciscanus but not S. droebachiensis. Water velocities were reduced by 90% within 5 cm of adults where juveniles sheltered. Field surveys confirmed that juvenile abundance is positively correlated with increased water motion. However, results indicate that juveniles compete with adults for kelp, displaying lower growth rates when exposed to adults. Juvenile S. franciscanus displayed consistent movement towards a secondary chemical signal released by adults exposed to P. helianthoides, whereas adults did not respond in the same manner. These results highlight the importance of post-settlement processes and call for the inclusion of juvenile stages in predictions of recruitment and population dynamics.