Spatial patterns in early post-settlement processes of the green sea urchin Strongylocentrotus droebachiensis

Abstract

Small juvenile benthic invertebrates are exposed to a variety of organisms that may affect their growth and survival; however, most studies focus on larger, mobile predators. This study examined if the often disregarded suite of cryptic macro-benthic invertebrates increased the mortality and decreased the growth of small juvenile sea urchins Strongylocentrotus droebachiensis (1-3 mm) in a caging experiment executed in Passmaquoddy Bay, Bay of Fundy, Canada. The sea urchins had greater mortality (4.1% wk-1) across all sites when the suite of small animals was present in experimental cages than when it was removed (2.4% wk-1). The addition of food (kelp and associated biofilms) had no effect on their mortality. Growth of the juvenile sea urchins in the cages ranged from 3.3-8.1% wk-1 across the treatments, and varied across sites (spatial scale of 100s of m). Small juvenile sea urchins had greater growth when the suite of animals was removed (6.6 vs. 4.3% wk -1), as well as with the presence of food (5.6 vs. 5.1% wk -1). Sea urchin mortality and growth were related to the abundances of larger sea urchins, chitons, and scale worms (mortality only) (mortality general linear model [GLM]: Z131 < -2.95, p < 0.0032; growth linear model [LM]: F1, 131 > 15.29, p < 0.0001). The mortality patterns found in the cages were similar to natural patterns over the same time period at 4 of the 6 sites. Overall, the suite of animals living amongst cobbles generally resulted in an increase in mortality and a decrease in growth of small juvenile sea urchins in Passamaquoddy Bay, indicating that these small organisms can influence early post-settlement growth and mortality across spatial scales of 100 to 5000 m. © Inter-Research 2014.