Capture Efficiency of Various Species and Sizes of Drift Macrophytes by Red Urchins, Strongylocentrotus franciscanus

Abstract

In the San Juan Islands, drift macrophytes from shallow waters represent a significant spatial subsidy provided to the subtidal zone. This organic matter is a potential food source for deep dwelling herbivores like the red urchin Strongylocentrotus franciscanus. Little is known about how urchins capture and use drift material. A racetrack flume was used to examine the capture of drift macrophytes of various species and size by S. franciscanus. No species-specific selectivity was found, but urchins caught 100% of small macrophyte pieces. A significant decrease in capture efficiency was noted as drift algal size was increased, probably due to the urchins’ inability to fight current drag acting on pieces with larger surface area.