Cannibalism among green sea urchins Strongylocentrotus droebachiensis in the laboratory and field

The green sea urchin Strongylocentrotus droebachiensis plays an important ecological role as a consumer of kelp in temperate subtidal ecosystems, but is known to be omnivorous, and recent studies suggest it is potentially cannibalistic. This study examined cannibalism of small juvenile S. droebachiensis by larger conspecifics in the laboratory, and estimated rates of inter-cohort cannibalism in the field based on gut contents of adult sea urchins collected from subtidal sites in the Mingan Archipelago, St. Lawrence Estuary, Canada. Initial 48 h trials investigated factors (substratum, presence of food) that may influence cannibalism, and a subsequent 4 wk laboratory experiment found that adults and larger juveniles consumed 21 and 24% of small juveniles (3.5-6 mm test diameter), respectively, on cobble substratum when kelp was provided ad libitum. Frequencies of cannibalism from gut contents of urchins in the field were combined with an estimate of gut passage time determined in the laboratory and the densities of sea urchins at each site to estimate rates of cannibalism. The proportion of adults that had cannibalized ranged from (mean +/- SD) 0 +/- 0 to 0.31 +/- 0.12 across sites in 2013 and 2014, resulting in estimated rates of cannibalism from 0 +/- 0 to 68.3 +/- 0.60 small juveniles cannibalized m(-2) wk(-1). This is the first study to report size-structured cannibalism among S. droebachiensis, and suggests that cannibalism can be a major source of mortality for small juvenile sea urchins in subtidal barren grounds.