Sea Urchin (Strongylocentrotus droebachiensis) Aggregating Behavior Investigated by a Subtidal Multifactorial Experiment

Abstract

We performed a multilevel factorial field experiment to identify the effects of five factors on sea urchins' (Strongylocentrotus droebachiensis) aggregating behavior. The factors were (1) source of urchins (kelpbed or barrens), (2) density of urchins (high or low), (3) location of treatment (kelpbed or barrens), (4) the presence and type of invertebrate predators (crabs or lobsters), and (5) season (summer or winter). These manipulative experiments were performed in flexible, resilient cages designed to withstand the severe wave surge in Nova Scotia's shallow subtidal environment. Interactions identified by ANOVA among the various factors showed that urchins aggregate more at high (20∙m−2) than at low (4∙m−2) density. The presence of lobsters in the kelpbed, and of crabs in the barrens, triggered the formation of even larger aggregations. These aggregations remain in the open even in the presence of predators. We argue that this behavioral mechanism (a defensive aggregation response to lobsters in the kelpbed) is the trigger that precipitates widespread destructive urchin grazing and the transformation of kelpbeds to barrens. Lobsters thus play two opposite roles. At low urchin density lobsters keep urchins in hiding and thereby contribute to kelpbed persistence. At higher urchin density, lobsters trigger the formation of large, exposed urchin aggregations that graze destructively on kelp. Urchin responses to predators are probably mediated by a combination of conditioning and sensitivity to biochemical cues. The large-scale change in community structure, from kelpbed to barrens, can thus be understood in terms of the adaptive behavioral responses of individual organisms. We review the roles of other predators in this system and show that they have varying effects on the intensity of urchin grazing, depending on urchin density, season, and habitat type.

Influence of fishing and functional group of algae on sea urchin control of algal communities in the eastern Atlantic

“…This trend has been similarly reported with other key-herbivorous sea urchins at other coastal systems in a large body of scientific literature covering the cold-temperate mid-latitudes in both of the hemispheres (e.g. Andrew & Choat 1982, Bernstein et al 1983, Sala & Zabala 1996, Estes et al 1998, Sala et al 1998, Babcock et al 1999, Steneck et al 2002, Shears & Babcock 2003 by following the classical trophic cascades paradigm (as reviewed by Sala et al 1998, Pinnegar et al 2000.…”

Relationships between rocky-reef fish assemblages, the sea urchin Diadema antillarum and macroalgae throughout the Canarian Archipelago

“…These results suggest that a trophic-cascade may exist in the Canarian Archipelago, as has been demonstrated in other locations worldwide such as the Mediterranean (Sala & Zabala 1996, Sala et al 1998, tropical coral-reef habitats (Hay 1984, McClanahan & Muthiga 1988, McClanahan & Shafir 1990) and kelp-dominated areas (Andrew & Choat 1982, Bernstein et al 1983, Steneck 1997, Babcock et al 1999, Shears & Babcock 2003. We suggest that this cascade is at least partially related to overfishing of large macroinvertebrate-eating fish (Duggins 1980, Tegner & Dayton 1981, Breen et al 1982, Tegner & Levin 1983, Hay 1984, McClanahan & Muthiga 1988, McClanahan & Shafir 1990, McClanahan 1992, McClanahan et al 1994, Sala & Zabala 1996, Babcock et al 1999; although no data on fishing intensity in this area is available to support this assumption, as artisanal fishermen sell their captures directly to local markets without any sort of governmental control (Bas et al 1995.…”

Daily movement of the sea urchin Strongylocentrotus droebachiensis in different subtidal habitats in eastern Canada

“…In shallow subtidal habitats, urchins often form destructive grazing fronts at the lower edge of kelp beds, and such aggregations can advance through the kelp beds at a rate of 2 to 4 m mo -1 (Bernstein et al 1983, Gagnon et al 2004. Destructive grazing leads to the formation of extensive 'urchin barrens' with encrusting coralline algae and isolated patches of less preferred macroalgae which can persist for years to decades (Chapman 1981, Johnson & Mann 1982.…”