MEPS 317:87-99 (2006)  -  doi:10.3354/meps317087

ABSTRACT: We measured the movement and orientation of sea urchins at 2 sites in each of 4 habitats on urchin barrens in the northern Gulf of St. Lawrence, eastern Canada. We tagged the urchins measuring 25 to 70 mm in test diameter using a non-invasive technique involving a knot of fine monofilament thread tied around one spine. Smaller urchins (<25 mm) were tagged by tightening a loop around the test. Daily displacement increased markedly going from large juveniles (measuring 10–15 mm in diameter) to small adults (15–20 mm) and from small adults to large adults (>25–70 mm); however, no effect of size was detectable among large adults measuring from 25 to 70 mm. The increased movement with size was associated with a change in foraging, as gut analysis revealed a marked increase in the proportion of brown algae with increasing size. For large adults, the maximum net distance displaced per day varied among sites from 1.0 to 4.9 m d^–1, and mean displacement varied from 0.40 (SE = 0.07) to 1.72 (SE = 0.28) m d^–1. Distance moved also varied with habitat, and was greater in habitats that were further from kelp beds. Orientation was random and individuals showed frequent reverses in direction from day to day. Hunger state did not affect displacement, as there was no difference in distance moved between starved and fed urchins which were released in the barrens. However, urchins transplanted from kelp beds to the barrens were less active than urchins removed from and returned to the barrens. The shifts in movement with size are probably important in determining the size structure of urchins at different depths. The great distance covered by larger urchins as they move over open surfaces in search of favourable resources probably leads to their increased numbers in shallow habitats where food is abundant.

KEY WORDS: Strongylocentrotus droebachiensis · Movement · Foraging activity · Urchin barrens · Tagging