Werner and Gilliam’s (1984) model predicts that size-specific rates of
growth and mortality determine the size at transition for animals with an
ontogenetic habitat shift (OHS). Although animals are unlikely to calculate
mortality rate, they often respond to changes in predation risk. For many
social species, an individual’s risk of predation is reduced by
conspecific aggregation. We hypothesize that individuals in groups respond to
this reduction of predation risk and should shift habitats at a smaller size
than solitary individuals. We tested this hypothesis by altering food
availability, predation risk and conspecific presence for newly settled spiny
lobsters in mesocosms. Juvenile Caribbean spiny lobsters,
Panulirus argus, undergo an ontogenetic habitat shift
from algal dwelling to crevice sheltering concomitant with aggregation in
crevices. We found that juveniles raised with low food availability and low
predation risk underwent transition at a smaller size. We also found that
juveniles raised with conspecifics underwent transition at a smaller size than
did solitary lobsters under the same conditions. Our results suggest that the
ontogenetic habitat shift of algal-phase lobsters is accelerated when food is
scarce, predation risk is low, and conspecifics are present. In the absence of
conspecifics, algal lobsters wait until they are larger to change from algal
dwelling to crevice sheltering. This result suggests that ontogenetic habitat
shifts for gregarious animals are influenced by the proximity of conspecifics.
Ontogenetic habitat shift of an herbivorous crab: a chemically mediated defense mechanism?
