Habitat related differences in the survivorship and growth of juvenile sea urchins

Abstract

In northeastern New Zealand abundance of the echinoid Evechinus chloroticus is strongly linked to habitat. A survey of both adults and juveniles in 3 habitats showed that numbers were high, though variable, in Coralline Flat habitats (areas devoid of large brown algae) but low in Ecklonia radiata forest and Deep Reef habitats. A transplant experiment was done to assess the extent to which this pattern could be ascribed to differences in juvenile survivorship among these habitats. Juveniles transplanted into Deep Reef habitats suffered a high mortality rate with a mean of 70 % being dead after 18 wk, compared to only 3 % for Coralline Flat habitats. Mortality in Ecklonia forest treatments was intermediate, with a mean of 37 %. Presence of conspecific adults did not significantly influence survivorship of juveniles in any habitat. Cause of mortality in this experiment is unknown and warrants detailed investigation. Implications of these results in the interpretation of the impact of sea urchins on benthic communities is discussed. The great variation in survivorship of juveniles in experimental situations where predators have been excluded suggests that processes other than predation are capable of strongly influencing the abundance of echinoids. The effect of conspecific adults and density on the growth of juvenile E. chloroticus was also investigated within the Coralline Flat habitat. Density of juveniles did not significantly affect their growth; however, at both densities, juveniles in the absence of adults reached a larger size than did those caged with adults.