Resource allocation and growth rates in the sea urchin Evechinus chloroticus (Echinoidea: Echinometridae)

Abstract

The morphometry of the sea urchin Evechinus chloroticus from habitats of contrasting algal abundance but of similar urchin density was examined at two localities in southern New Zealand during 1993. Populations from habitats of high algal abundance (Dusky Sound) had similar relationships of demipyramid (jaws) to test diameter, but individuals had significantly smaller jaws relative to their test diameter than those from a locality where algal abundance was low (Arapawa Island). The body wall mass (in relation to total wet weight) was similar for populations from both localities but, for Dusky Sound populations, individuals from exposed sites had greater relative body mass than those from sheltered sites. The ratios of gonad weight:total weight were similar between localities. However, E. chloroticus from Arapawa Island reached reproductive maturity at a smaller size than those from Dusky Sound. Growth rates of E. chloroticus varied among localities in Dusky Sound. Growth was modelled by the Tanaka function, which allows for rapid early growth until reproductive maturity is reached and declining growth rates thereafter. The results show that sea urchins respond to low resource availability by increasing the size of the food-gathering apparatus, maturing at a smaller size, and growing to a smaller size than individuals from food-rich habitats.