The relation of food availability and food preferences to the field diet of an echinoid Evechinus chloroticus (Valenciennes)

Echinoids are important in causing disturbance in many marine communities yet little is known of their foraging behaviour in the field. This work investigates the field diet of an echinoid species, Evechinus chloroticus (Valenciennes), in relation to food availability and food preferences. E. chloroticus is a generalist grazer occurring abundantly in a variety of algal and encrusting communities dominated by sponges. In the field, urchins grazed the sponge species according to abundance. In the laboratory, food preferences were demonstrated for two populations of E. chloroticus when presented with 24 species of sponge. One population of urchins came from an intertidal reef where sponges were rarely encountered and the other from a sublittoral habitat where the encrusting community was dominated by sponges. The sponges used as food in the preference experiments came from two different areas: one where urchins were almost absent, the other (from which the second group of urchins were taken) where urchins were abundant and were observed to feed upon sponges. When presented with sponge species unfamiliar to both urchin groups, urchins ranked sponge species similarly. Food preferences did not relate to the diet of the urchins in the field. Reasons for the difference between experimental results and field observations are suggested.