QUANTITATIVE-ANALYSIS OF FORAGING BEHAVIOR IN A FIELD POPULATION OF THE SOUTH-AMERICAN SUN STAR HELIASTER HELIANTHUS

Foraging behaviour in the South American sun-star Heliaster helianthus (Lamarck) was investigated quantitatively on a subtropical rocky shor e in central Peru. H. helianthus feeds mainly on two mussel species, S emimytilus algosus (Gould) and Perumytilus purpuratus (Lamarck). A seq uence of foraging behaviour was described and observations were made o f the timing of attacks on mussel beds by sun-stars. Although H. helia nthus is capable of foraging out of water, an unusual trait for astero ids, the timing of such foraging appears to be well adjusted to avoid the risk of prolonged heat and desiccation. Foraging activity began 4 h 52 min before high tide and 3 h 19 min after high tide, with a mode between 2 h 30 min and 2 h before high tide. Fifty-two percent of all foraging activity began between 3 h 30 min and 2 h before high tide, w hile only 12.4% began after high tide. This suggests that H. helianthu s mainly relies upon changes in the rate of tidal increase as a cue to begin foraging. Foraging activity ceased between 3 h 40 min before an d 4 h 29 min after high tide, with 48% ending between 2 and 0.5 h befo re high tide. The duration of foraging ranged from 19 to 190 min, with values between 30 and 80 min accounting for 67.4% of all observations . The median duration was 62 min. No significant correlation was detec ted between the time when foraging activity commenced and its duration . The intensity of foraging activity varied on consecutive days, with a general pattern of decreasing intensity after a day of relatively hi gh activity. Foraging location in relation to a mussel bed was analyse d on a marked, 8 m stretch of rocky shore. The numbers of foraging H. helianthus observed on different sections of the shore were related ne ither to the width of the mussel zone nor to the vertical position of the lower edge of the mussel zone, indicating that sun-stars do not re ly upon these factors to assess prey availability and that ideal free distribution with regard to prey abundance does not occur on the spati al scale examined.