DO BARNACLE LARVAE RESPOND TO MULTIPLE SETTLEMENT CUES OVER A RANGE OF SPATIAL SCALES

Numerous physical and biological factors have been identified which af fect the probability of larvae settling on hard substrata. The spatial scale at which these factors operate ranges from km's to sub-mm's. Th e wide variety of cues that barnacle larvae respond to coupled with th e subtleties of cue response to factors like surface roughness, sugges ts that larvae are fastidious in their choice of settlement sites and thus, (i) settlement is not rapid and, (ii) larvae carry out search be haviour to sample settlement cues. An experimental frame with settleme nt pits untreated or with either barnacle settlement factor, or cyprid settlement factor, or a squashed cyprid larvae were exposed for a dur ation of 10 minutes during the Semibalanus balanoides settlement seaso n in the Clyde Sea, UK. A total of 102 of the 240 pits were settled wi thin the 10 minutes. More settlement occurred in the chemically treate d pits than the untreated pits suggesting that settlement can be both selective and rapid. Video-photography was carried out in the laborato ry of the tracks of S. balanoides cyprids prior to settlement in pits. With untreated pits little search behaviour was identified, cyprids t ended to encounter the pit and then settle. Pits treated with squashed cyprid showed a chemical cue-mediated behaviour with cyprids tending to slow down and carryout antennular crawling in the vicinity of the p it. The mean time from entering a 40 x 40 mm window around the pit and settlement was 24.9 s (n = 11, SE = 5.4). Within the last 1.25 s prio r to settlement, cyprids settling in untreated pits moved faster than cyprids settling in CL treated pits (P < 0.01), with a 4 times differe nce between the mean speeds These data suggest that settlement can be rapid and the pre-settlement track does not necessarily display search behaviour.