VISUAL AND CHEMICAL CUES STIMULATE MICRODIET INGESTION IN SEA BREAM LARVAE

The aim of the present study was to investigate the mechanisms by whic h the presence of Artemia nauplii in the rearing medium stimulates the feeding behaviour of gilthead sea bream, Sparus auratus. Sea bream la rvae were given a microdiet ad libitum in the complete absence or the presence of chemical or visual stimuli of Artemia nauplii in the follo wing way. The larvae were offered a microdiet in containers in which t hey had only visual contact with the nauplii. Alternatively, larvae we re given the microdiet in the presence of only the Artemia-rearing cul ture-medium, which served as a chemical stimulus. Finally, larvae were exposed to both stimuli: visual and chemical. The stimuli were given at different levels which were equivalent to Artemia concentrations ra nging from 3 to 12 nauplii ml(-1). The present study demonstrated that while 20-day old larvae fed at a rate of 2.78 mu g larva(-1) h(-1) in the absence of `Artemia stimuli', their feeding rate increased by 120 %, to 6.3 mu g larva(-1) h(-1) in the presence of both chemical and vi sual stimuli. The stimuli work synergistically with each other, where the additive effect of each of the stimuli given alone was smaller tha n the two stimuli given together. Artemia's chemical stimulation amoun ted to a 35% improvement in larval microdiet feeding rate as compared with the control. This stimulus could be attributed mainly to the pres ence of four metabolites that were found in abundance in the Artemia-r earing medium: betaine and the free amino acids, arginine, alanine and glycine. In conclusion, the results suggest that the feeding response of marine larvae on microdiets could be elicited by the presence of v isual and chemical stimuli. The latter could be added to the larvae-re aring medium separately or possibly could be made to leach out of the microdiets as they are fed to the larvae.