Visual and chemical cues stimulate microdiet ingestion in sea bream larvae

The aim of the present study was to investigate the mechanisms by which the presence of Artemia nauplii in the rearing medium stimulates the feeding behaviour of gilthead sea bream, Sparus auratus. Sea bream larvae were given a microdiet ad libitum in the complete absence or the presence of chemical or visual stimuli of Artemia nauplii in the following way. The larvae were offered a microdiet in containers in which they had only visual contact with the nauplii. Alternatively, larvae were given the microdiet in the presence of only the Artemia-rearing culture-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 ranging from 3 to 12 nauplii ml^-1. The present study demonstrated that while 20-day old larvae fed at a rate of 2.78 μg larva^-1 h^-1 in the absence of 'Artemia stimuli', their feeding rate increased by 120%, to 6.3 μg larva^-1 h^-1 in the presence of both chemical and visual stimuli. The stimuli work synergistically with each other, where the additive effect of each of the stimuli given alone was smaller than the two stimuli given together. Artemia's chemical stimulation amounted to a 35% improvement in larval microdiet feeding rate as compared with the control. This stimulus could be attributed mainly to the presence of four metabolites that were found in abundance in the Artemia-rearing 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 visual and chemical stimuli. The latter could be added to the larvae-rearing medium separately or possibly could be made to leach out of the microdiets as they are fed to the larvae.