Oral taste preferences and food-handling behaviour in the cave morph of Mexican tetra Astyanax mexicanus.

Since its discovery in 1936, the cave morph of the Mexican tetra, Astyanax mexicanus, has become a promising model for studies of compensatory adaptation. The loss of vision has driven the adaptive evolution of sensory systems in the cave morph. Studies of the gustatory system have mainly focused on the morphology and distribution of taste buds, but the functional aspects of this system remain unknown. In the present study, oral taste preferences of the cave morph of A. mexicanus were identified for the first time. Using a behavioural assay, it was found that more than half of the 21 proteinogenic L-amino acids and more than one-third of the 18 organic acids, as well as nearly all of the four basic taste substances, were either indifferent or repulsive. These findings are not consistent with expectations based on the opportunistic feeding mode of the cave morph. In addition, the threshold concentration of the most palatable substance, maleic acid, is within the range commonly found in epigean fish species. In summary, there is no evidence to suggest that compensatory improvement in oral taste perception has occurred in the cave morph. A single grasp is often sufficient for the cave morph to swallow or irreversibly reject an offered pellet. The relatively long retention time, especially for palatable pellets, is another characteristic of food-handling behaviour. Before swallowing, the cave morph retains pellets for up to eight to nine times longer, regardless of their taste quality.