FISH INNER-EAR OTOLITH SIZE AND BILATERAL ASYMMETRY DURING DEVELOPMENT

Size and bilateral asymmetry (i.e. size difference between the left an d the right hand side) of inner ear otoliths of larval mouthbreeding c ichlid fish were determined during the ontogenetic development of larv ae from hatching to the free swimming stage. Animals of two batches we re raised in aquarium hatch baskets. The basket containing one batch w as placed directly above aeration equipment, resulting in random water circulation within the basket, which constantly shifted the specimens around ('shifted' specimens). The second batch of animals was raised in parallel without shifting. Due to the weight of the yolk-sacs, thes e animals lay on their sides until. the yolk-sacs were resorbed ('stat ionary' specimens). The groups of larvae did not differ from one anoth er in respect of individual general development, nor in otolith size. Contrasting results were obtained regarding bilateral otolith asymmetr y: In both shifted and stationary animals, asymmetry of utricular and saccular otoliths (lapilli and sagittae, respectively) ranged at compa ratively low values throughout development. However, by comparison wit h shifted individuals, lapillar asymmetry of stationary animals showed a highly significant increase during early development when larvae we re forced to lay on their sides due to their prominent yolk-sacs. In l ater developmental stages, when they began to swim freely, a dramatic decrease in lapillar asymmetry was apparent. These findings indicate t hat development of lapillar asymmetry depends on the direction of the acting gravity vector relative to the positioning of the larvae, sugge sting that the size (or mass) of a given otolith is regulated via a fe edback mechanism. (C) 1998 Elsevier Science B.V. All rights reserved.