Lordosis induction in juvenile red sea bream, Pagrus major, by high swimming activity

Lordosis is correlated with absence or malfunction of the swimbladder. However, swimbladder abnormalities do not completely explain the occurrence of lordosis. We examined whether muscle activities from vigorous tail beat induced by removal of the caudal fin could induce lordotic malformation in juvenile red sea bream. We also attempted to determine the minimal current velocity for inducing lordosis in relation to total length (TL). For this purpose, we employed relative current velocity in terms of move of water in times of total body length per second (TL s(-1)). We compared effects of different water current (2 vs. 4 TL s(-1)) for 10 days on bone malformation using juvenile of 25-mm TL. Exposure to 4 TL s(-1) current velocity, but not 2 TL s(-1), induced lordosis. Lordotic juvenile red sea bream had a cuneiform centrum mainly at the 15th vertebra. Lordosis was observed in fish with normal swimbladders as well. Thus, swimbladder abnormalities alone cannot completely explain the occurrence of lordosis. We removed part or all of the caudal fins of fish of 40-mm TL and exposed these fish to 2 TL s(-1) current for 10 days. Control fish did not show deformations in the vertebrae. Fish with complete removal of the caudal fin swam with vigorous tail beats. However, fish with complete, upper or lower caudal fin removal (36%, 29% or 7%, respectively) displayed lordosis even at the current velocity of 2 TL s(-1). These findings suggested that the muscle activities from excess beat of the tail against excessive current, but not the mechanical action of the caudal fin, induced lordosis. (C) 2002 Elsevier Science B.V. All rights reserved.