Developmental plasticity in fish exposed to a water velocity gradient: a complex response.

Developmental plasticity plays a major role in evolution and provides an excellent context for unravelling the ecological implication of variation in abiotic factors to which developing fish are subjected. Water velocity has been known to induce plasticity in salmonids, but the ontogenetic component of these changes is poorly documented. Newly hatched specimens of rainbow trout (Oncorhynchus mykiss) were reared in four constant water velocity treatments (0.4, 0.8, 1.6 and 3.2 cm/sec) for a period of 100 days. Traditional and geometric morphometrics were used complementarily to characterize morphological changes among treatments throughout ontogeny and demonstrated early and complex developmental plasticity in O. mykiss. Certain traits display a constant direction of change, whereas other traits present a break point at 20-25 mm ( approximately 40-60 dph) indicating a modification in the nature of changes between developmental stages. Plasticity in response to water velocity was detected from very early on (15-20 mm; approximately 20-40 dph) and concerned mostly fin-related traits. Body-related traits did not respond conformingly to functional expectations as fishes reared in the fastest velocity treatment show a robust body shape more comparable to that of fish from the slowest treatment than do fish from the intermediate velocity. The noncongruence between treatment and response gradient suggests that different combinations of water velocity have the potential to lead to diverging interpretation of plasticity, both in an ecological and evolutionary context. Overall, the complex shape changes observed in O. mykiss stress the importance of paying better attention to actual functional requirements and adaptations that occur at levels other than hydrodynamics. (c) 2009 Wiley-Liss, Inc.