The swimming and feeding performance of marine animals depends on the interaction of fluid flow and body morphology. For example, an array of body appendages such as hairs, cilia or tissue lappets (a
) can serve either as a paddle or as a sieve (Koehl et al. 2001
), depending on the ratio of inertial versus viscous fluid forces as expressed by the Reynolds number:
where ν is the kinematic viscosity of water, b
is the appendage width and U
is the velocity of appendage relative to water. As the appendage is set in motion, the surrounding fluid does not slip with respect to the surface of the appendage. Consequently, a velocity gradient, i.e. boundary layer, forms in the water between the appendage and the free-stream flow (b
). The lower the Re
, the thicker becomes this boundary layer of sheared fluid relative to the gaps of the appendage array, eventually overlapping between the neighbouring edges and conferring additional paddle surface. Conversely, at increasing Reynolds numbers, the boundary fluid layer retreats, turning the array into a grate useful for filtering (c
). New functions may thereby arise from unchanged structures simply through a shift in velocity or scale (Koehl 2004
), whereas continuous function may be achieved when changes in fluid dynamics owing to scaling effects are compensated by behavioural or morphological responses (Yen 2000
). Furthermore, tuning morphogenesis to fluid dynamics may conserve resources as suggested by a recent study on ephyrae, i.e. juvenile scyphozoan medusae (Feitl et al. 2009
). In particular, boundary layer overlap allows ephyrae to propel themselves through the water with a lean, star-like formation of lappets instead of employing a full bell. As the animals increase in size, the reduction in boundary layer thickness is balanced by gap-narrowing tissue growth, ensuring complete boundary layer overlap throughout the development. These results indicate that jellyfish ontogeny exploits viscous effects to minimize the costs of tissue maintenance without compromising momentum transfer essential for swimming and feeding.
Figure 1. (a) Characteristic parameters of ephyra morphology. A, potential bell area; D, diameter; r, radial lappet position; b(r), lappet width at radial position r. (b) Model of boundary layer formation on solid surface. U, velocity. Arrows represent velocities (more ...)
However, given that ephyral development is tuned to scale-dependent changes in Reynolds number, it raises the question as to whether it is capable of adapting to variation in other factors affecting Re
, most notably water viscosity, which is strongly dependent on temperature. Such adaptive ‘phenotypic plasticity’, the environmentally sensitive production of alternative phenotypes by the given genotype (DeWitt & Scheiner 2004
) would be beneficial for globally distributed scyphozoan jellyfish species like Aurelia aurita
, which are subject to significant temperature (and thereby viscosity) variation.
Here, we extend the fluid-dynamical model of ephyral ontogeny proposed by Feitl et al. (2009)
, in conjunction with experimental measurements of ephyral morphology, fluid dynamics and swimming performance, in order to determine whether jellyfish morphogenesis is plastic in response to temperature conditions, and if so, whether the changes can be considered adaptive by facilitating economic and effective animal–fluid interaction.