Jet-propelled swimming in scallops: swimming mechanics and ontogenic scaling

Abstract

Scallop locomotion was investigated on the basis of an analysis of fluid forces acting on the body and the balance of the forces during swimming. A hydrodynamic model for unsteady jet propulsion was developed in which propulsion performance is characterized by three nondimensional parameters: the storage/discharge volume ratio, reduced clapping frequency, and reduced discharge frequency. Pulsed jet propulsion is designed to achieve high thrust, although not necessarily with low hydrodynamic propulsive efficiency, as was previously widely considered. Swimming in scallops is realized by orientating the body at a certain angle of attack and maintaining a minimum swimming speed to prevent sinking. The working frequency of the locomotor system is determined and adjusted by the swimming strategy (angle of attack, swimming speed, and trajectory angle). For Placopecten magellanicus, the optimum angle of attack is about 6° – 12°, at which swimming requires the lowest energy input (lowest frequency) and hydrodynamic behaviour is ideal (without severe separation and stall). To maintain level swimming, P. magellanicus, during almost all their life, must swim at 5 – 7 body lengths per second if postured at a 6° – 12° angle of attack. The estimated Froude efficiency decreases during growth from about 0.5 to 0.3 for level swimming and from about 0.4 to 0.2 for climbing at an angle of 25°. It is suggested that the heavy body and inferior hydrodynamic characteristics (low aspect ratio and imperfect planform shape) have prevented scallops from becoming good swimmers. These problems are enhanced as the animals grow.

Hydrodynamics of pulsed jetting in juvenile and adult brief squid Lolliguncula brevis: evidence of multiple jet `modes' and their implications for propulsive efficiency

“…The scaling of swimming dynamics is less well understood in aquatic invertebrates compared with fishes, although the hydrodynamics of swimming throughout ontogeny has been examined in brine shrimp (Williams, 1994), spiny lobster (Nauen and Shadwick, 1999), scallops (Cheng and DeMont, 1996;Dadswell and Weihs, 1990), jellyfish (McHenry and Jed, 2003) and squid (Bartol et al, 2001b;Thompson and Kier, 2002). In contrast to fishes, squids and many jellyfishes do not rely on undulatory mechanisms over their entire range of life history stages.…”

Jet propulsion in the cold: mechanics of swimming in the Antarctic scallopAdamussium colbecki

“…By rapidly clapping their valves together, these unusual clams expel jets of water from the dorsal edge of the shell. The resulting thrust propels the animals ventrally (Gould, 1971;Cheng and DeMont, 1996), allowing them to escape from both predators and environmental stress, and potentially allowing them to migrate (Morton, 1980).…”

“…The shell mass in swimming bivalves is reduced relative to their sedentary cousins, which both increases the power of thrust (Eqn·3) and simultaneously reduces the need for thrust by reducing the weight that must be lifted against gravity (Gould, 1971). Lastly, the resilium is formed from a stiff elastic material -abductin -that causes the shell to open rapidly after it has clapped shut (Gould, 1971;Cheng and DeMont, 1996), and the mechanical resilience of abductin (its ability to store the potential energy of deformation with little loss to viscous processes) reduces the damping of the system. Although these adaptations allow scallops to swim, these bivalves are nonetheless on the verge of failure.…”

Effects of sponge and barnacle encrustation on survival of the scallop Chlamys hastata

“…Barnacles compromise scallop swimming by decreasing the time and distance that a scallop is able to swim, by decreasing the height in the water column the scallop is able to attain, and by increasing the drag coefficient of the scallop (Donovan et al, 2003). Scallops also need to maintain a minimum swimming speed to stay in the water column (Cheng & Demont, 1996) and barnacles, if of sufficient size and/or number, may prevent this causing the scallop not to be able to swim at all. Indeed, some of the scallops in our study carried barnacles so large the scallop was no longer capable of swimming.…”