Outer Limits and Inner Structure: The 3-Dimensional Flow Field of Pleuromamma Xiphias (Calanoida: Metridinidae)

Copepods create feeding currents to entrain water over their sensory receptors and through their feeding appendages. The flow field also creates a hydrodynamic disturbance which may be detected by the animal's potential prey and predators. A method is described for the 3-dimensional analysis of the flow field of feeding copepods. The equipment used is a laser-illuminated flow visualization system and video recordings made from 2 perpendicularly mounted cameras which permitted the determination of the X, Y and Z coordinates of individual particles and their trajectories. Flow within narrow volumes of water are used to minimize the error in flow rate estimates associated with analysying flow within a large unspecified volume of water compressed into a 2-dimensional plane. Three planar views are combined to create a 3-dimensional model describing the shape and magnitude of the flow field of Pleuromamma xiphias. A maximum velocity of 38.00 mm⋅s-1 occurred at the base of the downward swing of 2nd antennae just lateral to the sides of the body. We found lateral symmetry in the flow field with water velocity decreasing exponentially from the head to the distal tips of the antennae exposing different parts of the sensor to different flow regimes. Asymmetry in the flow field was found between the dorsal and ventral side of the animal with the 1.00 mm⋅s-1 velocity isoline approximately 1.5 times further from the body ventrally than dorsally. The asymmetry between the dorsal and ventral flow is predominantly within the region of low flow, therefore there is relatively little difference in the amount of water entrained from each side. However, because the escape responses of mechanoreceptive prey may be elicited by steep velocity gradients, the asymmetry may allow prey to be entrained closer ventral than dorsal to the animal. The hydrodynamic disturbance defined by the 1.00 mm⋅s-1 velocity isoline was detected as far as 4.10 mm above the head, 4.60 mm lateral, 5.60 mm ventral and 3.60 mm dorsal to the animal. The lower extent of the 1.00 mm⋅s-1 isoline was not detected however the lower portion of the 7.00 mm⋅s-1 isoline was found 7.50 mm directly below the head. Using the above dimensions and a spheroid model, the volume of the hydrodynamic disturbance created by the animal (approximately 4 ml) is greater than 175 times the actual volume of the physical body of the animal. This created signal may reveal the identity and location of Pleuromamma xiphias to its predators