Crustacean Sensory-Motor Systems
See also Copepod sensory-motor pages.
Mechanosensory mechanisms of crustacean zooplankton
Pelagic (mid-water-dwelling) crustaceans have a special problem surviviing in a habitat with no cover from predators and a three-dimensional space to deal with instead of the two dimensions that their benthic (bottom-dwelling) cousins must negotiate. One problem arising from the fluid nature of the environment they inhabit is that water motions that might warn their benthic cousins of the presence of a predator or mate or food tend to carry them along with the movement, so unless they have a fixed external frame of reference, such as a solid object on which to fixate, they have difficulty detecting the water movement. Thus their receptors tend to be extremely sensitive to water movement or "hydrodynamic cues" as they are technically termed.
To detect these hydrodynamic cues, pelagic crustaceans mostly rely mostly on an array of sensory units consisting of a hair or "seta" that projects from the body into the surrounding water and is hinged at the base. Small water movements exert force on the seta which causes it to pivot on its hinge, which activates one or more sensory neurons attached internally to the seta (see the diagrams below showing the basic morphological components associated with the mechanosensitivity). Movement of the seta with respect to the sensory dendrite initiates a train of nerve impulses that travels along the attached axon to the central nevous system for processing.
Responses of crustacean sensory setae to water movement are governed by the same laws of physics that constrain all other physical systems. Thus a seta will be made to move by both the viscous friction of water flowing past it and by the impact of water hitting it directly. It will also resist being moved by that flowing water by its own inertia and by any springiness provided by the hinge at its base. The physics of moving setae both in water and in air has been nicely reduced to mathematical expressions by the work of Joseph Humphrey, Friederich Barth and their colleagues. The images below are graphs of computations based on their developments applied to the movement of crustacean setae immersed in water that is subject to a periodic sinusoidally varying movement.