Mechanisms for the snapping behavior of two alpheid shrimp,Alpheus californiensis andAlpheus heterochelis

Abstract

1. In the snapping movement ofAlpheus californiensis the opener muscle of the propus is first contracted to cock the dactyl of the snapping cheliped, then the closer muscle is excited by a high frequency train of impulses. This train continues until the force holding together disks on the dactyl and propus is overcome. At this point the dactyl snaps closed and the closer excitatory motoneuron becomes silent. 2. A reflex is demonstrated whereby the opener muscle is excited upon passive opening of the dactyl. This is opposite to typical passive limb reflexes and may be important in setting the cocking mechanism. 3. All of the characteristics of snapping can be found in the small pincer cheliped with the exception of the long duration high frequency train of impulses in the closer muscle. 4. Another alpheid shrimp,Alpheus heterochelis, uses a completely different mechanism to hold the dactyl open while the closer builds tension. Here the closer apodeme is lifted over the pivot point around which the dactyl closes. In this way the dactyl is locked until a second closer muscle contracts to pull the closer apodeme down.