Communication between individuals in salp chains. II. Physiology

Abstract

When stimulated, salp chains achieve rapid coordinated changes in locomotion by the spread of epithelial action potentials or outer skin pulses (o.s.ps) from one zooid to the next along the chain. This process involves alternating epithelioneural and neuroepithelial chemical synapses. Each zooid is linked to another in the chain by two asymmetric attachment plaques; these are polarized so that transmission of o.s.ps proceeds from one zooid to the next in one direction at one plaque, and in the reverse direction at the other plaque. Sensory cells at the plaques send axons to the brain; they are not electrically coupled to the conducting epithelium in which they lie. Input from the plaque sensory cells affects the swimming generator in the brain (causing locomotor changes) and evokes synaptic activity at neuroepithelial synapses around the brain. This gives rise to o.s.ps that are conducted around the whole of the outer epithelium of the zooid and are detected at the plaques by the sensory cells of adjacent zooids. Severe stimulation of a zooid in the chain induces all zooids to separate; possible mechanisms of separation are discussed.