Behavioural Physiology of the Colonial Hydroid Obelia

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Abstract
1. Electrical, mechanical or chemical (KC1) stimuli produce similar luminescent flashing in Obelia geniculata. Hydranth withdrawal also occurs upon electrical or mechanical stimulation. 2. Luminescent flashes in response to individual stimuli occur in bursts. The first few flashes of the burst facilitate in intensity and then decline. Successive flashes within each burst show a constant shape and duration but a variable intensity. 3. The decay phase of the luminescent flash has a rate constant of 50 s-1 at 20±1 °C and 29 s-1 at 127plusmn; 1 °C. From these figures an energy of activation of about 12 kcal can be calculated. 4. Upon stimulation, an excitation system, the luminescent potential (LP) system, initiates the luminescent effector flashing. The LPs are monophasic, positive, and slow (duration about 120 ms) and are distinguishable from contraction potentials (KPs) mouth-opening potentials (MOPs) and tentacle contraction potentials (TKPs). LPs are all-or-none, non-decremental, through-conducting (at least over short distances) and show non-polar spread. 5. The LP systems can drive the KPs at the LP frequency. Coupling varies from none to very tight, and the degree of coupling is determined at each hydranth. 6. Several other campanulariids and probably most, if not all, luminescent hydroids possess similar physiological mechanisms for controlling stimulus-initiated luminescent responses.

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