BEHAVIORAL AND ELECTROPHYSIOLOGICAL STUDIES OF HYDRA. III. COMPONENTS OF FEEDING BEHAVIOR

¹ Department of Biology and Department of Biometry, Case Western Reserve University, Cleveland, Ohio 44106

The capture and engulfment of a single Artemia nauplius by Hydra consists of a series of complex behavioral sequences: (1) nematocyst discharge; (2) tentacular movements; (3) mouth opening, creeping over the prey and closure; and (4) inhibition of endogenous tentacle and body column contractions. The present study focused on two of these sequences: tentacular movements and inhibition of tentacle and column contractions.

On attachment of the prey to a tentacle by nematocysts, there is a latent period (1-3 sec in H. littoralis). Then the portion of the tentacle proximal to the prey contracts, sometimes accompanied by oral bending or inward spiralling when the prey attaches to the basal or distal tentacle regions, respectively. The latency is independent of the position of attachment or the type of tentacle movement. This implies that neither conduction time nor the time for the diffusion of chemical factors to the hypostome is a predominant component of the latent period.

As the prey nears the mouth on tentacle contraction, the surrounding tentacles concertedly flex orally (a concert). Concerts are frequent during and following the engulfment of prey. They are highly coordinated movements, unlike tentacle writhing, which is also frequently observed when hydra feeds. Concert frequency is markedly enhanced by exposure of the hydra to homogenates of Artemia, reduced glutathione (GSH) or analogs of GSH. In H. littoralis concert frequency increases with GSH concentration starting at 1 x 10^-10 M GSH and levelling off to a plateau value at about 8 x 10^-9 M GSH, above this concentration tentacle writhing is induced. No electrical correlates of concerts have been observed.

Column and tentacle contractions are inhibited during feeding. Such inhibition is also observed with extracts of Artemia and GSH. Concerts are negatively correlated both with tentacle and column contractions in several species of Hydra. However, the spontaneous frequencies of all three behavioral events decrease with starvation over a period of several days. GSH inhibits endogenous column contractions in H. pirardi without tentacles, but removal of tentacles significantly reduces inhibition of light-induced contractions in this animal.

Electrical potentials associated with tentacle contractions (TCP's) and column contractions (CP's) are suppressed when hydra feed on Artemia or are exposed to extracts of Artemia, or GSH. After such inhibition both types of potentials return together with contractions, at enhanced frequencies. The duration of the blockage of spontaneous CP's increases with GSH concentration over the range 1 x 10^-9 M to 10^-7 M, above which the length of inhibition is constant. A single Artemia fed to a hydra releases sufficient chemical factors into the medium to increase concert frequencies and suppress CP's in an attached bud of the fed animal, or in a second hydra some centimeters away from it.