Chemoreception In Sea Anemones: Betaine Stimulates The Pre-Feeding Response In Urticina Eques and u. Felina

Although the SSI plays an important part in the coordination of many behaviour patterns, no chemical that stimulates it has been identified. We have investigated SSI excitation during the pre-feeding response of Urticina felina and U. eques (previously known as Tealia felina var. coriacea and var. lofotensis, Manuel, 1981), using the technique developed by Lawn (1975). We have screened many possible activators, chosen from published analyses of known stimulatory foods, e.g. muscles from fish (Long, 1961), Nephrops (Robertson, 1961) and Mytilus (Bricteux-Grégoire, Duchâteau-Bosson, Jeuniaux & Florkin, 1964).

Anemones were bisected longitudinally, pinned cut edge down, and allowed to recover for 24 h. Test solutions were either applied in the bath to give a known final concentration or were drawn into a suction electrode (2 mm internal tip diameter) that was then attached to the column. Suction electrodes on tentacles were used to record electrical activity in all three conducting systems. As spontaneous SSI pulses were rare, responses to the applied chemical were obvious.

The chemicals tested, at concentrations between 10⁻⁶ and 10⁻¹moll⁻¹ (final bath concentration or concentration in electrode) were AMP, alanine, arginine, asparagine, aspartic acid, betaine, carnitine, cysteine, cystine, glutamic acid, glutamine, reduced glutathione (GSH), glycine, histidine, hypoxanthine, inosine, isoleucine, lactic acid, leucine, lysine, methionine, ornithine, phenylalanine, proline, serine, taurine, threonine, trimethylamine, trimethylamine oxide (TMO), tryptophan, tyrosine and valine. Of these, betaine (trimethyl glycine) was the most effective SSI stimulant (Fig. 1). We conclude that betaine is the normal pre-feeding stimulant, as intact anemones, exposed to 10⁻²moll⁻¹ betaine, displayed typical pre-feeding movements. Both carnitine and TMO evoked some SSI activity, but were always less effective than betaine. These results suggest that the receptors detect the (CH3)3-N moiety. No other substance excited the SSI although some (proline in particular) stimulated the SS2 when placed in the bath, but not when applied to the column. There is some evidence that the SS2, although an endodermal conducting system, has ectodermal receptors in the tentacles and pharynx (McFarlane, 1975). GSH, in the bath, excited TCNN pulses.

Betaine was only effective when the electrode containing it was attached to the column: the other ectodermal regions tested, the tentacles and oral disc, were insensitive. The observed distribution of chemoreceptive sites agrees with the results obtained by Lawn (1975). The receptors appear to be widespread on the column but their nature is not known. Most properties of the SSI are consistent with a neural basis for the conducting system but a complete nerve net has not been described in the column ectoderm. In Metridium senile scattered sense cells are, however, present in this position (Batham, Pantin & Robson, 1960) and we propose that such cells are the SSI chemoreceptors.

A dose/response curve for the action of betaine on the SSI (Fig. 2), based upon the activity evoked during the first 2min of the response, shows that the SSI chemoreceptors are not particularly sensitive. High betaine concentrations, ranging from 10⁻² to 1·5×10⁻¹ molkg⁻¹ tissue water, are, however, found in most marine animals (Burton, 1983). The concentrations present in known pre-feeding activators are: Mytilus muscle, 5× 10⁻²molI⁻¹; Homarus muscle, l·5×10⁻²moll⁻¹; and fish muscle, 10⁻²moll⁻¹ (Bricteux-Grégoire et al. 1964; Beers, 1967; Long, 1961). These values, taken together with the dose/response curve, suggest that food must be close to the column to excite the chemoreceptors. Unlike Urticina eques, U. felina is a littoral species and, as high concentrations of activators might occur in tide pools, close proximity of food might not always be necessary for pre-feeding behaviour to be evoked.

It is not known how many SSI pulses are required to elicit the pre-feeding behaviour. Although the SSI sensory response was normally complete within a few minutes, cycles of column extension and bending could persist for several hours. Possibly the SSI causes long-term activation of phases of nerve net activity, as is known to occur in Calliactis parasitica (McFarlane, 1983).

This is the first time an identifiable chemical has been shown, by electrophysiological techniques, to excite the SSI. Other chemically induced, SSI-coordinated, behaviour patterns include shell-climbing behaviour in Calliactisparasitica, excited by mollusc shells (Ross & Sutton, 1961; McFarlane, 1976) and swimming in Stomphia coccínea, evoked by certain starfish and nudibranchs (Yentsch & Pierce, 1955; Robson, 1961; Lawn, 1976). In neither case, however, has the stimulant been identified. Some activators of true feeding behaviour in anemones are known (Lindstedt, 1971): tentacle contraction and mouth opening are triggered by amino acids, such as proline, glutamic acid and asparagine, or by the tripeptide reduced glutathione (GSH). Proline and GSH, applied to the mouth of C. parasitica, evoke SS2 pulses that lead to mouth opening and pharynx protrusion (McFarlane, 1975).

The only other identified excitatory chemical is the alarm pheromone, antho-pleurine, which is released by wounded Anthopleura elegantissima, and causes fast withdrawal contractions of nearby conspecifics (Howe, 1976a). In C. parasitica it is the TCNN that normally coordinates fast and slow muscle contractions triggered by mechanical stimulation or endogenous pacemakers (McFarlane, 1982); the alarm reaction may be a rare example of chemical excitation of the TCNN. Interestingly, like betaine, anthopleurine is a quaternary ammonium compound. The anthopleurine response differs from the betaine response, however, in two main ways. First, the anthopleurine receptors are extremely sensitive: they respond to concentrations as low as 10⁻⁹moll⁻¹. Secondly, anthopleurine receptors are on the tentacles, not on the column; this is to be expected as in all species so far studied the TCNN is found in the tentacle ectoderm but not the column ectoderm (Batham et al. 1960).

Anthopleura elegantissima also shows pre-feeding behaviour and Howe (1976b) suggested, but without supporting electrophysiological evidence, that proline was the stimulant. In Urticina eques and U. felina proline excites the SS2 but not the SSI. In both these species the SSI responds to betaine: this may reflect their close taxonomic relationship. It remains to be seen if other anemones employ other prefeeding activators.

In some decapod crustaceans the quaternary ammonium compounds betaine and TMO have been shown to stimulate receptors upon the dactyls and effect the orientation phase of feeding behaviour (Laverack, 1963).