Control of Cnida Discharge: I. Evidence for Two Classes of Chemoreceptor

Abstract

Appropriate chemical stimulation of cnidocytes along with mechanical stimulation is required to trigger discharge of cnidae. It has been generally assumed that such chemosensitization is mediated via specific chemoreceptors. Such chemoreceptors and their complementary ligands have never been identified. We now identify two groups of naturally occurring substances that chemosensitize cnida discharge in the feeding tentacles of the sea anemone, Aiptasia pallida. In addition, using a novel technique to quantify cnida discharge we demonstrate that these chemosensitizers act through at least two distinct classes of receptors. One class is broadly specific toward a variety of amino and imino acids and histamine (Ko 5 = 1 \-30nM), but is competitively inhibited by antihistamines (K, = 0. 1 -7.4 nM). A second class is specific for N-acetylated sugars (K0.5 = 0.1-1.5 nM), but not affected by antihistamines. Presumably, these chemoreceptors detect specific substances from potential prey. Thus, cnidocytes are sensitized to discharge their cnidae in response to mechanical stimuli originating from the prey.

Predation by hydromedusae on hydrozoan embryos and larvae: planktonic kin selection?

“…However, relatively little attention has been paid to patterns and mechanisms of prey selection by cnidarian zooplankton. The finding that 'food juices' and mechanical stimulation in combination cause nematocyst discharge (Pantin 1942) has only recently been re-addressed (Lubbock 1979, Thorington & Hessinger 1988a, despite an enormous amount of work on the glutathione 'feeding response' in Hydra (reviewed by Lenhoff 1974). Greene (1985) categorized zooplanktivores on the basis of their feeding mechanisms (e.g.…”

Prey selection mechanism of ambush-foraging hydromedusae

“…While encounter rates are largely affected by tentacle characteristics such as spacing, length and number (Mills 1981, Madin 1988, capture and transfer efficiencies are primarily determined by nematocyst properties (Purcell & Mills 1988, Thorington & Hessinger 1988, 1996, Colin & Costello 2007. Though impossible to confirm without observing actual nematocyst -prey interactions, we suggest that capture efficiencies were most likely influenced by the failure of nematocysts to discharge during encounters with small or unrecognized prey.…”

“…Thus, only items recognized as food were captured. Consequently, prey selection patterns of ambush-foraging hydromedusae have the potential to be highly specific, since nematocyst discharge may be highly sensitive to different mechanical thresholds and chemical cues (Thorington & Hessinger 1988, Watson & Hessinger 1991.…”

Chemically regulated feeding by a midwater medusa

“…Proteins and amino acids have been found to chemosensitize the cnidocytes on the tentacles of sea anemones (Lubbock 1979;Thorington and Hessinger 1988), whereas ammonium is a common catabolite release into seawater following the breakdown of proteins and amino acids by marine organisms. We therefore assumed that even if proteins, amino acids, or ammonium are not natural attractants, production of chemical stimuli that induce feeding behaviors might be coupled with their release.…”