Frequency specificity of vibration dependent discharge of nematocysts in sea anemones

Hair bundles on tentacles of sea anemones are similar to vertebrate hair bundles in terms of structure and function. Anemone hair bundles are involved in regulating discharge of nematocysts, "stinging capsules," used to capture prey. N-acetylated sugars from the prey includ- ing N-acetylneuraminic acid (NANA) induce hair bundles to elongate while shifting vibration dependent discharge of nematocysts to lower frequencies matching prey movements. In the present study, we find that vibration dependent discharge of nematocysts exhibits sharp frequency dis- crimination to within one Hz. Testing at one-Hz intervals over the range of frequencies spanning 1-75 Hz, we find that seven of these are stimulatory in seawater alone. A total of twenty-six frequencies are stimulatory in the presence of NANA. Stimulatory frequencies in NANA are lower than those in seawater alone. We find that antagonists of ryanodine receptors including ryanodine, procaine and tetracaine shift discharge to lower frequencies. Fluorescently tagged ryanodine labels numerous small loci in the apical cytoplasm of supporting cells. We propose that calcium induced calcium release (CICR) via ryanodine receptors may sharpen frequency specificity and/or cause shortening of hair bundles to shift frequency specificity to higher fre- quencies. J. Exp. Zool. 281:582-593, 1998. © 1998 Wiley-Liss, Inc. Sea anemones are slow moving, predominantly sessile invertebrates that rely on prey locomotion or water currents to bring prey into contact with their tentacles. Anemones with small, fine ten- tacles or long, filamentous tentacles are prima- rily planktivorous (Shick, '91). Prey are captured by nematocysts and other cnidae, complex secre- tory products consisting of capsules containing eversible tubules (Mariscal, '74, '84). In response to appropriate chemical and mechanical stimuli (Pantin, '42), effector cells called cnidocytes trig- ger cnida discharge, rapid eversion of the tubule (Skaer and Picken, '65; Holstein and Tardent, '84). Depending on the type of cnida, the everting tubule may adhere to the surface of the prey, en- tangle its appendages, or penetrate its integu- ment to inject potent toxins (Mariscal, '74, '84). In anemones, most of the published work on the regulation of cnida discharge has concerned microbasic p-mastigophore nematocyts, selected for study because this nematocyst type is abun- dant in the cnidom (complement of cnidae) of the tentacle. Furthermore, microbasic p-mastigophore nematocysts discharged into gelatin coated test probes are relatively easy to count using phase contrast optics (e.g., Watson and Hudson, '94). Mechanical stimuli alone are sufficient to trig- ger discharge. Relevant mechanical stimulation can comprise contact alone or contact with a vi- brating test probe. Contact between a tentacle and nonvibrating test probe elicits discharge of a baseline number of nematocysts. Provided the test probe is vibrating at a preferred frequency (here- after referred to as a "key" frequency), about twice as many cnidocytes discharge nematocysts into the test probe as baseline. The number of nema- tocysts discharged into test probes vibrating at frequencies other than key frequencies is compa- rable to baseline (reviewed in Watson and Mire- Thibodeaux, '94). Thus far, two chemoreceptors are known to affect discharge of nematocysts. These chemore- ceptors bind N-acetylated sugars and amino com- pounds, respectively, compounds derived from prey (Thorington and Hessinger, '88). N-acety- lated sugars are contained in prey mucins and other glycoproteins. Amino compounds are con- tained in hemolymph of prey. Chemical stimuli