Nematocyst Discharge Research Articles

Efferent Mechanisms of Discharging Cnidae: II. A Nematocyst Release Response in the Sea Anemone Tentacle.

Feeding behavior in cnidarians is a sequence of coordinated responses beginning with nematocyst discharge. The nematocyst response produces prey capture by envenomating prey and attaching prey to the tentacle. The strength of attachment of discharged nematocysts to the tentacle is termed intrinsic adherence and is calculated from measurements of adhesive force. Following prey capture, the feeding response involves movement of the tentacles toward the mouth and mouth opening. For ingestion to occur, nematocysts attaching the prey to the tentacles must be released from the tentacle. A nematocyst release response has been proposed, but never documented nor measured. Our criterion for a nematocyst release response is that the intrinsic adherence of discharged nematocysts must decrease to zero. The unit of nematocyst discharge in sea anemone tentacles is the cnidocyte/ supporting cell complex (CSCC). The nematocyst response includes nematocysts discharged from Type C CSCCs by physical contact alone and nematocysts discharged from the more numerous Type B CSCCs that require both chemosensitization and physical contact. We identify two prey-derived substances, N-acetylneuraminic acid (NANA) and glycine, both of which chemosensitize nematocyst discharge from Type B CSCCs at low concentrations. At higher concentrations NANA stimulates the release response of Type Cs, and glycine stimulates the release response of Type Bs.

Different nematocytes have different synapses in the sea anemone Aiptasia pallida (Cnidaria, Anthozoa).

Sea anemones feed by discharging nematocysts into their prey, but the pathway for control of nematocyst discharge is unknown. The purpose of this study was to investigate the ultrastructural evidence of neuro-nematocyte synapses and to determine the types of synaptic vesicles present at different kinds of nematocyst-containing cells. The tip and middle of tentacles from small specimens of Aiptasia pallida were prepared for electron microscopy and serial micrographs were examined. We found clear vesicles in synapses on mastigophore-containing nematocytes and dense-cored vesicles in synapses on basitrich-containing nematocytes and on one cnidoblast with a developing nematocyst. In addition, we found reciprocal neuro-neuronal and sequential neuro-neuro-nematocyte synapses in which dense-cored vesicles were present. It was concluded that : (1) neuro-nematocyte synapses are present in sea anemones, (2) different kinds of synaptic vesicles are present at cells containing different types of nematocysts, (3) synapses are present on cnidoblasts before the developing nematocyst can be identified and these synapses may have a trophic influence on nematocyst differentiation, and (4) both reciprocal and sequential synapses are present at the nematocyte, suggesting a complex pathway for neural control of nematocyst discharge. J. Morphol. 238:53-62, 1998. © 1998 Wiley-Liss, Inc.

Immunohistochemical studies of GLWamides in Cnidaria.

GLWamides are a recently described, novel family of neuropeptides in Cnidaria. Antibodies specific for the GLWamide terminus have been raised and used to evaluate the occurrence and localisation of immunopositive material in various Cnidaria in order to determine whether GLWamides are present and to obtain a first impression of the possible regulatory role of these neuropeptides. GLWamide immunoreactivity has been found in all species tested and is not confined to distinct life stages but is present during most of the life cycle of the Cnidaria. Additionally, GLWamides are expressed by different nerve cells at different life stages. GLWamide-immunoreactive cells constitute a subset of the neural equipment. Overall our data suggest that GLWamides generally occur in the nervous system of Cnidaria and that these peptides are multifunctional. Putative functions other than the control of development include the regulation of nematocyst discharge, muscle contraction and the regulation of gastric function.

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 including 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 discrimination 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 frequencies.

Frequency specificity of vibration dependent discharge of nematocysts in sea anemones

Frequency specificity of vibration dependent discharge of nematocysts in sea anemones

Cellular Origins of Kinocilia, Stereocilia, and Microvilli on Tentacles of Sea Anemones of the Genus Calliactis (Cnidaria: Anthozoa)

Sea anemones feed by orchestrated movements of tentacles covered with ciliary cones overlying both nematocytes and sensory neurons and mediating nematocyst discharge into prey. The purpose of this study was to investigate the functional morphology of tentacle sensory receptors on Calliactis parasitica and C. tricolor with a combination of scanning electron microscopy (SEM) of surface features and transmission electron microscopy (TEM) of internal cellular details. We found that the ciliary cones of two types of nematocysts differed: mastigophore-containing nematocytes had an inner ring of large-diameter stereocilia, and basitrich-containing nematocytes had small-diameter stereocilia. Two types of sensory neurons were identified: one had a ciliary cone with an inner ring of large-diameter stereocilia, and the other had a long kinocilium surrounded by a basal ring of short, stubby microvilli. Varying lengths of stereocilia on peripheral supportive cells contributed to both ciliary cones of nematocytes and sensory neurons. Both the supportive epithelial cells with stereocilia and the nonsupportive epithelial cells without stereocilia had a single long kinocilium with a basal body, an accessory centriole, and a thick striated rootlet. Gland cells had a short cilium surrounded by even shorter microvilli. Spirocysts formed apical domes on spirocytes, which had two peripheral rings of short microvilli but no cilium. We believe this combined SEM and TEM study of sensory hairs on tentacles of two species of Calliactis adds to our knowledge of ciliary structures and their cellular associations in sea anemones. Additional key words: ciliary cones, nematocytes, sensory cells, gland cells Sea anemones have feeding tentacles covered with ciliary cones which overlay nematocysts and sensory neurons (Pantin 1942). The ciliary cones have in common a kinocilium central to several rings of stereocilia of varying lengths derived from adjacent supportive cells (Mariscal 1974a,b; Peteya 1975; Mariscal & Bigger 1976). The kinocilium is characterized by a typical 9x2 + 2 pattern of microtubules, whereas the stereocilia have filamentous cores of actin similar to those of vertebrate hair cells (Watson & Roberts 1995). Ciliary cones of sea anemones have been compared to vertebrate hair bundles both morphologically and physiologically (Mariscal 1974a; Watson & Roberts 1995; Mire & Watson 1997). A directional displacement of a vertebrate hair bundle that stretches tip links between adjacent stereocilia and opens transduction channels causes depolarization of the hair cell (Osborne et al. 1984; Markin & Hudspeth 1995). Bundle deflection of sensory neuron ciliary cones having tip links between adjacent stereocilia induces ion channels a Author for correspondence. E-mail: westfall@vet.ksu.edu to open, resulting in depolarization of supportive cells on one side of the complex (Mire & Watson 1997). Ciliary cones of nematocytes and of sensory neurons in sea anemones appear similar. At first, it was believed that chemoreception of N-acetylated sugars by supportive cells caused nematocyte ciliary cones to elongate and frequency tune to movements of swimming prey, leading to discharge of nematocysts (Watson & Hessinger 1989, 1991). This view supports the independent effector hypothesis of nematocyst discharge (Pantin 1942). More recent work, using better optics, has demonstrated that the sensory neuron/supporting cell complexes respond to N-acetylated sugars and elongate their hair bundles to tune to movements of swimming prey (Mire-Thibodeaux & Watson 1994; Watson & Roberts 1995). Those studies suggested that the nervous system plays a role in nematocyst discharge. In the present study, we have used a combination of SEM and TEM to distinguish ciliary cones of sensory neurons from those of the nematocytes and to further differentiate between various kinds of nemaThis content downloaded from 157.55.39.144 on Wed, 21 Sep 2016 05:31:38 UTC All use subject to http://about.jstor.org/terms Sensory structures on tentacles of sea anemones tocytes in the genus Calliactis. We have also investigated the cellular origins of other sensory structures such as kinocilia, stereocilia, and microvilli.

Hair bundles of sea anemones as a model system for vertebrate hair bundles

Sea anemones are marine invertebrates that use hair bundles to detect swimming movements of prey. Prey are captured by nematocysts (stinging capsules) that discharge into the prey. To further characterize anemone hair bundles and to compare hair bundles in anemones with hair bundles in vertebrates, we investigated fine structure and cytochemistry of anemone hair bundles. In addition, using a biological assay based on counting nematocysts discharged into vibrating test probes, we examined sensitivity of vibration detection to aminoglycoside antibiotics, Ca2+-free seawater, and amiloride. Like vertebrate hair bundles, anemone hair bundles are composed of stereocilia, possess lateral linkages between stereocilia whose preservation for transmission electron microscopy is enhanced by ruthenium red, and possess tip links morphologically similar to vertebrate tip links. Furthermore, vibration-dependent discharge of nematocysts is reversibly inhibited by 10−4 M streptomycin and abolished by brief exposure to Ca2+-free seawater. However, unlike vertebrate hair bundles, anemone hair bundles appear to be insensitive to amiloride since vibration-dependent discharge of nematocysts is unaffected by up to mM amiloride. Thus, anemone hair bundles may serve as a useful model system for vertebrate hair bundles with the interesting feature of being insensitive to amiloride.

Imaging of electrically induced fast motion by video microscopy and triggered flash illumination

A simple and inexpensive method is described for imaging with video microscopy fast transient events that can be triggered electrically. An electronic system was developed that triggered stroboscopic illumination and generated an electrical step function. The essential feature was precise control of a sub-millisecond time delay between electrical stimulus and the following short pulse of light. With this technique (15 μs time resolution, 50 frames/s) different phases of the exocytosis and discharge of nematocysts from Hydra culgaris can be visualized. It was shown that not only stenoteles, but also desmonemes, can discharge in less than 0.5 ms.

Evidence for intracellular stores of calcium ions involved in regulating nematocyst discharge

AbstractIn sea anemones, nematocyst discharge is regulated in part by chemical substances derived from prey. Adding N‐acetylated sugars or proline to seawater sensitizes cnidocytes to discharge nematocysts. Extracellular calcium ions are required since discharge is inhibited by reducing the Ca2+ concentration in artificial seawater. Known inhibitors of L‐type Ca2+ channels, nifedipine and verapamil, reduce discharge sensitized by N‐acetylated sugars but not by proline. Conversely, known inhibitors of certain Ca2+ channels at intracellular storage sites, ryanodine and procaine, reduce discharge sensitized by proline but not by N‐acetylated sugars. Thapsigargin, an agent that inhibits uptake of Ca2+ into vesicles, sensitizes discharge. Discharge is sensitized upon incubating specimens in a caged analog of inositol 1,4,5‐trisphosphate (InsP3) and subsequently photoactivating it. Furthermore, following preincubation of specimens in certain low concentrations of caged InsP3 and subsequent photoactivation, lower concentrations of proline are required to maximally sensitize discharge. W7, an inhibitor of Ca2+/calmodulin (CaM), and KT5926, an inhibitor of CaM‐kinase II, reduce discharge sensitized by both N‐acetylated sugars and proline. Apparently, sugar receptors activate dihydropyridine‐sensitive Ca2+ channels, whereas proline receptors stimulate the production of InsP3, resulting in InsP3‐initiated release of Ca2+ from intracellular stores. This process may trigger Ca2+‐induced Ca2+ release from InsP3‐insensitive channels, which can be blocked by ryanodine or procaine. With either receptor, elevated intracellular Ca2+ binds calmodulin to form an active complex. CaM activates CaM‐kinase II, which, presumably, phosphorylates unidentified substrates, leading to sensitization of discharge. © 1995 Wiley‐Liss, Inc.

Chemoreceptor‐mediated polymerization and depolymerization of actin in hair bundles of sea anemones

Hair bundles located on tentacles of sea anemones are morphodynamic mechanoreceptors employed to regulate discharge of nematocysts into swimming prey. Activation of chemoreceptors for N-acetylated sugars is known to induce anemone hair bundles to elongate while shifting discharge to lower frequencies matching those produced by calmly swimming prey. In the continued presence of N-acetylated sugars, activation of proline receptors is known to induce hair bundles to shorten while shifting nematocyst discharge to higher frequencies presumed to correspond to movements produced by wounded, struggling prey. In the present study, N-acetylneuraminic acid (NANA) causes stereocilia to become more intensely fluorescent in confocal optical sections of phalloidin-stained specimens, suggesting that receptors for N-acetylated sugars initiate processes to increase the density of F-actin within stereocilia. Computer analysis of electron micrographs is consistent with this interpretation for large diameter stereocilia but not for small diameter stereocilia. In the continued presence of NANA, proline causes fluorescence intensity of phalloidin to decrease to or below control levels. DNaseI uniformly stains large diameter stereocilia, suggesting that these stereocilia contain a pool of G-actin. Fluorescence intensity of DNaseI in stereocilia is significantly less bright in specimens exposed to NANA alone than in specimens exposed to proline in the continued presence of NANA. It appears that whereas activated receptors for NANA induce G-actin to polymerize in large diameter stereocilia, activated receptors for proline induce F-actin to depolymerize, restoring G-actin pools.

Cyclical morphodynamics of hair bundles in sea anemones: Second messenger pathways

AbstractDespite lacking sensory organs, sea anemones apparently integrate chemical and mechanical sensory input from prey in the regulation of nematocyst discharge. Exposing anemones to exogenous N‐acetylated sugars induces hair bundles to elongate while shifting nematocyst discharge to lower frequencies matching swimming movements of prey. A pharmacological approach was employed to examine the transduction events involved in the sugar induced “tuning” of anemone hair bundles. Photoactivation of caged cyclic‐AMP (cAMP) induces bundles to elongate to a maximum length by 4 min and subsequently to shorten to near control lengths by 8 min. Continuous stimulation with sugar produces a similar effect with the cycle continuing through at least 20 min. With sugar stimulation, nematocyst discharge into targets vibrating at 5 Hz also follows the same, cyclical pattern. Bundles do not elongate in the combined presence of N‐acetylated sugars and H89, an inhibitor of protein kinase A. Elongated bundles either do not shorten, or rapidly recover from shortening, if caged cAMP is photoactivated 3.5 min after the initiation of sugar stimulation. Furthermore, bundle shortening is blocked by caffeine, an inhibitor of phosphodiesterase, W7, an inhibitor of calmodulin, nifedipine, an inhibitor of certain calcium channels, or cypermethrin, an inhibitor of protein phosphatase 2B (PP2B). Thus, chemodetection of sugars may involve an immediate elevation of intracellular cAMP, leading to an activation of protein kinase A, inducing bundles to elongate. Subsequently, calcium channels may be activated leading to a calcium/calmodulin dependent enhancement of phosphodiesterase and an activation of PP2B, inducing bundles to shorten by degrading cAMP and dephosphorylating proteins. © Wiley‐Liss, Inc.

Localization of proline receptors involved in regulating nematocyst discharge

AbstractAt least two distinct classes of chemoreceptor are known to regulate nematocyst discharge in sea anemones. One class of receptor binds N‐acetylated sugars and the other class binds certain amino compounds including the imino acid, proline. Previously, receptors for N‐acetylated sugars were localized using colloidal gold conjugates of bovine submaxillary mucin. The sugar receptors are known to cycle by receptor‐mediated endocytosis from the apical surface of supporting cells. In this study, colloidal gold conjugates of polyproline were used to determine the distribution of proline receptors. We find that polyproline‐gold, like mucin‐gold, localizes to the apical surface of supporting cells. Binding of polyproline‐gold to supporting cells is blocked by excess free polyproline or free proline. After appearing at the cell surface, polyproline‐gold enters endosomes, followed by multivesicular bodies. To test for possible artifact due to colloidal gold, trafficking of cationized ferritin (CF) was monitored in the presence of proline. CF appears at the cell surface, followed by endosomes, multivesicular bodies, and lysosomes. Some evidence is presented that proline receptors are sorted from N‐acetylated sugar receptors in endosomes. We propose that receptor‐mediated endocytosis functions in anemone tentacles to enhance the temporal resolution of chemosensing. © Wiley‐Liss, Inc.

Anemonefish symbiosis: Vulnerability and resistance of fish to the toxin of the sea anemone

Protein toxins (20 kD molecular mass) causing lysis of human erythrocytes were isolated from sea anemones ( Heteractis magnifica, Madang, Papua New Guinea, and Entacmaea quadricolor, Red Sea), which host anemonefish ( Amphiprion sp.). These toxins are also ichthyotoxic. Freshwater and marine fish exposed to toxin concentrations of 0.5 μg/ml water were killed within 2 hr and exhibited extensive pathological alterations of the gill filaments. Amphiprion species, e.g. clarkii and percula, which live in the sea anemones Heteractis crispa and Stichodactyla mertensii, were highly vulnerable to the Heteractis magnifica toxin, whereas A. percula from the sea anemone H. magnifica proved to be toxin resistant. However, another species, A. perideraion also living in H. magnifica, was highly sensitive to the toxin. The two toxins exhibited cross-reactivity: Amphiprion, resistant to H. magnifica toxin, was also resistant to Entacmaea quadricolor toxin; but all fish were killed by other membrane-active substances such as gramicidin, saponin and latrunculin. The results of the study indicate that resistance to toxins secreted by the sea anemone has evolved in some Amphiprion species, but it is not an essential or a major factor in the anemonefish symbiosis. The skin's mucus layer seems to provide protection from nematocyst discharge.

Calcium- and voltage-dependence of nematocyst discharge in Hydra vulgaris

In Hydra vulgaris, discharge of stenotele nematocysts was induced by contact with prey, electrical stimuli, or increase in the external potassium concentration. In each case 10-4M calcium was required in the culture medium. The results indicated a voltage- and calcium-dependent mechanism different from mechano- or chemoreception allowing calcium influx from the external medium. A threshold for activation was suggested by the steep increase of the rate of electrically induced discharge in external fields of 3.5 kV/m. Although organic antagonists for vertebrate calcium channels were ineffective in blocking the calcium-induced nematocyst discharge, inorganic divalent and trivalent cations competitively inhibited the process, with a sequence (Co2+ < Ni2+ < Cd2+ < La3+ < Gd3+) similar to that seen for antagonism of calcium influx through voltage-dependent channels. Magnesium, an intracellular calcium antagonist, decreased nematocyst discharge, while strontium replacing calcium supported the discharge at a lowered rate. It is concluded that in the nematocyte a voltage-activated influx of calcium through apical ion channels initiates the discharge of the nematocyst in an exocytotic process.

Do anemonefishes use molecular mimicry to avoid being stung by host anemones?

Anemonefishes are known to have a protective mucous coat that allows them to contact the tentacles of their host anemone without being stung. There are two conflicting hypotheses as to the source and biochemical properties of this mucous coating. One hypothesis proposes that anemonefishes acquire anemone substances from their hosts during the behavioral process of acclimation, that protect the fish from being stung. Anemonefishes are considered to use anemone mucus as “chemical camouflage” or “macromolecular mimicry” to avoid recognition as “not-self” by the anemone, and possible subsequent stinging. Another hypothesis is that anemonefishes produce their own protective mucus coat, which lacks substances that elicit cnida (nematocyst and spirocyst) discharge by their hosts. The present study used immunological techniques to test whether the anemonefish Amphiprion clarkii (Bennett) (which was innately protected from two of the anemones used in the present study) has a mucous coat that resembles the external mucus of anemones. Polyclonal antibodies were prepared to the mucus of four species of anemones [ Heteractis crispa, (Ehrenberg) Stichodactyla haddoni (Saville-Kent), Macrodactyla doreenensis (Quoy and Gaimard), and Condylactis gigantea (Weinland)]. Ouchterlony (double immunodiffusion) tests showed that different antigens were present in the mucus of the four anemone species. Anemone antigens were not detected in the mucous coating of either naive (fish that had never before encountered sea anemones) or associated (those living with sea anemones) anemonefishes in Ouchterlony tests. However, more sensitive ELISA (enzyme-linked immunosorbent assay) tests showed that anemone mucus antigens were present in the mucous coating of associated anemonefish, but not naive fish. This showed that an innately protected A. clarkii does not produce a mucus coat that is biochemically similar to that of anemones, but that the same fish does acquire anemone substances in its mucus coat when it associates with anemones in aquaria. It remains to be shown whether these anemone substances actually provide the initial protection for those anemonefishes that must undergo acclimation behavior in order to keep from being stung by their host sea anemones, or additional protection for innately protected fishes.

Morphodynamic hair bundles arising from sensory cell/supporting cell complexes frequency‐tune nematocyst discharge in sea anemones

Discharge of nematocysts from cnidocytes occurs in response to appropriate chemical and mechanical stimulation. In sea anemone tentacles, activating chemoreceptors for N-acetylated sugars shifts maximal discharge into vibrating targets to low frequencies corresponding to prey movements and induces hair bundles to elongate by approximately 1-2 microns. Until now, only indirect, correlative evidence linked these two events. Using cytochalasin D, we provide evidence that bundle elongation is necessary for the frequency shift. Moreover, we find that only bundles associated with sensory cell/supporting cell complexes elongate with chemosensitization, and not bundles associated with cnidocyte/supporting cell complexes as was previously thought. Cytochemical labeling of sensory cells, purported to be bipolar neurons connected to the nerve net, suggests that sensory cells may interconnect with each other and with cnidocytes. Taken together, these findings are incompatible with the classical view that cnidocytes are independent effectors of nematocyst discharge and, furthermore, implicate the involvement of morphodynamic neurons in fine-tuning vibration-dependent discharge of nematocysts into swimming prey.

Frequency and amplitude tuning of nematocyst discharge by proline

AbstractIn sea anemone tentacles, discharge of nematocysts into vibrating targets is regulated by hair bundles. N‐acetylated sugars are known to induce hair bundles to elongate while tuning nematocyst discharge to low frequencies and small amplitudes corresponding to movements produced by calmly swimming prey. Proline is known to induce hair bundles to shorten while tuning nematocyst discharge to higher frequencies. In this report, we provide evidence suggesting that proline linearly tunes nematocyst discharge. Nematocyst discharge is tuned by increases of 10−18 M proline above background levels ranging from 10−12 M to 10−8 M proline, but only after anemones adapt to the background levels of proline. Anemones adapt more rapidly to 10−12 M proline than to 10−10 M, followed by 10−8 M proline. L‐proline and D‐proline comparably tune nematocyst discharge. Proline tunes discharge to higher frequencies and/or to larger amplitudes at the same lower frequencies produced by prey. We propose that N‐acetylated sugars tune nematocyst discharge to the movements of calmly swimming prey. After the prey is wounded by nematocysts, it releases proline into the seawater. This proline tunes nematocyst discharge to higher frequencies and/or larger amplitudes at low frequencies corresponding to movements produced by struggling, wounded prey. Thus, the greatest numbers of nematocysts may be discharged into calmly swimming, fresh prey and into vigorously struggling, wounded prey. © 1994 Wiley‐Liss, Inc.

Evidence for calcium channels involved in regulating nematocyst discharge

In the tentacles of sea anemones, nematocyst discharge is regulated by cnidocyte/supporting cell complexes (CSCCs) of which three functional types have been identified: A, B and C. Type A CSCCs respond to contact by vibrating targets. Types B and C CSCCs respond to contact by static targets. Whereas type C CSCCs respond to contact alone, type B CSCCs require that surface chemoreceptors bind ligands before becoming responsive. Reducing Ca 2+ levels in artificial seawater to below 1 mM inhibits discharge from each type of CSCC. The calcium channel inhibitors, nifedipine or verapamil, selectively inhibit discharge from type B CSCCs. The calcium channel activator, Bay K-8644, mimics the biphasic dose response of type B CSCCs to natural chemosensitizers such as N-acetylated sugars. Discharge from type A CSCCs is unaffected by inhibitors of L-type calcium channels, but is selectively inhibited by the aminoglycoside antibiotics, gentamicin and streptomycin. While each type of CSCC requires extracellular calcium, the calcium channels employed may vary according to CSCC type.

ANTAGONISTIC FREQUENCY TUNING OF HAIR BUNDLES BY DIFFERENT CHEMORECEPTORS REGULATES NEMATOCYST DISCHARGE

Sea anemones capture prey by discharging nematocysts into them. Chemical and mechanical cues identify suitable prey to sensory receptor systems on the anemone. Conjugated N-acetylated sugars from prey bind to chemoreceptors on cnidocyte/supporting cell complexes to tune hair bundles on the complexes to lower frequencies matching prey movements. The hair bundles regulate discharge of microbasic p-mastigophore nematocysts into vibrating targets. Provided that proline receptors are activated after those for N-acetylated sugars, nematocyst discharge is tuned to much higher frequencies. Thus, anemone hair bundles are tuned to either higher or lower frequencies by antagonistic chemoreceptors. Chemoreceptors for proline can adapt to 10(-8) mol l-1 proline and yet respond to increases in proline concentration of less than 10(-15) mol l-1. Under these conditions, too few molecules of proline are added to activate chemoreceptors on all responding cnidocyte/supporting cell complexes. Evidence indicates that the extreme sensitivity of anemones to proline may be attributed, in part, to intercellular communication.

Direct Monitoring of Intracellular Calcium Ions in Sea Anemone Tentacles Suggests Regulation of Nematocyst Discharge by Remote, Rare Epidermal Cells.

In tentacles of sea anemones, cnidocytes and adjacent supporting cells are believed to be independent receptor-effector complexes that regulate nematocyst discharge in response to exogenous N-acetylated sugars. When sugar chemoreceptors on supporting cells are activated, nematocyst discharge is two- to threefold greater than discharge without chemosensitization. To examine the role of Ca2+ as a second messenger in chemodetection of sugars, we used fluo-3 to monitor Ca2+ levels in epidermal cells of intact anemone tentacles. Certain epidermal cells exhibit relatively high Ca2+ both with and without chemosensitization. With chemosensitization, a two-to threefold increase occurs in the abundance of relatively rare cells exhibiting the highest Ca2+ levels. Timecourses depicting abundances of these rare cells in chemosensitized specimens show positive correlations to timecourses for nematocyst discharge from chemosensitized specimens and for labeling of chemoreceptors. Cnidocyte/supporting-cell complexes discharging nematocysts are about three times more abundant than the rare cells exhibiting the highest intracellular Ca2+ levels. One interpretation of these data is that the Ca2+-dependent regulation of nematocyst discharge occurring with chemosensitization involves intense Ca2+ signaling by remote, rare cells. This interpretation is inconsistent with the current model that portrays cnidocyte/supporting-cell complexes as independent effectors of nematocyst discharge.