Two slow conduction systems co-ordinate shell-climbing behaviour in the sea anemone Calliactis parasitica.

1. Pulses in two slow conducting systems, the ectodermal SS 1 and the endodermal SS 2, were recorded during shell-climbing behaviour. The mean pulse interval of SS 1 pulses was 7-4 s and that of SS 2 pulses was 6-4 s. Activity in both systems may arise as a sensory response of tentacles to shell contact, but the SS 1 and SS 2 may not share the same receptors. 2. Electrical stimulation of the SS 1 and SS 2 together, at a frequency of 1 shock every 5 s, elicits shell-climbing behaviour in the absence of a shell. 3. Low-frequency nerve-net activity (about 1 pulse every 15 s) accompanies column bending during both normal and electrically elicited responses. This activity probably arises as a result of column bending and is not due to a sensory response to the shell.     

Control of mouth opening and pharynx protrusion during feeding in the sea anemone Calliactis parasitica.

1. Activity in all three known conducting systems (the nerve net, SS1, and SS2) may accompany feeding in Calliactis. The most marked response is an increase in pulse frequency in the SS2 (the endodermal slow conducting system) during mouth opening and pharynx protrusion. 2. Electrical stimulation of the SS2 at a frequency of one shock every 5 s elicits mouth opening and pharynx protrusion in the absence of food. 3. A rise in SS2 pulse frequency is also evoked by food extracts, some amino acids, and in particular by the tripeptide reduced glutathione, which produces a response at a concentration of 10(-5) M. 4. Although the SS2 is an endodermal system, the receptors involved in the response to food appear to be ectodermal. 5. The epithelium that lines the pharynx conducts SS1 pulses, but there is some evidence for polarization of conduction.     

The transmission of impulses in the ectodermal slow conduction system of the sea anemone Calliactis parasitica (Couch).

1. The SS 1 fatigues in response to repetitive electrical stimulation. This fatigue is manifested by an increased conduction delay and a decreased SS 1 pulse amplitude. 2. Continued repetitive stimulation leads to the failure of the system. Recovery may take many seconds. Narrow strips of column fail more rapidly than wide strips. 3. The increased conduction delay is explained in terms of a decrease in the population of spiking cells. 4. A computer model is described and analysed. It suggests that conduction between electrically coupled ectoderm cells could be the basis for the SS1. The SS 1 may have properties not so far experimentally demonstrated; for example, under certain conditions it could behave as a local system.     

Calitoxin, a neurotoxic peptide from the sea anemone Calliactis parasitica: amino acid sequence and electrophysiological properties

We have isolated a new toxin, calitoxin (CLX), from the sea anemone Calliactis parasitica whose amino acid sequence differs greatly from that of other sea anemone toxins. The polypeptide chain contains 46 amino acid residues, with a molecular mass of 4886 Da and an isoelectric point at pH 5.4. The amino acid sequence determined by Edman degradation of the reduced, S-carboxymethylated polypeptide chain and tryptic and chymotryptic peptides is Ile-Glu-Cys-Lys-Cys-Glu-Gly-Asp-Ala-Pro-Asp-Leu-Ser-His-Met-Thr-Gly-Thr- Val-Tyr - Phe-Ser-Cys-Lys-Gly-Gly-Asp-Gly-Ser-Trp-Ser-Lys-Cys-Asn-Thr-Tyr-Thr-Ala- Val-Ala - Asp-Cys-Cys-His-Glu-Ala. No cysteine residues were present in the peptide. Similarly to other sea anemone toxins, calitoxin interacts, in crustacean nerve muscle preparations, with axonal and not with muscle membranes, inducing a massive release of neurotransmitter that causes a strong muscle contraction. The low homology of CLX with RP II and ATX II toxins has implications regarding the role played by particular amino acid residues.     

Delayed initiation of SS1 pulses in the sea anemone Calliactis parasitica: evidence for a fourth conducting system.

1. Single electrical shocks to the column sometimes elicit a series of 1-6 pulses in the SS1 (ectodermal slow system) but the first pulse does not appear until 5-28 s after stimulation. These pulses occur in addition to the early SS1 pulse which follows every shock and which has a conduction delay of less than 1 s. 2. The threshold of the delayed SS1 response is different from the thresholds of the three known conducting systems (through-conducting nerve net, SS1, and SS2). 3. In the case of stimulation of the column, the delayed SS1 pulses do not arise at the point of stimulation but probably originate in the tentacles or upper column. The pulse origin can shift during a single burst. 4. The pathway from the point of stimulation to the site of origin of delayed SS1 pulses is endodermal. We propose that this pathway represents a fourth conducting system (Delayed Initiation System--DIS). The DIS must connect, across the mesogloea, with the ectodermal SS1. The long pulse delay and repetitive firing may derive from pacemaker activity in the DIS. The DIS pacemakers closely resemble the pacemakers connected to the through-conducting nerve net. The DIS may be neuronal. 5. Delayed SS1 pulse bursts from unattached anemones showed an earlier onset, and more pulses/burst, than those from attached anemones. 6. Delayed SS1 pulses can also be evoked by electrical, and in some cases mechanical, stimulation of the pedal disc, tentacles, and pharynx, but there are regional differences in the number of pulses evoked, in their delay, and in their site of origin.     

Spontaneous contractions and nerve net activity in the sea anemone calliactis parasitica

Suction electrodes attached to tentacles of the sea anemone Calliactis parasitica record regular bursts of activity associated with the through‐conducting nerve net. Most bursts consist of 10–15 pulses at a frequency of 1 every 4 sec to 1 every 10 sec. The interval between bursts is usually 10–20 min. Regularity in pulse number and frequency in successive bursts suggests that the activity originates from a pacemaker. Bursts are always followed by slow contraction of endodermal longitudinal (parietal) muscles after a short delay, and endo‐dermal circular muscles after a long delay. A simple model for nervous pacemaker control of rhythmic contractions cannot be proposed as slow contractions can also occur in the absence of recorded nerve net activity.     

The sea anemone Calliactis tricolor and its association with the Hermit crab Dardanus venosus

The association of certain sea anemones and hermit crabs is established in different ways according to the species involved. The present study shows that the behaviour patterns of the two partners in associations between Calliactis tricolor (Lesueur) and Dardanus venosus (H. M. Edwards) in the Caribbean are similar to those seen in the Mediterranean C. parasitica and D. arrosor .
Although about half the crabs display an active behaviour pattern in laboratory trials, the anemone frequently settles on shells unaided and most C. tricolor respond to molluscan shells by clinging with their tentacles until the pedal disc can be attached. As a rule it is necessary for the anemone to relax and to cling to the shell if the crab is to be successful in transferring the anemone to its shell.
The behaviour patterns of D. venosus include a distinctive tapping of the edge of the base of C. tricolor after which the anemone is pulled or lifted off and transferred to the shell. An experimenter can also cause the anemone to relax and to detach itself by tapping the edge of the base with plastic rods after the manner of the crab.
The mechanisms by which the tentacles of Calliactis cling to, and by which the base settles upon, shells still remain to be elucidated. The participation of nematocysts in these processes could not be demonstrated in this study.
C. tricolor is found on some other pagurid and non-pagurid crabs in various localities. These associations need to be investigated fully in order that the behaviour patterns of C. tricolor may be correctly interpreted and compared with those of other species of Calliactis .     

The influence of feeding on behavior and nematocyst discharge of the sea anemone Calliactis tricolor

A decrease in basitrichous isorhiza and spirocyst nematocysts is observed in a fully fed sea anemone, Calliactis tricolor, as compared to unfed animals. Discharge of tentacle nematocysts of an intact living animal is inhibited by anesthetics and a decrease in temperature. The chemical (food origin) threshold for discharge of basitrichs is higher than for spirocysts and the implications of these results are discussed. The most probable cause for the observed decrease in nematocyst discharge is a combination of food present in the gastrovascular cavity and a physical stretching of the cavity.     

Ultrastructure of the tentacle nerve plexus and putative neural pathways in sea anemones

Abstract. Neurons of sea anemone tentacles receive stimuli via sensory cells and process and transmit information via a plexus of nerve fibers. The nerve plexus is best revealed by scanning electron microscopy of epidermal peels of the tentacles. The nerve plexus lies above the epidermal muscular layer where it appears as numerous parallel longitudinal and short interconnected nerve fibers in Calliactis parasitica . Bipolar and multipolar neurons are present and neurites form interneuronal and neuromuscular synaptic contacts. Transmission electron microscopy of cross sections of tentacles of small animals, both C. parasitica and Aiptasia pallida , reveals bundles of 50–100 nerve fibers lying above groups of longitudinal muscle fibers separated by intrusions of mesoglea. Smaller groups of 10–50 slender nerve fibers are oriented at right angles to the circular muscle formed by the bases of the digestive cells. The unmyelinated nerve fibers lack any glial wrapping, although some bundles of epidermal fibers are partially enveloped by cytoplasmic extensions of the muscle cells; small gastrodermal nerve bundles lie between digestive epithelial cells above their basal myonemes. A hypothetical model for sensory input and motor output in the epidermal and gastrodermal nerve plexuses of sea anemones is proposed.     

Ultrastructure of neuro‐spirocyte synapses in the sea anemone Aiptasia pallida (Cnidaria,Anthozoa, Zoantharia)

Using transmission electron microscopy of serially sectioned tentacles from the sea anemone Aiptasia pallida, we located and characterized two types of neuro‐spirocyte synapses. Clear vesicles were observed at 10 synapses and dense‐cored vesicles at five synapses. The diameters of vesicles at each neuro‐spirocyte synapse were averaged; clear vesicles ranged from 49–89 nm in diameter, whereas the dense‐cored vesicles ranged from 97–120 nm in diameter. One sequential pair of synapses included a neuro‐spirocyte synapse with clear vesicles (81 nm) and a neuro‐neuronal synapse with dense‐cored vesicles (168 nm). A second synapse on the same cell had dense‐cored vesicles (103 nm). An Antho‐RFamide‐labeled ganglion cell and three different neurites were observed adjacent to spirocytes, but no neuro‐spirocyte synapses were present. Many of the spirocytes also were immunoreactive to Antho‐RFamide. The presence of sequential neuro‐neuro‐spirocyte synapses suggests that synaptic modulation may be involved in the neural control of spirocyst discharge. The occurrence of either dense‐cored or clear vesicles at neuro‐spirocyte synapses suggests that at least two types of neurotransmitter substances control the discharge of spirocysts in sea anemones. J. Morphol. 241:165–173, 1999. © 1999 Wiley‐Liss, Inc.     

Receptor-mediated endocytosis of a chemoreceptor involved in triggering the discharge of cnidae in a sea anemone tentacle

Collodial gold coated with the glycoprotein, bovine submaxillary mucin (BSM-gold), was used to localize chemoreceptors known to be involved in triggering the discharge of cnidae in sea anemones. BSM-gold binds exclusively at the apical surface of the supporting cell, the cell adjacent to the cnidocyte (Watson and Hessinger, 1986). Subsequent to binding, BSM-gold is internalized into endosomes and then translocated to multivesicular bodies (MVBs) and lysosomes. At cold temperature (4 degrees C), BSM-gold appears in endosomes near the surface of the cell but not in endosomes located more medially in the cell, nor in MVBs or lysosomes. The kinetics and sequence of intracellular translocation of BSM-gold were studied by fixing animals at various intervals following incubation in BSM-gold. Unlike that for supporting cells adjacent to non-cnidocytes, the amount of gold at the surface of supporting cells adjacent to penetrant cnidocytes does not seem to change despite considerable internalization of the mucin-probe. Apparently, free receptors replace receptor-ligand complexes in a one-for-one fashion in these cells.     

Ultrastructural evidence for two-cell and three-cell neural pathways in the tentacle epidermis of the sea anemone Aiptasia pallida.

Sensory and ganglion cells in the tentacle epidermis of the sea anemone Aiptasia pallida were traced in serial transmission electron micrographs to their synaptic contacts on other cells. Sensory cell synapses were found on spirocytes, muscle cells, and ganglion cells. Ganglion cells, in turn, synapsed on sensory cells, spirocytes, muscle cells, and other neurons and formed en passant axo-axonal synapses. Axonal synapses on nematocytes and gland cells were not traced to their cells of origin, i.e., identified sensory or ganglion cells. Direct synaptic contacts of sensory cells with spirocytes and sensory cells with muscle cells suggest a local two-cell pathway for spirocyst discharge and muscle cell contraction, whereas interjection of a ganglion cell between the sensory and effector cells creates a local three-cell pathway. The network of ganglion cells and their processes allows for a through-conduction system that is interconnected by chemical synapses. Although the sea anemone nervous system is more complex than that of Hydra, it has similar two-cell and three-cell effector pathways that may function in local responses to tentacle contact with food.     

Ultrastructure of the mesoglea of the sea anemone Nematostella vectensis (Edwardsiidae)

Abstract. Cnidarians have extracellular matrix, or mesoglea, situated between an outer epidermis and an inner gastrodermis. In this article, we describe the ultrastructure of the mesoglea of polyps of Nematostella vectensis during development and regeneration. The column wall of recently metamorphosed polyps had basal laminae composed of a meshwork of thin filaments underlying each epithelium and a network of unstriated thick (20–25 nm in diameter) and thin fibrils (～5 nm) decorated with particulate matter. In juvenile polyps with eight tentacles, the system of thick fibrils was concentrated near the gastrodermis. In the column wall and mesenteries of the adult there were bundles of thick fibrils that ran parallel to the myonemes. In regenerating polyps 2 days after transection, the network of thin fibrils and particulate material as well as the basal lamina largely disappeared in the healing part of the oral, but not aboral, half. In the regenerating portion of the aboral half 1 and 2 days after transection, the bundles of thick fibrils were smaller and less organized, and the basal laminae were thicker than in the column wall of untransected polyps. In both regenerating halves, the general organization of the mesoglea of normal polyps was reattained by 5 days after transection. At all stages the mesoglea contained cellular processes that may belong to amebocytes; nucleated amebocytes with a range of shapes were present in the mesoglea of the column wall and mesenteries of adult polyps. Certain features of the mesoglea of members of N. vectensis and Hydra are similar, especially the ultrastructure of the basal laminae, but the fibrillar systems of these two model cnidarians are different. Temporal and spatial differences in the composition of the mesoglea of N. vectensis point to different roles for its components during development and regeneration.     

Neuroglandular synapses in the pharynx of the sea anemone Aiptasia pallida (Cnidaria, Anthozoa)

Scanning and transmission electron microscopy of the pharynx of the sea anemone Aiptasia pallida revealed a heavily ciliated epidermis and two types of gland cells not known previously to be innervated. By tracing serial cross sections of the pharynx, we located and characterized two types of neuroglandular synapses (i.e., those having clear vesicles and those with dense-cored vesicles). The diameters of the vesicles at each synapse were averaged; clear vesicles ranged from 70 to 103 nm in diameter and were observed at synapses to both mucous and zymogenic gland cells. Dense-cored vesicles ranged from 53 to 85 nm in diameter and were observed at synapses to two mucous gland cells. One mucous gland cell had three neuroglandular synapses, one with clear vesicles and two with dense-cored vesicles. The occurrence of either clear or dense-cored vesicles at neuroglandular synapses suggests that at least two types of neurotransmitter substances control the secretion of mucus in the sea anemone pharynx. To date, only clear vesicles have been observed at a neurozymogenic gland cell synapse in the pharynx. No evidence of immunoreactivity to phenylethanolamine-N-methyl transferase was observed at neuroglandular synapses, suggesting that adrenaline is not a transmitter in the pharynx of A. pallida.     

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.