J. exp. Biol. 104, 231-246 (1983) 2 3 1Erinted in Great Britain © The Company of Biologists Limited 1983

NERVE NET PACEMAKERS AND PHASES OFBEHAVIOUR IN THE SEA ANEMONE CALLIACTIS

PARASITICA

BY IAN D. McFARLANE

Department of Zoology, University of Hull, Hull HU6 7RX

(Received 9 November 1982 —Accepted 31 January 1983)

SUMMARY

Bursts of through-conducting nerve net (TCNN) pulses, 20—45 minapart, were recorded from Calliactis attached to shells. Within 15—25 minof the anemones being detached the TCNN bursts suddenly became morefrequent (only 4—11 min apart). Such bursts continued for several hours ifre-attachment was prevented. In an attached anemone simultaneouselectrical stimulation of the TCNN and ectodermal slow system (SSI) with20-30 shocks at one every 5 s also led to more frequent TCNN bursts,whether or not detachment took place. If, however, the anemone remainedattached, the intervals between bursts returned to the normal resting dura-tion after about 90 min. In all cases the decay of the 4—11 min intervalTCNN bursts involved a reduction in pulse number, not an increase in burstinterval. Partial activation of the TCNN pacemakers followed stimulationof the SSI alone. It is suggested that in sea anemones the change from onebehavioural phase to another is associated with a change in the patternedoutput of nerve net pacemakers.

INTRODUCTION

The most obvious behavioural response of a sea anemone is that it contracts whentouched. These fast contractions are triggered by a through-conducting nerve net(TCNN), (e.g. Pantin, 1935a; Josephson, 1966; Pickens, 1969). Other more com-plex, but still obvious, behavioural responses such as swimming in Stomphia coccineaand shell climbing in Stomphia and Calliactis parasitica involve additional conduct-ing systems, the slow systems SSI and SS2 (Lawn, 1976a; Lawn& McFarlane, 1976;McFarlane, 1976). These complex behaviour patterns are also evoked by externalstimuli.

Far less noticeable are the slow, rhythmical, spontaneous shape changes shown byall species of sea anemone. These contractions are important as they may be the basicprimitive form of neuromuscular activity in anemones (Ross, 1957) and componentsof these rhythms are incorporated into the complex behaviour patterns. Slow contrac-tions have received less attention than the fast reflexes because the time scale is somuch longer. Techniques such as kymograph or time-lapse cine' recording have shownthat a particular sequence of muscular contractions (a phase) may be repeated for

words: Pacemakers, nerve net, sea anemone.

232 I. D. MCFARLANE

several hours but even a brief stimulus can trigger a new phase that may persistseveral more hours (Batham & Pantin, 1950). The responses to stimuli may vaccording to the phase the animal is in.

The only neurophysiological analysis of the coordination of slow contractions todate has shown that bursts of TCNN pulses in half-animal preparations of Calliactisparasitica are followed by a sequence of parietal and circular muscle contractions(McFarlane, 1974a). In this paper, neurophysiological evidence is presented for achange in behavioural phase in intact Calliactis. It is shown that detachment of thepedal disc leads to a long-lasting activation of TCNN bursts.

MATERIALS AND METHODS

Calliactis parasitica, attached to Buccinum shells, were supplied by the PlymouthMarine Laboratory and kept at 15-19 °C. They were starved for a week before use.Animals were detached from the shells by being gently pulled. The pedal disc of adetached anemone will quickly re-attach to any object it touches, so to prevent thisthe anemone was suspended free of such contact by a large diameter suction electrodeattached to the mid-column region. Recordings were made with polyethylene suctionelectrodes attached to tentacles. To minimize mechanical stimulation, only onerecording electrode was used. This made pulse identification difficult (previousstudies always compared pulses at two electrodes) and required selection of in-dividuals where all pulse types were clearly recognizable. Electrodes sometimesremained attached for several hours but usually there was a gradual deterioration insignal size, presumably due to tissue damage, after about 1 h. Consequently theelectrode had to be moved periodically and this caused a short break in the record asa few minutes must elapse before pulses can again be confidently identified. Pulseidentification was facilitated by connecting the pre-amplifier output to theoscilloscope via a Datalab DL902 transient recorder in the roll mode with a samplerate of 1 KHz. This gave a 2s period in which to identify each pulse. When a pulsewas seen a contact was made which deflected a slow moving plotter pen. Differentpulse types were indicated by different sized deflections. SSI pulses are relativelylarge (around 10-30/iV) and easily recognized. TCNN pulses have a distinctiveshape but during a TCNN burst they usually become smaller and some may have beenmissed. SS2 pulses are small (often only 5 fiV) and are often lost in backgroundactivity (Fig. 1). The results are based upon more than 160h of recording from 10anemones.

RESULTS

Detachment causes frequent nerve net bursts: a pre-settling phaseThe spontaneous electrical activity of an anemone attached to a shell could gener-

ally be predicted by the anemone's appearance. If it was 'alert' (i.e. well expanded withtentacles outstretched) a typical recording was as shown in Fig. 2A. This record showsthe distribution of electrical activity in the three conducting systems (TCNN, SSIand SS2). For this, and subsequent Figures, the activities of the systems are sum-marized in Table 1.

Nerve net pacemakers in Calliactis 233

Fig. 1. Single suction electrode recordings from a tentacle of Calliactis. The first deflection in eachrecord is the stimulus artefact. (A) Single shock (40 V, lms) to base of column elicits through-conducting nerve net (TCNN) pulse ( • ) , probably a muscle action potential, an ectodermal slowconduction system (SSI) pulse (A) , and an endodermal slow conduction system (SS2) pulse (A) .(B) Same size shock, 30s later, evokes same pulses but the SS2 pulse is lost in a burst of complexpulses. Such complex activity is usually localized and associated with a twitch contraction of thetentacle. The SS2 pulse often has an amplitude of only 5 fiV and many may be missed duringcontinuous monitoring of spontaneous activity, particularly during and just after TCNN bursts,when complex activity is common. Time scale: 500 ms.

SSI pulses were rarely detected in an attached, unstimulated anemone. GenerallySS2 activity was at a low frequency — about one pulse every 25 s. There were alsooccasional short bursts of TCNN activity, usually 20-45 min apart and rarely contain-ing more than seven pulses. The pulse frequency in the bursts was about one pulseevery 4 s. A small, slow sphincter muscle contraction was visible after most bursts.The pattern of electrical activity recorded from an unstimulated, attached anemonewill be termed 'resting phase' activity in this paper.

Occasionally, anemones looked 'limp' with flaccid tentacles. Recordings thenshowed a much higher level of SS2 activity — around one pulse every 8 s. TCNNbursts were rarely detected (less than 1 h"1). This is atypical and such anemones werenot used in the experiments described below. Electrical stimulation of the SS2 cancause a similar loss of muscle tone (McFarlane, 1976).

When an anemone was detached by being pulled off the shell there was a markedchange in TCNN activity. After a delay of 15—20 min, the widely-spaced TCNNbursts seen in the attached anemone were suddenly replaced by more frequent bursts,often only 5 min apart (Fig. 2B) and rarely as much as 11 min apart (Fig. 2C). Thebursts contained more pulses than resting phase bursts; containing 9-12 pulses onaverage in different anemones (Table 1). A wide range of pulse intervals was seen

n the bursts (3—6 s). There was rarely any noticeable change in the overall level

I. D. MCFARLANE

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Fig. 2. Electrical activity patterns of attached and detached Calhactis. In each section the recordsare continuous and each line of the record covers 260 s. Pulses are shown by different-sized deflectionsof the plotter pen: TCNN > SSI > SS2. Periods of complex activity, sufficiently large to mask SS2pulses, are shown by long-duration marks at SS2 amplitude. The long-duration marks at TCNNamplitude show where fast contractions occurred. A break in the record signifies a period whenelectrode contact was lost. (A) Resting phase: activity in an anemone attached to a shell. Two short-duration TCNN bursts are shown. The spontaneous activity of the resting phase is characterized bywidely-spaced TCNN bursts and regular firing of the SS2. (B) Pre-settlement phase: activity in ananemone pulled off a shell 10 mm before the start of the record. Note the frequent TCNN bursts thatin this case start 17min after detachment. (C) Another detached anemone, showing TCNN burstssome 11 min apart. This record starts 20min after detachment. Time scale: 1 min.

of SS2 activity but there were always more SSI pulses (Table 1), probably caused bymechanical stimulation of the detached pedal disc margin (McFarlane, 1976).

TCNN bursts in detached anemones were followed by a small slow sphinctermuscle contraction, a peristaltic wave which passed down the column, and

Nerve net pacemakers in Calliactis 235

1. Changes in conducting system activity following mechanical detachment andelectrical stimulation

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bursts so TCNN activity is here expressed as the interval between bursts (measured from the start of one burstto the start of the following burst) and the number of pulses per burst. For SSI and SS2, activity is shown asthe number of pulses per hour.

of the pedal disc. This will be termed the 'pre-settlingphase' in this paper as it appearsto increase the chances of contacting a surface on which to settle.

Pre-settling phase TCNN bursts sometimes persisted for several hours if theanemone was prevented from re-attaching. A kymograph was used to record the rhyth-mic movements (Fig. 3) by connecting the anemone to a light isotonic lever attachedto a pin hooked into the sphincter muscle. The movements shown in Fig. 3A occurredat 5 min intervals and were probably contractions of the sphincter and circularmuscles. On rare occasions the rhythm stopped (here for 30 min) and then restarted.Fig. 3B shows contractions of the same anemone 2h after it had re-attached to a shell;contractions were fewer, smaller and less rhythmic. Fig. 3C shows pre-settling phasecontractions in another detached anemone. Note that the contractions vary in size: thismay reflect variations in pulse number or frequency in the TCNN bursts.

In 15 trials only two animals failed to show frequent contractions when detached.In both cases more than 60 % of the pedal disc was still covered by periostracum from

shell. In all other trials there were never more than a few scraps of periostracum

236 I. D. MCFARLANEstill adhering. Perhaps contact with shell or periostracum inhibits activation ofTCNN pacemakers.

Change from pre-settling phase to resting phase

The onset of the TCNN bursts after detachment was delayed but sudden: there wasno evidence for a gradual build-up of TCNN activity. By contrast, the terminationof the pre-settling phase was far less abrupt. Fig. 4 shows what may be a transitionbetween the pre-settling phase and the resting phase. Here the pre-settling burstsstopped about 5 h after the anemone was detached. Note that although the bursts atthe end of the pre-settling phase contained far fewer pulses and were much shorterthan earlier bursts, the interburst interval was not increased. In fact the last few burstswere about 280 s apart whereas the first few bursts were 310 s apart (Table 1). Ap-parently the termination of the phase involves reduction in TCNN pulse number andnot a gradual increase in interburst interval. Possibly the processes which controlburst interval and burst content are only loosely linked. In this recording SS2 activitywas particularly clear and there was obviously very little SS2 activity towards the endof the pre-settling phase, but a marked increase on return to the resting phase (Table1). It is not clear if the increase in SS2 activity was the cause or the result of the phasechange. In half-animal preparations, evoked or spontaneous SS2 pulses during theTCNN bursts can cause an increase in subsequent nerve net pulse intervals (McFar-lane, 19746). This inhibitory effect was not obvious in the pre-settling phase bursts

Fig. 3. Kymograph records of spontaneous contractions of the sphincter and upper column circularmuscles in attached and detached Cailiactis. (A) Detached anemone. (B) Same anemone after re-attachment to a shell. (C) Another detached anemone. Time scale: ISmin.

Nerve net pacemakers in Calliactis 237

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Fig. 4. Decay of pre-settling phase TCNN activity in a detached Calliactis. (A) Record starts 20 minafter anemone was pulled off shell. The TCNN bursts are unusually long, possibly due to an inhibi-tory action of SS2 activity during the bursts. (B) Continues 169 min later and shows what appear tobe the last five bursts of the pre-settlement phase followed by a 40 min delay before the first TCNNburst of the resting phase. The return to the resting phase activity pattern is here associated with anincrease in SS2 activity. Note that the last few bursts of the pre-settling phase still occur at 5-minintervals but they are shorter and contain fewer pulses than bursts earlier in the phase. Time scale:1 min.

238 I. D. MCFARLANE

but, as pointed out before, it is often difficult to identify SS2 pulses during aburst. Such an inhibitory action may explain some of the more irregular pulse ivals seen in some bursts (e.g. Fig. 4).

Electrical stimulation of the pre-settling phase

Activity similar to the pre-settling phase TCNN bursts can be evoked by simul-taneous electrical stimulation of the TCNN and SSI. Prolonged SSI stimulationcauses pedal disc detachment (McFarlane, 1969), but in the present study theanemone had to remain attached to control for the possibility that detachment itselfcauses TCNN burst activation. This can be done by stimulating on the oral side ofa shallow cut around the mid-column region: SSI activity cannot pass this cut andreach the pedal disc. This method, however, damages the anemone and was not used.Sometimes just 20 shocks, at one every 4 or 5 s were used, as this is insufficient to causedetachment. Alternatively, if anemones were repeatedly detached by electrical stimu-lation (say once a day for a week) they then failed to detach in response to 30 shocks,

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Fig. 5. Activation of the prc-setthng phase pacemakers by simultaneous electrical stimulation of theTCNN and SSI (30 shocks at one every 5 s). The anemone did not detach. The record startsimmediately after stimulation. Stimulation is followed here by 14min with very little activity of anysort. The bursts, believed to be part of the pre-settlement phase, are numbered 1 — 11. The twounnumbered TCNN bursts are thought to be part of the resting phase. Note that SS2 pulses werevery small and difficult to detect during most of the recording period: there were probably many morethan shown here. Time scale: 1 min.

Nerve net pacemakers in Calliactis 239

f fcesibly due to depletion of the chemicals that cause shedding of the pedal disc cementyer.In Fig. 5 the mid-column region was stimulated with 30 shocks at one every 5 s at

an intensity sufficient to excite both the TCNN and the SSI, but not the SS2. Theanemone remained attached. At the beginning of the record very little SS2 activitywas detected, compared with a mean frequency of one pulse every 25 s in the 15-minperiod immediately prior to stimulation (Table 1). The first TCNN burst came17 min after stimulation, but, judging by its duration and its relation to subsequentbursts, it was not part of the pre-settling phase. The first burst of the pre-settlingphase came 28 min after stimulation. A total of 11 bursts was recorded, the last comingsome 90 min after stimulation. In some replicates of this experiment the bursts lastedjust over 2h, but never for as long as with detached anemones. Apparently detach-ment is necessary for the activity to be maintained over an extended period.

As in Fig. 4 the recording shows the decay of the phase. Again the interburstintervals remained quite constant (for the 11 bursts they are 390, 360, 430, 365, 410,370, 410, 400, 370, 390 s), but the bursts shortened and the number of pulses fell,again to a minimum of three or four. Unfortunately SS2 pulses were not very clearover the latter half of this recording period and it is not known if the return to theresting phase was accompanied by increased SS2 activity. Note that a possible restingphase TCNN burst appeared before the first pre-settling phase burst and also betweenbursts 9 and 10. This independence of the two types of burst implies that they

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Fig. 6. Stimulation of the TCNN alone (30 shocks at one every 5 a) is followed by a marked increasein SS2 activity. The two TCNN bursts are short and 800s apart: they are probably resting phasebursts. Stimulation took place during the first break in line 6. Time scale: 1 min.

240 I. D. MCFARLANE

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Fig. 7. Stimulation of the SSI alone (20 shocks at one every 5 8) is followed by a marked increase inSS2 activity and irregularly-spaced, short-duration TCNN bursts. They may be pre-aettling phasebursts as the interburst interval is less than lOmin. Stimulation took place during the first break inline 4. Time scale: 1 min.

originate in separate pacemakers, rather than in a single pacemaker that switches fromone type of output to another.

Stimulation of the TCNN or SSI alone was only partially successful in activatingthe pre-settling phase burst rhythm. Stimulation of the TCNN alone, with 30 shocksat one every 5 s, always led to slow sphincter muscle contraction and loss of electrodecontact. In cases where the electrode could be quickly replaced it was obvious thatcontraction was followed by a marked increase in SS2 activity (Fig. 6 and Table 1).This increase was not seen when the SSI and TCNN were stimulated together. It maybe a sensory response to the muscular contractions that followed stimulation. Eventu-ally TCNN bursts appeared, 36 min after stimulation. In no case did the burstscontain more than six pulses and were thus more likely to be resting phase bursts thanpre-settling phase bursts. Also the interval between bursts was quite long - just over16 min in this case.

Stimulation of the SSI alone was more successful in evoking TCNN bursts, butsomewhat less effective than stimulation of both systems together. In Fig. 7 the SSIwas stimulated by an electrode attached to a shallow ectodermal flap (McFarlane,1969). Here just 20 shocks at one every 5 s were given — too few to evoke detachment.SSI stimulation causes circular muscle contraction (McFarlane, 1976) and this £

Nerve net pacemakers in Calliactis 241

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Fig. 8. Electrical activity patterns of a detached anemone change when a shell is placed against thepedal disc. The pre-settling TCNN bursts continue but as the anemone settles additional, short, high-frequency TCNN bursts are seen (arrows). Time scale: 1 min.

have led to the observed increase in SS2 activity (Table 1). This time the first TCNNburst did not appear until 58 min after stimulation. Several bursts followed but atirregular intervals (95 s, 610s, 735 s).

Electrical activity during settlement of the pedal disc

The pre-settlement phase is somewhat unnatural in that the anemone is preventedfrom re-attaching. If the pedal disc is allowed to start to settle there is a further changein the electrical activity to what may be termed the settlement phase. If a shell is gentlybrought into contact with the pedal disc when it is expanded there is immediateadhesion, perhaps due to sticky secretions from the ectodermal cells. The area ofcontact is then gradually increased and cycles of pedal disc expansion and contractionensue as the anemone adjusts its position on the shell. This is equivalent to the laststage of shell-climbing behaviour, after the tentacles have released their hold on theshell (Ross & Sutton, 1961).

Recordings made during this settlement phase show that pre-settlement TCNNbursts continue unchanged but in addition there are short, high-frequency burstspreceding alternate pre-settlement bursts (Fig. 8). The last high-frequency burst seenhere only contained two pulses but its position suggests it is related to the two prece-ding bursts. The high-frequency bursts come at 550 s intervals whereas the pre-settlement bursts are 260-290 s apart. Possibly the high-frequency bursts representactivation of another pacemaker. There is also a marked increase in SSI activity, thepulses tending to come in bursts, particularly in the interval between one pre-settlement TCNN burst and the subsequent high-frequency TCNN burst. Futurework will attempt to relate this complex pattern of electrical activity to the behaviour

flkthe anemone.

242 I. D. MCFARLANE

DISCUSSION

The behavioural correlates of the spontaneous TCNN bursts have not been clearlyestablished. Needier & Ross (1958) described a sequence of rhythmic contractions,presumably evoked by TCNN bursts, in attached Calliactis parasitica. A sequenceof parietal and circular muscle contractions has been described following TCNNbursts recorded from half-animal preparations of Calliactis (McFarlane, 1974a).Rhythmic behaviour in Calliactis has also been detected as cyclical variations inoxygen consumption, with cycle lengths of l lmin and 34min (Brafield, 1980);presumably this is directly related to contraction cycles. A 34 min respiratory rhythmalso occurs m Metridium senile and Actinia equina (Jones, Pickthall & Nesbitt, 1977).

Many rhythmic movements in animals are thought to be coordinated by patternedoutput from the central nervous system. The cells that produce the rhythm are calledcentral pattern generators (CPGs). Phasic sensory inputs or tonic excitatory inputssometimes maintain the rhythm (Delcomyn, 1980; Selverston & Miller, 1980). In theleech, identified CPGs are small groups of synaptically-linked interneurones(Friesen, Poon & Stent, 1978; Peterson & Calabrese, 1982).

Methods which can distinguish peripheral sensory feedback from central rhythmorigin (Delcomyn, 1980) cannot be applied easily to sea anemones. NeverthelessTCNN bursts are thought to have a central origin as an intercalated TCNN pulseresets the intraburst rhythm (McFarlane, 19746). Also the pre-settling burstsprecede, rather than follow, visible movements.

TCNN bursts probably arise in CPGs somewhere in the nervous system. The mostlikely site is within the network of endodermal multipolar nerve cells. These areparticularly obvious in the column of the swimming sea anemone Stomphia cocdneawhere the soma are up to 30 /im across and bear up to twelve processes; there may bemore than 10 000 such cells in a small Stomphia (Robson, 1963). The network is lessdeveloped in Calliactis parasitica, where the column multipolars are 10-15 /zm acrossand have three to five processes (Robson, 1965). Even so, at the quoted density of50-100 per mm2 in fixed sphincter muscle preparations one can expect well over10000 even in a small Calliactis. These cells appear to connect with sense cells andwith the bipolar neurites of the through-conducting system (Robson, 1961).Metridium senile has a poorly developed multipolar system consisting of scatteredtripolar cells in the mesenteries (Pantin, 1952). Metridium does not swim like Stom-phia or somersault onto shells like Calliactis, so perhaps restricted behaviour can becorrelated with restricted neural networks.

Assuming that the multipolar cells are the CPGs and that their output appears asTCNN bursts, what is the switch that changes their output as behaviour moves fromone phase to another? In the case of the pre-settling phase bursts it appears to besimultaneous stimulation of the SSI and TCNN. Both systems can be excited by touch(McFarlane, 1969), with the lower column being particularly sensitive, so both mustbe stimulated when the anemone is pulled off the shell. There is anatomical evidence(Robson, 1961, 1965) for connections between the bipolar cells of the TCNN and themultipolar cells. There is no such evidence of SSI connections onto the multipolarcells, indeed we still do not know the cellular basis of the SSI. There is, however,physiological evidence for links between the ectodermal SSI and endodermal regior^

Nerve net pacemakers in Calliactis 243

t;son & McFarlane (1976) showed that an endodermal conducting system, termedDelayed Initiation System (DIS), connects with the SSI. It was suggested thatthe DIS is the multipolar nerve net. Also we know (McFarlane, 1976) that SSIstimulation causes contraction of endodermal circular muscles: possibly this actionoperates via the multipolar net. In Stomphia coccinea the SSI has transmesogloealconnections with the pacemaker system that coordinates swimming (Lawn, 1980).

Possibly then the multipolar cells receive input from at least two sources — the SSIand the TCNN. The Results show that simultaneous SSI and TCNN stimulation ismore effective in evoking pre-settlement bursts than stimulation of either systemalone. This may be because the inputs summate or because the pacemakers areprevented from full expression by the increased SS2 activity that accompanies stimu-lation of the systems separately. Earlier work has shown that SS2 activity has aninhibitory action on TCNN pacemakers (McFarlane, 19746). A puzzling feature ofthe response is the long delay, usually 15-25 min, between stimulation and the startof bursting. Whilst this may not be long in sea anemone terms, representing only oneTCNN cycle at the resting phase rate, it does seem difficult to reconcile with theproposed model for multipolar cell activation, and may imply that pacemaker activa-tion is an indirect (hormonal?) action of the conducting systems.

As Croll & Davis (1982) point out, animals can perform more than one behaviourwith the same set of muscles so there must be a number of CPGs capable of feedingdifferent outputs to the same set of neurones. In Calliactis parasitica there may beno completely independent motor supplies: a pulse in the TCNN reaches all musclegroups. There are many reasons why this arrangement does not lead to behaviouralinflexibility. First, muscle groups differ in their speed of contraction and theirstimulus - response delay (Pantin, 1935a; Batham & Pantin, 1954). Secondly, theTCNN has both excitatory and inhibitory actions (Ewer, 1960; Lawn, 19766). Third-ly, different muscle groups respond optimally at different stimulus frequencies (Pan-tin, 1935a; Ross, 1957; McFarlane, 19746). Finally, muscles are affected by otherconducting systems. Local contractions, in only part of a muscle group, may becoordinated by a 'primary nerve net' (Pantin, 19356). The two slow systems, SSI andSS2, may inhibit inherent muscular activity (McFarlane & Lawn, 1972; McFarlane,1974a). Clearly then, considerable behavioural flexibility can result from changes inthe TCNN burst pattern (e.g. burst length, pulse interval and interburst interval).Presumably a given phase represents a given output pattern from the TCNNpacemakers and whatever alters the output of these CPGs can be regarded as a switchwhich redirects the anemone's behaviour.

Switching between behavioural phases is seen in other cnidarians. In Hydra at-tenuata changes in light intensity modulate the rhythm of contraction pulse trains(Taddei-Ferretti & Cordelia, 1976). A number of swimming activity rhythms havebeen detected in the hydromedusan Sarsia tubulosa (Leonard, 1982) and it wassuggested that the animal can 'choose' from the set of available rhythms. In theactinian Stomphia coccinea, contact with a starfish evokes a sensory response lastingonly a few seconds, followed by swimming flexions lasting for several minutes (Lawn,1976a). The pacemakers are probably the multipolar cells of the column endoderm(Robson, 1963) but here they feed activity directly to the muscles responsible for

^fc and not into the TCNN. Bursts of TCNN pulses are, however, recorded

244 I. D. MCFARLANE

from Stomphia during the latter stages of another behavioural phase, that of s'settling (I. D. Lawn & I. D. McFarlane, unpublished observations). Perhaps thare at least two groups of pacemaker cells, only one of which connects with theTCNN. Alternatively there may be only one group of cells but the output could beswitched: such capabilities are present in molluscan multipolars (Haydon & Winlow,1982).

It is probable that the rhythmic contractions of burrowing anemones are co-ordinated by TCNN bursts. In the burrowing anemone Calamactispraelongus, theremay be three different pacemaker systems with outputs to the TCNN (Marks, 1976).The pacemakers are activated by light but none are concerned with burrowing. Aburrowing pacemaker has, however, been proposed in Phyllactis concinnata (Man-gum, 1970) where the digging rhythm is similar to the pre-settling rhythm in Calliac-tisparasitica, the contractions coming every 4 min at 21 °C. Such contractions can bemaintained for as long as 6 h. No form of electrical stimulation was found which wouldelicit burrowing in an inactive animal.

In structural terms actinians lack a centralized nervous system but the proposednetwork of multipolar cell pacemakers, able to produce various types of patternedoutput in response to a variety of inputs, shows one of the main functions associatedwith central nervous systems. It seems reasonable therefore to say that actinians havea diffuse central nervous system. A network of multipolar cells, extending as a singlecell layer, can be regarded as an extended ganglion.

Indeed, rather than look for tasks that can be performed by this simple nervoussystem it might be more instructive to ask what the system cannot do. Perhaps itcannot make associations between stimuli. Although habituation is a feature of thenervous system in actinians (Logan, 1975) claims of associative learning (see reviewby Ross, 1965) have not been rigorously tested. Another limitation of the anemonecentral nervous system is that each species has a restricted behavioural repertoire.Although some can swim by body flexions, some swim by tentacle flexions, someclimb onto shells, some crawl, some fight, and some burrow (see review by Shelton,1982), no single species is capable of all of these activities even though all species havemore or less the same arrangement of muscles. Of course some behaviour patternsrequire particular specializations of the muscles and not all available behaviour isappropriate to the life style of a given species.

I propose that restrictions are placed upon the behavioural repertoire of individualspecies and that such restrictions are to be found within the controlling neural net-works and the range of available sensory inputs. Whereas the density of neurones andtheir interconnections in a three-dimensional ganglion may allow for complex switch-ing based upon a wealth of information provided about the state of the body and theenvironment, the simple two-dimensional central nervous system of sea anemones,with its restricted sensory input, has only a limited ability to switch betweenbehaviour patterns.

Sensory limitation can be seen in the case of the SSI. The SSI is mechanicallysensitive but also responds to chemicals (different chemicals in different species). InTealiafelina var. lofotensis, now to be known as Urticina eques (Manuel, 1981), SSIchemoreceptors respond to food (McFarlane, 1970; McFarlane & Lawn, 1972). Alowfrequency discharge (about one pulse every 10 s) produces oral disc expansion

Nerve net pacemakers in Calliactis 245

^ parts of the pre-feeding response. In Calliactisparasitica the SSI again showsalow frequency response to food extracts and evokes a pre-feeding phase but it canalso be activated by a chemical in molluscan shells (McFarlane, 1976). In this case,however, SSI pulses appear at a much higher frequency, about one pulse every 5 s,and are accompanied by SS2 pulses. The pulses lead to pedal disc detachment andother parts of shell-climbing behaviour. Apparently Calliactis achieves a degree ofbehavioural flexibility by using SSI pulses to activate two behavioural phases - oneat low frequency, one at high frequency. In captivity this strategy is not alwayssuccessful: a hungry anemone sometimes detaches when fed. The most developed useof the SSI sensory response is seen in Stomphia coccinea, where a short, high-frequency, burst of SSI pulses evoked by contact with certain starfish, triggers swim-ming behaviour (Lawn, 1976a). Swimming is an escape response and thus must beexecuted quickly to be effective: hence the small number of SSI pulses required.Presumably the SSI in Stomphia is relatively insensitive to food or touch, if this werenot the case, inappropriate stimuli might too readily evoke swimming.

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