The Water Relations of Snails and Slugs: III. Factors Determining Activity in Helix Pomatia L.

This paper leaves out of account the special case of hibernation, and deals with the activity phases of snails at other times.

It has long been known that both the water content and the activity of Helix pomatia vary very greatly, and there appears to be a broad parallelism between the two. Aestivating snails have a lower total water content, and a lower osmotic pressure of the blood, than active (Duval, 1930; Arvanitaki & Cardot, 1932; Meyer & Thibaudet, 1937). Even among non-aestivating animals, the correlation is said to apply; thus Duval (1930) writes of the blood of snails collected in the field, ‘on voit très nettement, on observant les animaux avant de prélever leur milieu intérieur, que plus la concentration de ce milieu est grande, moins l’animal est actif. C’est ainsi qu’un individu donc le sang se congelait a −0·39° était presque inerte, à moitié rentré dans sa coquille. Ceux que l’on rencontre au contraire se promenant dans les herbes mouillées par la rosée du matin ont un sang se congelant aux environs de −0 30°. ‘We may infer that water content determines activity, or that activity determines water content, or that both are determined by other factors.

The view that activity depends on hydration was championed especially by Duval (1930), who describes as follows the alternation of aestivation and. activity : ‘L’animal étant collé, inerte, le Δ de son sang est inférieur à −0·40° et s’abaisse peu à peu. Une cause hydratante intervient-elle, pluie, rosée etc., la concentration du sang s’abaisse et quand elle devient inférieure à celle correspondant à Δ = −0·40°, l’animal s’éveille, sort de sa coquille, rampe, cherche sa nourriture, s’alimente.… Lorsque la cause d’hydratation a cessé, l’animal se déshydrate assez rapidement et quand le A de son sang atteint −0·40° il redevient inerte, rentré dans sa coquille. Il restera dans cet état d’engourdissement jusqu’à ce qu’une cause d’hydratation vienne de nouveau faire remonter le Δ. Si cette cause ne se produit pas, l’animal se déshydrate progressivement et finit par mourir sansetre redevenu actif.’

If this account be true, it should be possible, by experimentally varying the snail’s water content, to control its activity. Attempts to do so, using H. pomatia and H. aspersa, were described in a doctorate thesis on ‘la vie ralentie’ by Fischer (1931). His conclusions are briefly summarized, without further experimental substantiation, in a preliminary note by Fischer & Duval (1931). This note has been widely quoted in favour of the view that water content may determine activity, e.g. by Pieh (1936) and by Schlieper (1936) ; the more extensive thesis, on which the note is based, is less well known. The present writer does not believe that Fischer’s actual experimental results justify his conclusions ; the point is of sufficient importance to deserve full discussion, particularly as Fischer’s thesis is somewhat inaccessible.

Fischer’s experiments on ‘l’influence de la teneur en eau ‘on respiratory rate were inspired by a paper by Mayer & Plantefol (1925), who found that the oxygen consumption of mosses increased with hydration up to a maximum at a water content of 62–64%, above which it decreased again. He concluded from his results that the same general law held for snails, the maximum respiration occurring in this case at a total water content of 85–86%, and wrote : ‘Il était intéressant d’étendre à un organisme animal les faits établis pour les mousses, et dont la portée générale avait été pressentie par les auteurs.’

This conclusion rests upon four propositions, in which he summarizes his actual findings. The fourth of these, about the respiratory quotient, does not now concern us ; the others are as follows :

‘ 1. A une hydratation de 80%, la respiration est moins active qu’à une hydratation de 85–90%.

‘2. Si chez un Escargot peu nche en eau (80% par exemple), il se produit une déshydratation, celle-ci est accompagnée d’une diminution de l’intensité respiratoire; l’hydratation produit un résultat inverse.

‘3. Unehydratation dépassant 88% est accompagnée au contraire d’une diminution de l’intensité respiratoire. ’

If, now, we turn to the descriptions of the actual experiments, we find that the evidence for the view that water content determines oxygen consumption is in reality exceedingly tenuous. The three propositions will now be examined one by one from this standpoint.

This statement is based on a comparison between the oxygen consumption of hibernating snails (water content about 80%) with that of active ones (85–90%). In other words, the two groups of animals were not strictly comparable. It is well known that hibernating snails can be awakened by immersion in water, but, as Fischer himself points out in another chapter, the same thing can be done by exposure to light, warmth, vibration or low oxygen tension; ‘l’humidité n’est pas un facteur nécessaire du réveil.’ He also repeats and confirms an experiment by Schuurmans-Stekhoven (1920), who found that mere removal of the epiphragm of a hibernating snail nearly doubled the respiratory rate. Hibernation seems to resemble sleep, from which one can be aroused by divers stimuli. As the first thing the snail does, when it emerges, is to seek water and drink it, the higher activity of nonhibernating specimens may be the cause, rather than the result, of their raised water content.

The first clause is based on the records of three hibernating snails, which slowly lost weight, mainly by evaporation of water, over a period of several weeks. In one of the three, the oxygen consumption steadily fell at the same time (three determinations were made). In the other two, whose water loss was assisted by P₂O₅, the respiratory rate fell on the whole, but with notable fluctuations, so its dependence on water content was apparently not at all strict. The second clause is based on a single experiment in which a hibernating snail, put into water, absorbed water and became active; the same criticism applies to this case as to proposition 1.

This is supported by the records of three active animals. One hydrated itself spontaneously to a total water content of 90·05% ; the others were artificially hydrated by immersion in water. The difficulty here is that the depression of respiration does not coincide very well in time with the increase of water content: ‘La diminution de la consommation d’oxygène peut se produire avec un certain retard, et persiste quelque temps même si l’hydratation cesse.’ Evidently, if there is a relationship between the two variables, it is by no means a simple and straightforward one.

The short note by Fischer & Duval (1931) adds nothing to the above; it simply states Fischer’s general conclusions, and cites a portion of one of his protocols. It will, I think, be clear from the above account that the experiments of these workers have failed to provide convincing proof that hydration directly influences oxygen consumption in snails.

A very different picture of the relationship between water content and activity was given by Arvanitaki & Cardot (1932), in a paper on the variations in blood concentration of four Helix species. They showed that hydration depends in a rather indirect way on the atmospheric moisture, and very directly on the availability of food. Two experiment of great importance in the present context will be cited.

In the first experiment, a number of active H. vermiculata were collected from a garden at Tamaris, 18 hr. after a strong nocturnal rain, and the concentration of their blood was determined by measuring the conductivity. Another group, which had been kept for 15 days in the same garden, but confined in a cage (‘nasse’), was similarly treated at the same time. Both groups had experienced identical atmospheric conditions, including exposure to rain. The only difference was that those in the cage had been unable to reach food. The blood of the fasting animals proved to be as concentrated as it is ‘au cours des périodes d’estivation sévère’, while that of the others was nearly twice as dilute.

In the second experiment, a great number of H. pisana were collected from the same garden as they fed after a heavy rain, which followed on a prolonged period of drought. They were then kept in dry vessels in the laboratory. They slowly lost weight from the moment of collection, a loss to which evaporation of water contributed, but their blood (which was very concentrated at the time of collection, although the animals had been active for some hours) became more and more dilute for 2 days, after which it rose quite rapidly to the usual aestivating concentration ; at the same time, the cessation of copious defaecation showed that digestion was completed.

Arvanitaki & Cardot conclude from these observations that ‘la dilution de l’hémolymphe après la pluie débute avec la digestion des aliments ingérés et cesse dès que celle-ci prend fin. Les fortes concentrations humorales de l’hibernation et de l’estivation sont la conséquence immédiate et directe du jeûne.’ From these results, it appears that rain, after drought, does not so much hydrate an aestivating snail as stimulate it to emerge, eat and drink; that tissue hydration follows after a lag of over 12 hr. due to the slowness of absorption from the gut; and that the hydration is only measurable if the animal has fed.

In the views discussed above, the pace-maker controlling snail activity is environmental moisture, acting either as a stimulus to feed or by hydrating the tissues. But activity may fluctuate very widely in a manner apparently independent of any environmental variable. Howes & Wells (1934a) found that, if H. pomatia are kept under reasonably favourable conditions they undergo enormous oscillations in weight, the graphs presenting an irregularly cyclical character with a period of several days. The weight rises are due partly to feeding and partly to drinking. In the absence of food, similar fluctuations, but of reduced amplitude, appear. Activity varies with weight, for the snails creep about when their weight is high, and often aestivate, or at least withdraw into their shells, when it is low. Animals kept together in the same small container often show weight peaks on different days; the fluctuations are therefore of spontaneous, and not environmental, origin. The fluctuations were shown by every one of 114 snails studied, and even appeared, although with reduced amplitude, in some kept in atmospheres saturated with water vapour; Howes & Wells concluded: ‘We have come to believe that a constant water content never occurs in the active phases of the life of this species.’ Activity appears, then, to be determined, partly by environmental factors, especially moisture, and partly by an internal rhythm.

Similar cycles, of even greater amplitude, occur in newly hatched H. pomatia kept at 18–20° in atmospheres saturated, or nearly saturated, with water vapour (Howes & Whellock, 1937).

The experiments described below were made from time to time since the completion of Howes & Wells’s paper, to throw further light on the nature of the cycles. After their detailed description, the discussion of activity in relation to internal and external variables will be resumed.

Howes & Wells (1934a) regarded the weight cycles as due primarily to factors intrinsic to the snail, and not as responses to varying environmental conditions, because different animals kept in the same container showed peaks on different days. To confirm this conclusion, I kept a number of H. pomatia under constant conditions (of humidity, temperature, illumination, food supply and water supply) and weighed them daily.

The apparatus was originally designed by Stoughton (1930, 1931) and set up at Rothamsted for studying plant growth. It was later moved to University College, London, and used for the snail experiments. Only the upper half of Stoughton’s apparatus was used. It consists essentially of a large box, with walls and roof of two layers of glass (enclosing an insulating air space) in wooden frames. The air inside is warmed by a coil of resistance wire, controlled by an electric thermoregulator and relay. A current of air is continually driven into the box; water evaporates into this air current from a piece of wet muslin lying on an electrically heated plate ; the heater is controlled, through a relay, by a hair hygrometer inside the box. A combined recording thermometer and hygrometer, standing in the box, provides a check on the regulating devices. The snails were kept in the box, in containers of glass and perforated zinc (Howes & Wells, 1934b, Fig. 1). They were taken out and weighed daily.

The snails were received from France, in the hibernating condition, on 13 March 1934, and kept in a moderately cool refrigerator. When needed, they were ‘brought out’ by removal of the epiphragm and exposure to warmth and light in a shallow layer of water. Immediately after removal of the epiphragm, the hibernating weight was determined, and each snail was thereafter distinguished by a number painted on its shell. Following the routine of Howes & Wells (1934a), all subsequent weighings were converted into percentages of the hibernating weight of the animal concerned ; this means that measurements made on different individuals are roughly comparable.

Two experiments were made. In the first, the animals were ‘brought out’ on 4 April 1934, and studied in the box from 24 April to 8 June. Some of these snails were given food (mixed carrot and cabbage) and a vessel of drinking water—both renewed daily—until 23 May, after which date they received water only. Others received water, but no food, from the time when they were ‘brought out ‘. It is a remarkable fact that, of the latter group, two (out of nine) survived the whole experiment, although they had taken no nourishment since they began to hibernate. This experiment was run at 16° C. and 90–93% R.H. (except that, on 25 April, owing to a failure of the relay, the R.H. fell to 86%). At first the animals were continually illuminated, but, after 3 May, they were kept in darkness, except, of course, for the daily weighing.

In the second experiment, another set of animals from the same hibernating stock was ‘brought out’ on 20 September 1934, and at once put in the box, some receiving carrot, cabbage and water and the others water only. The experiment was run until 14 October at 23 ° C. and 89–92% R.H. (except that the relay failed again on 3 October, and the R.H. fell to 82%). The snails were in darkness.

Two typical extracts from the records are shown in Fig. 1. The white and black circles give a rough idea of the activity of the snails when taken for their daily weighings. Actually, a number of grades of activity were noted on the protocols. The snail might be feeding or creeping along ; or sitting still with most of its body out of the shell but its head more or less completely withdrawn ; or sitting on the hinder half of the foot only, the rest being withdrawn ; or withdrawn into the shell except that the tip of the heel was still visible, though not in contact with the substratum; or completely withdrawn, so that only the mantle could be seen in the mouth of the shell. In the latter case, the opening of the shell might or might not be closed with a film of dried mucus. For the purposes of the diagram, this series of states has been arbitrarily divided into ‘active’ (white circles) and ‘resting’ (black) ; the former group includes all states in which any part of the foot is applied to the substratum.

In all essentials, the records resemble those already published by Howes & Wells (1934a). The fed snails show enormous, irregularly cyclical variations in weight, the fasting ones show similar cycles of lesser amplitude, and both tend to be active on the weight peaks and withdrawn in the troughs. As before, there is no correlation, in either experiment, in the days on which different individuals peak. The conclusion that the cycles are not determined by varying environmental factors is therefore fully confirmed.

The results of Howes & Wells (1934a), and those described in the last section, show that there is some sort of relationship between weight and activity, for the snails tend to withdraw into their shells in the troughs of the weight curve. To get more precise information on this relationship, a number of oxygen consumption measurements were made on H. pomatia, at different stages of the weight cycle.

The measurements were made with a manometric apparatus already described (Wells, 1938). The general method of a single measurement was as follows. A snail was weighed, then put in the apparatus, where it found itself in a fairly spacious cylindrical glass container, at a constant temperature which varied in different experiments from 22 to 24 ° C. The air was exposed to 20% KOH, which absorbed CO₂ and, incidentally, regulated the humidity at about 85% R.H. After an hour and a half, during which time temperature equilibrium was reached and the snail could get used to its surroundings, the taps were closed and the oxygen consumption was followed manometncally for one hour. The snail was then returned to its vivarium.

The whole of the data are plotted in Fig. 2. Each point is a single measurement, made as just described. Weight is given as percentage of the hibernating weight. Oxygen consumption is given as c.c. (N.T.P.) per kilogram of hibernating weight per hour.

The details of the various experiments are as follows :

1. A single preliminary measurement was made on each of fourteen snails. These were received from France in the hibernating state on 13 March 1934, and kept in the refrigerator until 20 September, when they were ‘brought out ‘, after determination of their hibernating weights, with warmth, water, light and food. The respiration tests were made on 6 –13th December, after they had been active for over 2 months. The results are shown as large white circles in Fig. 2.

2. A stock of snails was received, hibernating, on 17 December 1934, and put in the refrigerator. On 29 March 1935 they were moved into the laboratory and divided into two groups, one (27 animals) receiving food and water and the other (13 animals) water only. Three measurements were made on each snail—the first soon after emergence from hibernation (on 31 March to 2 April), the second after about five days (5 –7 April) and the third a week later (11–14 April). The resulting points are shown as small circles on Fig. 2; the consecutive measurements on any one snail are joined by straight lines; the fed animals are shown as white circles while those which had had no food since they began to hibernate are black.

3. After the experiment just described, all the animals were kept with food and dnnk until 21 July, when two groups were selected. The first group, of seven snails, continued to receive food and water; the second, of five, got water only. Measurements of each of these animals were then made every day for 10 days. The results of the first 2 days were later discarded, in the case of the fasting snails, as representing a transition between the fed and the fasting conditions. All these measurements, except the discards just mentioned, are shown on Fig. 2 as small circles not joined by lines; as before, the fasting animals are black. The histories of six typical animals, from this experiment, are also plotted in Fig. 3.

   Fig. 3.

   

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   Day-to-day histories of three fed and three fasting snails. The circle—white for fed and black for fasting animals—indicates the first measurement of each senes.

   Fig. 3.

   

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   Day-to-day histories of three fed and three fasting snails. The circle—white for fed and black for fasting animals—indicates the first measurement of each senes.

4. Another stock was received on 27 March 1935, and kept in the refrigerator until 7 December, when they were ‘brought out’ and fed. Measurements of these snails were made by Mr A. N. Rowan, using the author’s apparatus, at various times from January to June of the following year. His points appear as medium-sized circles in Fig. 2; they in-clude in all seventy-seven measurements, on forty-one animals, each of which was used from one to four times.

When the data are considered, the following results emerge:

1. In Fig. 2, the points are scattered over a broad diagonal belt of the diagram, so it seems that weight and activity are in fact related. The area covered is about the same for the different stocks studied in different years. There is a tendency for the 1936 data (medium circles) to fall below the 1935 ones (small circles), but the degree of overlap is more striking. There’ is also a tendency for the snails of any one batch to become lighter and less active as the months go by, but that is unimportant from our present standpoint.

2. If the lines joining the points for any one animal are studied, it will be seen that they mostly follow the general drift of the belt of points ; this is particularly clear in the case of fed animals of high weight and activity. But, in many cases, the lines strike across the general drift. The scatter, then, is not simply due to the fact that measurements made on a great number of snails are included in the diagram, but is caused to a considerable extent by the behaviour of the individual animals. Clearly, the relationship between weight and activity is not very close.

3. On comparing fed with fasting animals (white and black circles), it is at once seen that the latter never reach a high respiratory rate, and that the relationship between weight and activity is less well marked in their case. On withholding food from an active snail, it drops more or less rapidly towards the left-hand bottom comer of the diagram, where it wanders about irregularly (Fig. 3). This is confirmed if the fed and fasting points of Fig. 2 are separately divided into weight groups and the average oxygen consumption is calculated for each weight group; activity increases more rapidly with increasing weight in the fed animals. It would therefore appear that water content cannot be the main factor determining activity ; if it were, the activity : weight relationship should be clearer in the fasting snails, where water uptake is the only means of increasing the weight. Evidentiy, it is food—or, to be quite accurate, the combination of food and drink— which boosts the oxygen consumption of a snail.

A typical aestivating snail may be recognized as follows: the animal is wholly withdrawn into its shell, and the opening of the latter is closed by a film of dried mucus, which either runs directly across it or from the rim of the shell to the substratum.

In the course of the present work, and in that of Howes & Wells (1934a), animals were often found fully withdrawn, but without any film of dried mucus. Such animals seem to be in the same physiological condition as typical aestivating ones (see Howes & Wells, 1934a, Table I), and in the following paragraphs, as in Fig. 1, the two groups are taken together, and described as ‘resting’.

Aestivation is generally regarded as a response to unfavourable environmental conditions. Thus Kühn (1914), in contrasting hibernation with aestivation, points out that snails hibernate in autumn even if the conditions are favourable for active life, ‘wâhrend Beginn und Dauer jeder Trockenstarre ausschhes-shch durch àussereEinwirkungen bestimmtwerden’. A citation to the same effect from Duval (1930) is given above, on p. 79.

Against this view, Howes & Wells (1934a) pointed out that their animals often showed typical aestivation, especially in the troughs of the weight curves ; as the weight fluctuations are not due to environmental variables, aestivation evidently occurs, sometimes at least, as a consequence of internal processes.

This conclusion is confirmed by the experiments illustrated in Fig. 1 ; the snails aestivated from time to time, although the environment was constant. Howes & Wells (1934a) kept H. pomatia for several weeks in atmospheres so saturated with water vapour that moisture continually condensed on the walls of the containers, and a curious unpublished point from the records of this experiment may here be mentioned : the snails were often found completely withdrawn and, on one or two occasions, they even threw a sheet of mucus (which, of course, could not dry) across the mouth of the shell. The resting state occurred less frequently in moist than in ordinary room air; nevertheless, the fact that it occurred at all in the humidors shows that ‘Trockenstarre’ is an inappropriate name for the condition.

To bring out more clearly the relation between aestivation and the weight cycles, Fig. 4 was plotted from some of the data used in Fig. 2. The fasting animals were omitted, for the intention is to study the incidence of aestivation under comparatively favourable conditions. A small number of measurements on fed animals, in which the state of the snail was not noted, had also to be left out. The remaining points fall into four groups ; it may be recalled that a respiration measurement ends nearly three hours after the snail was taken from the vivarium.

In thirty-two cases (black circles), the snail was resting when taken from the vivarium and remained so during the whole respiration experiment. These were animals of low weight and low oxygen consumption; with one exception, all the points lie below the oblique line on the diagram.

In 149 cases (white circles), the snail was active (as defined on p. 81) when taken, and remained so during the respiration experiment. All of the resulting points, with only six exceptions, lie above the oblique line.

In thirty-seven cases (bisected circles, white over black), the snail was resting when taken but emerged and became active during the respiration experiment. These animals were on the whole of low weight—with few exceptions, the points lie to the left of the vertical line, drawn at 130% of the hibernating weight—but their oxygen consumptions, as one might expect, show a scatter comparable with that of the snails which were active throughout.

In six cases (bisected circles, black over white), the snails were active when taken but withdrew during the respiration experiment. These points are all to the left of the 130% weight line.

The results show very clearly that the tendency to aestivate becomes more and more marked as the bottom left-hand comer of the diagram is approached. Above a weight of 130%, aestivation seldom occurs; below about 115%, it is the rule rather than the exception. The general impression gained by the inspection of a series of records such as Fig. i is therefore confirmed.

That aestivation depends, to a large extent at least, on internal conditions is indicated from another angle by the following considerations. According to Duval (1930), the blood of an aestivating H. pomatia is much more concentrated than that of an actively creeping one; according to Arvanitaki & Cardot (1932), dilution of the blood in H. pisana accompanies the digestive process, which lasts for a couple of days ; it therefore appears, if the results on the two species may be put together, that digestion and aestivation are incompatible.

Confirmation of this view comes from an unpublished experiment, made in 1933 by Howes & Wells, which will now be briefly described. A number of H. pomatia were kept with abundant food and water. After noPng their condition (resting or active), they were weighed, dissected and classified into ‘digesting’ or ‘non-digesting ‘. In the digesting animals, the crop was full of food; in the others, it contained only the usual reddish fluid, coloured by helicorubin, although in some cases there was still food in the rectum. The animals grouped themselves as follows :

1. Ten animals were digesting. All of these were active. Their weights ranged from 128 to 170%, with the mean at 148%, of the hibernating weight

2. Of the non-digesting animals, sixteen were active. Their weights ranged from 11410 151%, with the mean at 131%.

3. The remaining fifteen animals were nondigesting and withdrawn into their shells, with or without mucous films. Their weights ranged from 104 to 146%, with the mean at 121%.

These results suggest, first, that feeding and digestion only occur on the peaks of the weight curve— a conclusion already reached on other grounds by Howes and Wells (1934a, Fig. 6)—and secondly, that an animal which has recently fed is unable to aestivate, at least until the digestive processes have gone so far that the crop is cleared of food.

From the available data, laced with a certain amount of speculation, we can draw the following picture of a snail’s activity in relation to internal and external variables :

1. If the snail is kept under reasonably favourable conditions, with plentiful supplies of food and water, it undergoes somewhat irregular weight cycles of considerable amplitude (Fig. 1, lower half). On the weight peaks, it feeds and digests, its blood is consequently dilute, its behaviour is active and its oxygen consumption 13 high. In the troughs, on the other hand, its digestive organs are resting, its blood is more concentrated, its behaviour is sluggish and its oxygen consumption is low; at these times, it tends to withdraw into its shell and aestivate. The fluctuations are due in the main to its internal processes.

2. If no food is available, but other conditions are as above, the snail still shotvs cycles of weight and activity (Fig. i, upper half), but the weight changes are comparatively small. In the active phases, failing to find food, its metabolic rate never rises to great heights, and we may infer from the results of Arvanitaki & Cardot (1932) that its blood is never very dilute. It spends more of its time in the resting condition than does a fed snail.

3. If no water is available—e.g. in time of drought —the snail fails to rise out of the first weight trough into which, in consequence of its cycle, it sinks. Howes & Wells (1934a, Fig. 6) found that snails offered food but no water spent all their time withdrawn in their shells, their weight gradually falling, for three weeks—except only for an exceptional day when the experiment had to be moved into another building; ‘the snails, stimulated presumably by the shaking, crept around a little and nibbled at their food’. But they ate very little on this occasion, and, finding no water, they soon retired into their shells again. In this state, the oxygen consumption is very low, and there is a gradual loss by evaporation of water, with increase in the concentration of the blood. If rainfall follows a period of drought, the snail becomes active and embarks on new cycles; it seems perfectly clear, from the results of Arvanitaki & Cardot (1932) and those reported in the present paper, that the immediate mechanism of this activation is not, as Duval (1930) suggests, the hydration of the tissues, but a sensory stimulation, operating in much the same way as the move to another building in the incidental observation of Howes & Wells. The raindrops knock on the door, and the snail comes out. Hydration follows after, when it has eaten and drunk, and may then perhaps make a secondary contribution to its great rise in metabolic rate.

I wish to thank Dr N. H. Howes, who read and criticized the manuscript and allowed me to discuss, in § iv, some unpubbshed results which we obtained in collaboration, and Mr A. N. Rowan, who made a number of the respiration measurements discussed in § III.