REACTIONS OF RADIATA (BOM) () TO SALINITY CHANGES

BY (MIss) SUI,:UNDA SUKUMARAN AND S. KRISHNASWAMY,F.A.Sc. (Zoological Research Laboratory, University of Madras) Received April 5, 1961

IT is known that some gastropods, like limpets living in the intertidal region respond to splashing of sea-water. Arnold (1957, 1959) has shown that the limpet, vulgata, reacts to dilute sea-water as well as to various ions. Casual observations made in the Madras University Laboratory showed that Cellana radiata, the common limpet found at Madras, responds to salinity changes in the environment, by raising or lowering the shell. An analysis of this behaviour is presented in the present paper.

MATERIAL AND METHODS

Individuals of Cellana radiata (Born) were collected from the rocks and concrete blocks on the northern wall outside the Madras Harbour. All the were collected at about the high tide level. Preliminary observa- tions showed that there was no significant difference in the behaviour of the animals collected from the high or low tide levels. Animals were care- fully removed by inserting a sharp scalpel between the foot and the substratum, causing the foot to relax its grip on the substratum. They were then kept in shallow aquaria containing fresh sea-water which was aerated with an air compressor. Animals injured in any way were not used for experiments. Before the experiments were started, each was allowed to attach itself to a small plastic cup of 3.5 cm. diameter and having an opening at the base, fitted with a rubber tube and a pinch-cock for draining. The ani- mals were thus kept out of water for a period of 4 hours. Longer periods of exposure to the atmosphere resulted in the death of the animal, although Arnold (1957, 1959) exposed P. vulgata for 8 hours without ill-effects. The shell movements were recorded with the help of a weighted lever, to one end of which, a cotton thread was attached from the anterior end of the shell and therefore the upward movements were recorded as such. The Kymo- graph drum was rotated at a very slow speed (0-72 ram. per rain.) Every 3 minutes the animal was splashed with about 2 c,c. of the test solution, 122 Reactions ofCellana radiata (Born) (Gastropoda) to Salinity Changes 123 namely, sea-water or salt solution. This was done with a medicine dropper. After the exposure of the animal to the test solution for the required period, the test solution was drained off and the animal rinsed with distilled water to remove traces of adhering salts. Washing with distilled water did not produce any apparent ill-effects on the animal. Sea-water with a salinity of 32-34~o was taken as normal and diluted with distilled water to give 75, 50, 25 and 10~o concentrations. Isotonic solutions of NaCI, KC1, MgSOa, NaHCOs, Na~SO~, Sucrose and Urea were prepared according to the formula given by Pantin (1948). The animal was allowed to remain in about 2 c.c. of each solution for a period of 3 minutes only, since longer exposures proved fatal. The osmotic pressure of the body fluid was determined by the com- parative melting point method of Gross (1954). C. radiata reacts to shadows thrown on it, by closing the shell. This was used as an index to the condition of health of the animal, and in all experiments only animals which exhibited this shadow reaction, were used.

EXPERIMENTS AND RESULTS Normally, Cellana radiata adheres to the substratum very firmly when exposed to air. Under laboratory conditions when a weighed animal was left in a dry, shallow glass dish for a period of 4 hours, at a room tem- perature of 29 ° C., it was found that the animal loses about 29/0 of its body weight at the end of the period. Prolonged exposure for more than 4 hours, resulted in the death of the animal and therefore all animals were exposed to the atmosphere for a period of 4 hours only. If after this period of drying, sea-water is splashed on the animal, C. radiata responds by raising the anterior end of the shell. Arnold (1957) noticed a similar behaviour in Patella vulgata, When C. radiata is immersed in sea-water, the anterior end of the shell is first raised, followed by an upward movement of the rest of the shell and the whom is lowered again after a few seconds. This movement is repeated as shown in the Kymograph tracing (Fig. 1). This slow vertical oscillation appears to be the normal response in all but a few, in which the shell remained raised. The maximum height to which the shell is raised varied and tended to increase with an increase in shell length (Table I), although this relation- ship is not clear. If the sea-water to which the animal has been exposed is drained and replaced with distilled water, C. radiata responds by lowering the shell and adhering to the substratum firmly. Prolonged exposure to lower salinities however, results in the animal getting adapted to them and therefore distilled water does not elicit a closing response. In the same way, C. radiata reacts to sea-water diluted to 75, 50, 25 and 10~ of the original strength, by raising 124 (MIss) SUKUNDA SUKUMARANAND S. KRISHNASWAMY

TABLE I The relationship between the height attained by the shell and the shell length

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Lengthsof 1.1 1-1 1 1.3 1-6 1.7 1.6 1.9 1-6 2 1.9 1.8 1.5 1.8 animals (ram.)

Mean height 0.75 0.5 0.75 0.75 0.75 1 1 1.5 1.5 1.5 1.7 2 2 3 attained (ram.)

the shell; but the maximum height reached varied with the salinity (Fig. 2). Table II shows the reaction of C. radiata to dilutions of sea-water. TABLE II Reaction due to dilution of medium, as indicated by the height of the rise of shell

Dilution Reaction Remarks

Sea-water Positive Characteristic vertical oscil- lations of the shell seen. 75% ,, Height attained is lower than in sea-water. 50% ,, Distinct drop in height of shell (Fig. 2). 25% ,, Height attained is similar in both. Negative reaction lO% ,, to distilled water is not so great as those in sea-water (Fig. 2).

Prolonged exposure to lower dilutions results in the death of the animal. Reactions of C. radiata to NaC1 solutions of different dilutions and solutions of various ions, isotonic with normal sea-water are shown in Table III. C. radiata reacts positively to Sodium sulphate and height attained by the shell is similar to that in potassium chloride (Fig. 3). Arnold (1959) found that Patella vulgata did not respond to sodium sulphate solution. In artificial sea-water as well as in a solution free of CP ions, the animal behaved as in normal sea-water but the height was always lower, Reactions ofCellana radiata (Born) ( Gastropoda) to Salinity Changes 125 TABLE III Reactions to dilutions of NaCl and solutions of various ions, isotonic with normal sea-water

Chemicals Strength Reaction Remarks used

NaCI Isotonic Positive. Maximum rise followed by minor peak (Fig. 4)

]% Response similar to Vertical oscillations as in sea- those in 25~o and water are absent (Fig. 3). 10~o sea-water

,, 0.01~ Positive Heights attained are equal in ,, 0-00001% ,, both (Fig. 5).

,, Isotonic ,, Height as in normal sea-water. Osmotic pr. raised with sucrose

KC1 Isotonic ,, Height lower than in NaC1, with no shell movements (Fig. 3).

CaCI .... Height as in NaCI, with a different pattern. No res- ponse to distilled water (Fig. 4).

NallCOs ,, Negative (Fig. 3). As in Patella vulgata (Arnold, 1959).

NaSO4 Positive As in KC1 (Fig. 3).

MgSO4 ,, As in NaCI (Fig. 3).

When exposed to solutions of Sucrose or Urea isotonic with normal sea- water, the response was normal as far as the height was concerned, but the pattern was different (Fig. 6). It has been suggested by many, that the cephalic tentacles as well as the mantle fringe may be involved in detecting the osmotic pressure changes as well as those in ionic composition. In some animals, the tentacles were t26 (Miss) SUKUNDA SUKUMARAN AND S. KRISHNASWAMY

I II I

2

3 4 FIG. 1. Activity pattern in normal sea-water. Time 1.4cm,/2 minutes. FxG. 2. Response of Cellana radiata to dilute sea-water. FIG. 3, Effect of various ions on the response of C. radiata. FIG. 4. Effect of calcium ions on C. radiata. removed completely and these individuals were allowed to recover in sea- water. After complete recovery, indicated by response to shadow, dilute sea-water and solutions of various salts were tried. The response appeared to be normal regarding the pattern but the height reached was lower. The mantle fringe was also removed in some animals. This proved much more difficult and survival was poor. When tested with sea-water and the salt solutions after full recovery, the response was poor. The negative response to sodium bicarbonate was no longer noticeable. The height to which the shell was raised was the same in sea-water and in different ions (Fig. 7), indi- cating that the mantle fringe and the tentacles are involved in detecting the changes in the environment. Arnold (1959) has shown that the osphradium, the molluscan ch~emo-receptor, may not be involved in assessing the quality of the water entering the mantle cavity in Patella vulgata. All attempts to remove the osphradium of C. radiata without damaging the animal were unsuccessful. Reactions ofCellana radiata (Born) (Gastropoda) to Salinity Changes 127

5 fs fu 6

7 FIG. 5. Effect of chloride ions on C. radiata. FIe. 6. Reactions of C. radiata to Sucrose and Urea. Fro. 7. Reactions of C. radiata whose mantle fringe has been cut.

DISCUSSION The present studies have shown that C. radiata (Bom), the common limpet of Madras, reacts to changes in sea-water concentration and to various ions. This behaviour is in many ways similar to that reported by Arnold (1957, 1959) in Patella vulgata. C. radiata differs in its response to sodium sulphate solution which bring about the lowering of the shell in P. vulgata. The reac- tion to sodium bicarbonate was the same in both the species. In both, calcium ions appear to exert a stabilizing influence and interfere with permeability of the mantle. As the animals in his experiments treated with calcium chloride, failed to elicit any reaction with distilled water, Arnold (1959) pointed out that the reactions shown by P. vulgata may be attributed to its sensitivity to chloride ions. The present studies have shown that C. radiata responds to very low dilutions of NaCI (0.0001~), clearly indicating the importance of these ions in the medium. But the positive response to a solution free ofCl-ions is puzzling. An intertidal bivalve, Scrobicularia plana, responds mainly to changes in osmotic pressure (Freeman and Rigler, 1957) whereas both Cellana and Patella are affected by changes in osmotic pressure and the various ions. The osmotic pressure of C. radiata is variable and in sea-water, it is hypotonic to normal sea-water (cal. 4-5~ NaC1) but after exposure to 128 (MIss) SUKUNDA SUKUMARANAND S. K-RISHNASWAMY air for a period of 4 hours, the body fluid showed an increase in concentration of about 4 times this value. It is known that the pedal ganglion of slugs is sensitive to osmotic pressure changes (Hughes and Kerkut, 1956), (Kerkut and Taylor, 1956), the spontaneous activity increasing with the lowering of osmotic pressure of the bathing fluid. This change in the osmotic pressure has been shown to affect the activity of the slug (Kerkut, 1958). The slugs are active when hydrated and less so, when dehydrated. It is possible that in C. radiata the changes in the osmotic pressure as well as those in the ionic composition of the medium, affect the central nervous system as in slugs and bring about this characteristic response. The mantle fringe is known to absorb ions (Morton, 1958). In Scrobicularia, the receptors that are sensitive to the changes in osmotic pressure are situated along the mantle which protrudes (Carthy, 1958). Arnold (1959) has suggested that the mantle fringe may play a part in detecting changes in the environment and the experiments carried out by the present authors on C. radiata with mantle fringe removed also suggest the same. The ecological significance of the behaviour pattern in P. vulgata, has been pointed out by Arnold (1957, 1959). The reaction to splashes of sea- water would enable the animal to feed even during low tide. Lasaea, an intertidal lameUibranch, also responds to changes in sea-water concentration by opening and closing the shell (Morton, Boney and Corner, 1957). As pointed out by these authors, the ability to maintain its response to wetting in sea-water diluted by rain between tides and concentrated by evaporation, is perhaps true of all intertidal animals.

SUMMARY 1. Celtana radiata (Born) uncovered by sea-water at low tides reacts to splashes of sea-water by raising its shell from the position assumed in tight closure. The maximum height reached by the shell decreases with decreasing salinity. 2. Isotonic solutions of NaCl, KC1, Na2SO4, Urea and Sucrose bring about a positive response, but the response is negative in NaHCO3. 3. The animal reacts positively to very low concentrations of NaC1. 4. Animals in which the mantle fringe and tentacles have been removed react differently. 5. The ecological significance of these behaviours is discussed. Reactions of CeUana radiata (Born) (Gastropoda) to Salinity Changes 129

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