Laboratory 2 Locomotion, gravity responses and trail following in a terrestrial snail

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Maurice Arnold
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1 Laboratory 2 Locomotion, gravity responses and trail following in a terrestrial snail Introduction: Gastropods (including limpets, whelks, slugs and snails) generally move slowly and sluggishly. For that reason, their locomotion is relatively easy to study. Under certain conditions they move consistently in one direction, and apparently well-oriented with respect to gravity. Also, the direction of their movements often seems to be influenced by the paths taken by other nearby snails. This laboratory exercise concerns the study of three aspects of the locomotory behaviour of a terrestrial snail Helix apersa: (1) undisturbed movements, (2) gravity responses, and (3) the tendency to follow trails left by other individuals. Helix apersa is commonly found in gardens, hedges and woodlots. This snail is normally inactive during daylight and under dry conditions, and moves and feeds especially at night or after rain. It feeds on living vegetable material, from which fragments are abraded by means of a toothed radula. In winter and during prolonged dry periods, Helix seals up the aperture of its shell with a film of mucus hardened with calcium phosphate. Many, if not all, snails will be quiescent at the start of the laboratory session, and it is essential to stimulate them and have them moving about actively immediately before use. This is best done by immersing the snails in tepid (lukewarm) water for a few minutes. As soon as a snail shows signs of activity, it should be removed from the water, dabbed dry, and introduced to the experimental set-up. Handle the snails gently, especially when lifting them from an attached position. They are not likely to move in a natural, consistent way if you roughly tear them loose from a substrate. The laboratory exercise: Materials: Per pair of students - several snails from a stock container - sheet of Plexiglas (30 cm x 30 cm) - magnifying lens - water container - clamp and stand - ruler and protractor - twine - stopwatch - thermometer - hand digital counter - graph paper 1

2 2 PART A: Observation of locomotion (1). Observe the foot of the snail in action. (2). Correlate the speed of movement of the snail with the rate of passage of ripples along the muscular foot. (1). Remove one snail from the stock container and place it in a bowl of warm water. As soon as it becomes active, place it in the centre of a clean piece of Plexiglas. When the snail has taken a firm hold of the glass surface and is moving with the foot fully extended, gently and slowly turn the glass over and clamp it firmly in a horizontal position. With the aid of the hand lens examine carefully the underside of the foot of the snail as it moves across the undersurface of the glass. Describe in detail, the locomotory movements of your snail. If it becomes inactive replace it with another one. (2). While observing the movements of your snail, count the number of muscular ripples passing along the foot of your snails for 5 minutes. At the end of this period, determine the distance travelled by the snail in 5 min. by plotting to scale on graph paper the path travelled by the snail from the beginning to the end of the 5- min interval. Correlate the rate of passage of foot ripples (ripples per min.) with the speed of movement of the snail (cm/min.) by using the class data. Plot this relationship on graph paper. What can you conclude from the observed relationship? PART B: Gravity responses (1). Study the movements of snails in relation to gravity. (2). Determine the mean angle of orientation (relative to a horizontal plane) of the snail s path and its mean speed of movement in relation to the vertical angle of inclination of a plane. (1). Free-living Helix can be found on the ground and, especially after rain, climbing on vegetation. Movements in relation to gravity (geokineses or geotaxes; see attached table and references) are to be studied as follows.

3 (2). Place a fresh snail in warm water until it is active, then place it on the centre of a clean Plexiglas plate that has a grid of squares drawn on the reverse side. Clamp the plate in a vertical position with one edge on the bench top (angle of inclination = 90º). As soon as the snail begins moving, start a stop watch, and plot its course to scale on a separate sheet of graph paper. After 5 minutes, or when the snail has reached the edge of the glass plate (whichever occurs first), remove it. Determine the angle of orientation of its path with respect to the side of the plate in contact with the bench (see diagram below). Do this by drawing a straight line on the graph paper that represents as accurately as possible the direction of the snail s path. If a sharp change in direction occurred during the test, two separate angles of orientation should be determined. By using a piece of string to estimate the distance travelled by the snail, and the time between start and end of the test, determine its speed of movement (cm/min.). 3 glass plate snail s path angle of orientation of the snail s path (θ) angle of inclination of the plate (α) (3). Repeat the above procedure (step 2) with two more snails (thus yielding 3 replicates), being sure to wash the glass plate thoroughly after running each animal. (4). Then change the angle of inclination of the plane to 45º from the horizontal (see above diagram) and repeat the above procedure (step 2) with the same three snails (or fresh animals if the supply is large enough). (5). Finally, repeat again step 2 with the plate at 70º and 25º inclination (3 snails each) for a total of 12 separate tests. (6). Prepare graphs showing the mean angle of orientation and the mean

4 speed of movement of your snails in relation to the angle of inclination of the plate. Each mean value should be based on 3 replicate measurements. 4 (7). In discussing your results consider the following: What are the sensory structures that enable a snail to detect changes in gravity? Where are they located? What are some environmental variables that could affect the behaviour of snails in this laboratory exercise? What might be the significance of the observed responses to Helix in nature? Are they taxes or kineses? PART C: Trail following (1). Study trail following by determining the degree to which the mucous trails of a blazer and a follower snail coincide. (1). The tendency for one animal to follow the recently laid mucous trail of another conspecific has been demonstrated in several gastropods, including terrestrial snails. Examine trail following in Helix as follows. (2). Place one active snail (the trail blazer ) at the centre of a piece of clean Plexiglas lying flat on the bench top. Plot to scale the path of this snail until it has moved cm (within about 5 minutes). Then remove it and place a second snail (the follower ) at the same starting point, and oriented in the same direction as the blazer. Observe and plot the path of the follower on the same graph paper until it has moved at least as far as the blazer did. Remove the follower and determine the coincidence index (C.I.) of the two mucous trails using the following equation: C.I. = Lc Lb x Lf where Lc is the length of the follower trail that coincides with the blazer trail, Lb is the length of the blazer trail, and Lf is the length of the follower trail. Use string to estimate each of these distances. (3). Repeat the above procedure (step 2) with two more pairs of snails, making sure that the glass plate is washed between tests. Summarize your data in a table, showing values of Lc, Lb, Lf and C.I. for each pair of snails and an overall mean (± SD) C.I. Discuss the possible functional significance of trail following in free-living Helix.

5 5 References: Chase, R., K. Pryer, R. Baker and D. Madison Responses to conspecific stimuli in the terrestrial snail Achatina fulica (Pulmonata: Sigmurethra). Behav. Biol., 22: Cole, W.H Geotropism and muscle tension in Helix. J. Gen. Physiol., 8: Fraenkel, G.S. and D.L. Gunn The orientation of animals. Kinesis, taxes and compass reactions. Dover Publ. Inc., N.Y. (pp only). Jaremovic, R. and C.D. Rollo Tree climbing by the snail (Cepaea nemoralis L.): a possible method for regulating temperature and hydration. Can. J. Zool., 57:

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