The Behaviour Of A Supralittoral Gastropod Biology Essay

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The species chosen for this experiment (or rather set of experiments) is Melarhaphe neritoides. This is a very common (small) species of snail found distributed along the rocky Maltese shores. It is listed in the Phylum Mollusca (Class Gastropoda) and grows to about just under 1cm. Its sides are flat (unlike the more common rounded shell found amongst molluscs) and has a high pointed spire. An operculum covers an oval aperture and a white periostracum leads to the dark blue/black shell. Its niche is located in a very specific stretch on the shore labelled the supralittoral zone. This is that area located just above the high tide mark. It is not submerged but is frequently splashed by sea spray when it is windy/stormy (in fact it is also known as the splash or spray zone). It is an unforgiving environment and organisms living here must be very well adapted to its instability. The Melarhaphe neritoides snail must be able to withstand; high temperatures, freshwater, salt and brine water, desiccation and exposure to air and of course any shore line animals which might prey upon the snail. In the summer months, the sea round the Maltese islands is very calm and the snail's environment is rarely wetted. Also the snail lives in direct contact with the hard rocky surface which reaches high temperatures up to 50 degrees easily (which for most organisms this would be lethal). On the other hand during the winter months, storms are frequent and wave action is very violent on the supralittoral zone. Not only this but when there are no waves, pools of fresh water may form in these rocky patches which for most creatures adapted to a salty (high water potential) environment will cause osmotic problems. As opposed to the summer months, the temperature of the rocks in winter falls drastically some times even below freezing point. In fact as the mollusc is very well adapted to this environment, it is the dominant macro-faunal organism found there. To accommodate such drastic changes in its environment, Melarhaphe neritoides has many behavioural adaptations. Such adaptations include; becoming inactive, taking refuge in pits/rocky overhangs, aggregating in groups and becoming active only when conditions are suitable. To be able to accurately avoid the harshness of the environment, the mollusc must have some kind of sense as to when to actually begin aestivation periods or when to come out of them, which spot is suitable (offers enough protection) to take refuge in etc. It is these behavioural adaptations that this experiment investigates. A set of different habitats and conditions are prepared and a number of snails tested to see their reaction and preference. Such an experiment must be conducted as accurately as possible as there are many factors which induce errors. In fact the test subjects where freshly caught and a number (10) of individuals were tested with each method to ensure usable and explainable data. Also the individuals tested where seen to be about 0.7mm in height. This is the height of juveniles not too young or too old as it is in this part of its life time in which an animal is most probably going to respond as expected in individuals seen in the wild (as old or young specimens may be less active or inhabit different regions of the environment).

Procedure:

Apparatus

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Petri dishes Graph paper Plastic aquariums

Ruler Timer Acrylic plates (drilled)

Measuring cylinder Black bags Glass Rod Gravel Fine Sand Coarse Sand Circular glass trough

Materials

Calcium Chloride Sea Water Distilled water

Vaseline grease Blu-Tack

Method

Experiment.1

A plastic Petri-dish was partitioned by means of thin plastic into 3 separate parts. One part filled with fine sand, another with coarse sand and the other with gravel. Ten inactive snails were scattered across on these 3 different sabstrates and any movement made noted every 1,2,6,24,36 hrs.

Experiment.2

Two identical Petri dishes had their bottom ruled to form a 4x4 grid at 1 cm intervals. One snail was placed on each grid line intersection. One dish contained a container full of Calcium chloride, whilst the other housed distilled water. Both dishes were sealed and observed over a period of 3 days.

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A plastic Petri-dish was floated over a pool of water in a circular glass trough. Ten inactive snails were placed in the Petri-dish which was left opened, but the glass trough was covered. After one hour, the cover was removed just enough to get the snails out. These were tapped gently with a glass rod a few times and put back in the trough. The number of active snails after the treatment was noted.

Experiment.3

Two identical Petri dishes as in exp.2 (with a 4x4 grid) were prepared with one inactive snail per intersection. One dish is filled up to about 1mm with sea water, whilst the other one was left dry. Both dishes were sealed with Vaseline grease and observed over a period of one hour. This procedure was repeated only using fresh water instead of sea water.

Snails which had been left in dry air for one hour were tapped sharply on the shell and immediately placed in 1cm of sea water. The time taken for the first noticeable movement of the snail's operculum was noted.

Experiment.4

Two identical measuring cylinders were filled with sea water one to a depth of 5cm and the other to a depth of 20cm. Individual inactive snails were placed in each cylinder, and the time taken for the snail to move 5 cm up the wall in each container was noted.

Experiment.5

Two identical measuring cylinders are filled to a depth of 3cm with seawater. Each cylinder was marked at 3 cm intervals starting from the water surface. One cylinder was stoppered tightly whilst the other was left open. The movement of each snail up the cylinder was noted with time.

Experiment.6

Two identical plastic aquaria are filled to a depth of 1.5cm with sea water. Regular plastic plates that were drilled with a pattern of regular holes were attached to the walls of one of the aquaria. The aquaria were marked off at 3cm intervals starting from the water surface. Ten inactive snails were placed in each aquarium and covered with a lid. After 3 hours the number of snails at each level was noted.

The procedure above was then repeated but instead of the walls, the drilled plate was placed at the bottom of the aquarium. The number of individuals remaining submerged was counted at intervals of 1,2,6,24,36… hours.

Experiment.7

The procedure of experiment 5 was repeated only this time both the cylinders were tightly stoppered and with 10 snails in each one. One of the cylinders is placed in an opaque black bag whilst the other one is left in the light. The number of snails at each level for both cylinders at intervals of 1,3,6 and 24 hours was recorded.

Precautions

Snails that were used for an experiment were not reused but placed separately in a container to note that they have already undergone some treatment. This was done so as not have active snails from a previous experiment ruin the results of the next experiment.

The snails were all freshly caught (not more than 2 days) so as to have an accurate result as possible.

In most experiments a good number of individuals were used (like 10) and others were possible were replicated.

Enough time was left to elapse for results to be collected as the stimuli that activate the snails may be over a long period of time.

Snails used were chosen to be of similar size (0.7cm shell height) and handled very gently.

Errors

Handling of snails from capture site to lab and from tray to the experiment may have activated the snails prior to the actual experiment taking place.

Most experiments could have been done only once to the long waiting time, and with a relatively small number of individuals (ten snails may not yield a representative result).

The experiment tried to replicate the conditions that the snail would be in the wild. This can never be fully achieved and so the experiment its self is not so accurate.

Movement of apparatus or activity on the bench could have changed snail position in other experiments or activating them due to the vibration not to the variable tested.

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The snails themselves may have moved other snails in experiment 2 and 3 giving errored results.

Results

Experiment 1

Time / hr

Fine sand

Gravel

Rough sand

0

3

3

4

1

3

3

4

2

3

3

4

6

3

4

3

24

2

5

3

36

2

5

3

48

1

5

4

No snails were noted to have accumulated on the smooth plastic surface of the Petri-dish.

Experiment 2

Part 2A:

Time/hr

RH=0%

RH=100%

0

0

0

6

1

2

30

1

2

54

1

3

Part 2B:

Snails active:

Before tapping

After tapping

 

0

2

Experiment 3:

Part 3A:

Time/hr

Number of snails moved

0

0

1

5

6

12

24

14

32

15

48

16

56

16

Part 3B

Time for first discernible movement of operculum after left in:

Dry conditions

Dry conditions followed by tapping

Immersed in seawater

1

40

8

2

2

14

8

3

3

29

20

4

4

33

16

40

5

28

9

20

6

19

7

53

7

8

38

62

8

11

45

1

9

21

14

13

10

12

10

50

Part 3C

Time/hr

Number of snails moved

0

0

1

0

6

0

24

0

32

0

48

0

56

1

Experiment 4:

Replicate

5cm water

20 cm water

1

2340s

4140s(69 mins)

2

2400s

86400s (1day)

3

9000s

>2 day

4

9900s

>2 day

5

86400s

>2 day

6

>2 day

>2 day

7

>2 day

>2 day

8

>2 day

>2 day

9

>2 day

>2 day

10

>2 day

>2 day

Experiment 5:

A total of 2 snails were placed in each measuring cylinder. The numbers in the table show the number of snails recorded at each level marked.

Closed

Open

Time/hr

1

2

6

24

32

48

56

1

2

6

24

32

48

56

0-3cm

2

2

2

2

2

2

2

2

1

1

0

0

3-6cm

0

0

0

0

0

0

0

0

1

0

1

1

0

6-9cm

0

0

0

0

0

0

0

0

0

1

1

1

9-12cm

0

0

0

0

0

 0

0

0

0

0

0

0

0

12-15cm

0

0

0

0

0

 0

0

0

0

0

0

0

1

15-18cm

0

0

0

0

0

 0

0

0

0

0

0

0

0

18-21cm

0

0

0

0

0

0

0

0

0

0

0

0

21-24cm

0

0

0

0

0

0

0

0

0

0

0

0

24-27cm

0

0

0

0

0

0

0

0

0

0

0

0

27-30cm (top)

0

0

0

0

0

0

0

0

0

0

0

0

0

Experiment 6

Part 6A

After 3 hours:

At the bottom of the tank (smooth)

In crevices

On smooth wall

Submerged in seawater

8

0

1

0-3cm above water

0

0

1

3-6cm above water

0

0

0

Part 6B:

Time/hr

1

2

6

24

32

48

56

submerged

In crevices

6

8

9

9

9

9

9

On smooth wall

4

1

0

0

0

0

1

Above seawater

In crevices

0

0

0

0

0

0

0

Not in crevices

0

1

1

1

1

1

0

Submerged (the only one required others are extra)

10

9

9

9

9

9

10

Experiment 7:

Light

Dark

Time/hr

1

2

6

24

32

48

56

1

2

6

24

32

48

56

0-3cm

10

10

10

10

10

10

10

5

4

4

4

4

3

3

3-6cm

0

0

0

0

0

0

0

1

3

1

0

0

1

1

6-9cm

0

0

0

0

0

0

0

0

1

2

2

2

1

1

9-12cm

0

0

0

0

0

0

0

2

0

0

0

1

0

0

12-15cm

0

0

0

0

0

0

0

0

1

1

2

1

3

3

15-18cm

0

0

0

0

0

0

0

0

1

1

0

0

0

0

18-21cm

0

0

0

0

0

0

0

0

0

0

0

0

0

0

21-24cm

0

0

0

0

0

0

0

0

0

1

1

0

0

0

24-27cm

0

0

0

0

0

0

0

2

0

0

1

1

1

0

27-30cm (top)

0

0

0

0

0

0

0

0

0

0

0

1

1

2

Discussion

The results were organized in the form of tables usually with length in movement or number of snails against time. In the first experiment, habitat preference was tested. Rough gravel and fine/coarse sand are the typical supralittoral substrates and snails may have a preference to one and not the other. As observed from the table up to the first few hours, no movement was noted. On the sixth hour a single snail had changed substrate from the fine sand onto the gravel. At the end of the experiment two of the 3 snails placed on the fine sand had moved onto the gravel or coarse sand and one from the coarse sand had moved onto the gravel as well. This indicates that the organisms somewhat dislike a loose substrate such as the fine sand (only 2 remained) but prefer rough gravel (5 snails remained). The coarse sand was somewhat in between the two with 4 snails remaining. This is the expected result as these organisms are found between small rocks and in crevices. The fact that not all of the travelled to one substrate could have been due to the lack of space as with 5 or 4 snails in one section, the Petri-dish became somewhat crowded. No snails were observed to go onto the smooth Petri-dish surface and this is also explained by the fact that their habitat preference is towards rocky terrain.

In the second experiment, the effect of humidity on the inactive snails was observed. In part A which consisted of the two Petri-dishes with the grid, the difference in humidity was created by using calcium chloride (anhydrous). This chemical can absorb the water present in the atmosphere creating dry conditions whilst the other had a tap with water giving the environment 100% relative humidity. In the dry dish 6% of the snails moved whilst in the wet dish, 18% of the snails moved. Although not so many snails moved the difference from dry to wet is already evident with about 3 times the snails moving in the 100% RH than the 0% RH. This shows that although it is not that strong of a stimulus, the relative humidity plays a part in the activation of the sails.

In part B of experiment number two, the snails were once again exposed to an atmosphere of 100% RH, but they were also tapped on the shell after one hour and placed back for a few more minutes. The tapping seems to have some effect on the snails as unlike the 100 % RH in part A where the snails took days to move, 20% of the snails in part B after only one hour were noticed to be active (which is roughly the same amount as in part A at 100% RH). This suggests that probably the wave action on the snails combined with the high humidity (as they are wetted) are effective stimuli to activate the snails.

Experiment 3 consisted of three parts. In the first part, the Petri-dish had a grid on the bottom where 16 snails were placed. In the one which contained the 1mm of sea water by the second day all the snails had moved. The period where most snails became active was between the 6th and 24th hour. This when compared to the previous experiment where only the humidity was at 100% shows that water is a much stronger activator as all the snails moved (the dry control had no noticeable movement). This would make sense as if there was wave action apart from high humidity and the mechanical force its self (both of which have shown positive results) , the snails would most likely become submersed in little pockets of sea water. Interesting to note that when as in part 3C the water used was fresh water, no snails moved up to 24 hours and only one moved just slightly over the 3 days period (probably due to humidity not the water its self). This is interesting to note. It can probably be explained by the fact that if it were to rain over the snails which in August or September (although rare) it might, although the snail would feel the mechanical force, be in an atmosphere of high humidity and covered in water it would not be beneficial to come out of dormancy as the environment would not yet be suitable and so it must be sea water to activate the snail as this would only come ashore from waves (indicating a suitable environment). In part B of experiment 3 the time taken for each snail to become active was noted. Prior to the experiment, they were sharply tapped. This tapping followed by the immersion in sea water instantly brought about a response from the snail which opened its operculum. This was very fast, in fact an average time of 17.5 seconds was recorded between the ten snails tested. This once again replicated wave action only a faster response was obtained due to the fact that actual sea water was used and not high humidity or tapping only (which continues to suggest that the snail becomes active after summer during the winter storms).

Up till experiment 3, sea water was known to bring about a response, the question then was (answered by experiment 4) if more water would bring about a faster reaction. Apparently this is not so as the time taken for the snails to travel a distance of 5cm up the wall of the cylinder containing 5cm of water was less than the cylinder containing 20cm of sea water. This can probably be explained by the fact that the snail is not adapted to live in deep water but it is actually semi-terrestrial only venturing into the sea to lay its eggs. Thus a higher hydrostatic pressure of 20cm would indicate an unsuitable environment and the snail will most likely remain dormant. Therefore only frequent wetting and not submerging (in more than 10cm of water) brings about a response (once again pools brought about by waves are usually not as deep as 20cm in small crevices where the snail aggregates).

Experiment 5 was somewhat baffling and probably should not be considered as a representative result. Only a few snails were activated and the snails which travelled most up the cylinder were in the one not stoppered! An expected result based on the other previous experiments and knowledge of the snail's habitat preference would be something as follows. Upon introducing the snails to the 3cm of seawater, they would become active and since they would be submerged find a way out which would be to climb onto the cylinder's side. Since their habitat lies a fair distance from the sea, the snail should continue to climb up to a good few centimetres. The snails in the stoppered cylinder should reach the top as the high humidity indicates that they are still very close to the water and travel up as far a possible. In the open cylinder having a dryer atmosphere, the snails would probably not move up to the top as they would be under the 'impression' that they are a fair enough distance from the 'sea' located at the bottom.

Experiment 6 was an extension of experiment 5 where the upward movement from a submerged place was tested only this time the variable was not humidity but terrain brought about by the drilled plates stuck to the sides of the tank. Again the data was not as expected so much so that the slightly different version experiment part 6B did not have the same results as A but was as expected. The reasons for experiments 5 and 6A being somewhat not accurate could be due to the errors mentioned in the sources of errors section above. If one were to follow the result brought about by experiment 6A it would be concluded that the snails prefer to remain submerged and on smooth surfaces rather than in the crevices. This of course is not the case as the snails prefer crevices in rocks as seen from experiment 1 and also in the wild these are found in crevices not submerged on smooth surfaces. In part B where the drilled plate was placed at the bottom, the snails aggregated in the pits and stayed submerged. This would indicate that they actually do prefer crevices and pits which offer protection against the elements. The pits were submerged, but the expected result was that despite this the snails should not go out of the water. This is because they were submerged under only 1.5cm and in the wild this would be something common for a snail in a small pit to experience a few mm of sea water. So in the pits and under water the mollusc is actually in its preferred environment.

Finally experiment 7 tested if the snail is photosensitive. According to the results obtained, when in light the snails (all 10 of them) became active faster (in the first hour even) and climbed a distance of 3cm, whilst in the covered cylinder half as much became active and over a longer time period. The strange thing was that in the light, the snails did not reach the top and in the dark only 1 did. Considering the high humidity in both, all the snails should have gone up to the top, the only difference (if there is) would be in the time taken.

Taking into consideration all the results obtained and considering the concordant data, it is noted that the snails become active via various stimuli. In summer humidity is low and the snail would be in a crevice seeking refuge from the direct sun. When the first storms come along, the crevice the snail would be in becomes wet with sea water and very humid (a sort of micro habitat). The pounding action of the waves also has an effect on the snail. The mollusc is at home under a few centimetres of water and in fact in November to March during high tide the snail travels from its habitat to the Eulittoral zone (which is covered in water during this period of the year) and lays its eggs. The snail is affected by hydrostatic pressure (experiment 4) and so only travels a specific distance into the sea (which would be the optimum place to lay its eggs). It also detects the relative humidity and only ventures up shore a fixed distance (up to the supralittoral zone) from the sea. In this zone lichens also grow in the winter and spring times. These are a prime source of food for the snail and may be another reason why it aestivates as in summer, this food source dries up.

Conclusion

Having done the experiments and observed the results, it can be concluded that there are various factors which play a part in activating the snail. Each factor affects the snail to a certain degree and the combination of all of them (humidity, mechanic wave action, sea water etc) brings about a reaction either to be inactive due to their lack or to be activated due to their presence.