The Experiment

 What is the Snail's Motivation for Clustering around rocks?


Observations/Preliminary Experiment

Experimental Design


Discussion of Results: Motivated by Shade?


The tiny snails (Batillaria minima) in the tide pool at Spittal Pond in Bermuda all seemed to be clustering around loose rocks or against vertical surfaces in the tide pool. I was curious as to their motivation for moving into this distribution pattern, hypothesizing that shade was the most likely explanation. The dominant shell color morph in the tide pool was almost entirely black. I determined that they would in fact move into the formation again if disturbed as long as it was a sunny day. I also placed experimental rocks into the tide pool to determine if there was anything else about the rocks in the tide pool they were attracted to. The snails displayed no preference for the natural rocks over the experimental rocks after 24 hours. The evidence has led me to conclude that the snails are motivated to move into this distribution pattern by shade, though further study is needed.



The tiny snails in the tide pool were exhibiting a strange distribution pattern.                                   

They all seemed to be clustered around and under loose rocks in the tide pool.

I only had a limited amount of time to spend with these snails, but I wanted to explore their

motivation for clustering around these rocks.


Preliminary Experiment

The first experiment I did was to move the rock about 4 cm away from its original place to see if the cluster would move back toward the rock. There were other snails or organisms in the area where the rock was moved or between the snails and the rock. Slowly but surely, the snails made their way across the 4 cm with visible speed (I actually caught this on video, but due to circumstances outside of my control the tape was lost! Otherwise, you too would be able to witness these adorable little snails crawling towards the rock). After thirty minutes about 90% of the cluster that had been under the rock before it was moved had made there way back to the rock. It is important to note that the weather on this day was sunny with a temperature around 30°C. For a second trial I moved a different rock, under which was a different cluster, about 10 cm away. This time the snails remained immobile for a much longer period of time, eventually spreading out in different directions toward any vertical surface or other rock.

When I returned the next day, the weather was overcast, with a temperature in the mid-20’s (°C). Again, I moved a rock 4 cm away from its original location and cluster of snails. This time the snails did not really react. After 30 min, hardly any of the snails had moved from their original location, and still almost none had moved after an hour.


Experimental Design

I wanted to test whether the snails were motivated by some kind of food or other substance on the rocks in the tide pool. I collected three rocks from the parking lot at Spittal pond (about a ¼ mile away from the tide pool), washed them with fresh water, and wiped them with a towel. I marked the tops of these “unnatural” rocks with an X using nail polish and randomly placed them in the tide pool. The day when the unnatural rocks were placed into the intertidal was sunny with temperature around 30°C.


I returned the next day (approximately 24 hr later) and counted all of the snails in the clusters underneath each of the “unnatural” rocks. In addition, I randomly selected three other naturally occurring rocks in the tide pool and counted all the snails in the cluster underneath these rocks. The cluster was considered any snail touching the group underneath the rock. The counts from the “unnatural” rocks were compared to the counts from the natural rocks using an unpaired, two-tailed t-test to test the null hypothesis that there was no difference between the number of snails that congregated under the rocks. The results showed no difference between the two, the snails showed no preference for either rock type. See Table 2 for summary of results.





  Natural Rocks Unnatural Rocks
Mean 6.28 5.21
Variance 8.67 0.94
Observations 3 3
Pooled Variance 4.80
Hypothesized Mean Difference 0
df 4
t Stat 0.599258158
P(T<=t) two-tail 0.581289818
t Critical two-tail 2.776450856
F 9.264127214  
P(F<=f) one-tail 0.097426696

Table 2 This table shows the results of a two-sample, two-tailed t-test to compare the number of snails underneath the naturally occurring rocks with the number of snails underneath the experimentally placed rocks.

You can take a look at the raw data


Discussion of Results: Motivated By Shade?

The majority of the snails in the tide pool were almost all black. Of the snails I counted from under both rocks I would estimate that more than 95% of the snails were 90% black. There were a few snails in the tide pool with other color morphs (as pictured). On the sunny day, the temperature of the water was also warmer than the temperature of the air. This led me to believe that the snails were clustering under the rocks to avoid being heated by the sun, their black shells promoting the adsorption of heat from the sun. The most convincing evidence to support this was their lack of motivation to move on the overcast day compared to their visible struggle to move back towards the rock on the hot, sunny day. The comparison between the natural and unnatural rocks also supports this to some degree by suggesting they were not motivated to seek the rock for food or other cue on the rocks in the intertidal; they showed no preference for the natural rocks to the foreign, cleaned rocks. It still remains that this is part of a mating practice in Batillaria minima. The problem with this theory is that they were still clustered around the rocks on the overcast day, even though when the rock was moved they showed no desire to move. Maybe they were just tired from a full day of mating the day before. I think the evidence points to shade being their motivation for movement, though further study is needed to support this claim.


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© 2002 Laura Brentner, for educational purposes only