A considerable number of different animals are known to huddle together, but most of them do it for two major reasons. The first of those is to aid with thermoregulation and prevent heat loss. One very well- known example of huddling together for warmth is found in the colonies of male emperor penguins during the Antarctic winter. Although this is much more famous in endothermic vertebrates, huddling together for warmth can also be found in a number of different species of bees and slugs. The second major reason for huddling together is to minimize predation risk. When an individual lives in a group it reduces the probability that it will be eaten should it encounter a predator as the overall risk of predation is shared over all members of the group. This kind of behaviour can be found in a number of different invertebrates such as water beetles and funnel spiders just to name a few.
In my study I wanted to look at whether cold stress or predation risk had a bigger hand to play in promoting huddling behaviour. To do this, I collected 200 green shore crabs (Hemigrapsus oregonensis) from Tower Beach on UBC campus. I divided them up into groups of five and painted each of them with a unique colour of nail polish in order to identify individuals. I put each group of five crabs into a glass tank with water that had been treated in one of four ways. In half of the tanks the water had been left at a normal 120C while in the other half the water was cooled to 60C. In half of the warm tanks the water contained a chemical predator cue from a Dungeness crab (Metacarcinus magister). Likewise, half of the cold treatment tanks received the predator cue while the other half did not. I allowed each group of five crabs to move around freely within each tank for 10 minutes while taking photographs of their position every minute. I then used this data to calculate how close the crabs within each group positioned themselves to each other over ten minutes and how quickly they aggregated under each of the four treatments.
I found that crabs were more likely to group together when they were exposed to cold water than when they were exposed to predation but that the rate in which they aggregated together was largely unaffected by either treatment. This was true for crabs of all sizes and both sexes. However, even though I found statistical significance in the both the cold water and predator treatments, the actual difference was very small. Because of the very large sample size that I used, the power of my experiment was high enough to detect very small differences in each treatment. So although I found statistical significance, the biological significance of these results is actually probably fairly minor.
To fill your crab carnage quota for the day here is an example of sting rays preying on massive aggregations of spider crabs in Australia