Some time during the mid 1800’s a European species of snail, called the common periwinkle (Littorina littorea), was carried to Nova Scotia, Canada. It is not known if they were introduced intentionally for food or if the larvae were carried in the ballast of ships. Either way, this snail found the northern Atlantic to be quite hospitable.
Today the common periwinkle is one of the most common intertidal species of animal in New England. It has spread from Canada down the coast to the mid-Atlantic states and is now the dominant herbivore on virtually every rocky shore in its range.
Normally it’s a given that exotic species do not help natives. But hermit crabs depend on a steady supply of new shells to move into as they grow. They will change shells two or three times a year and seem to be constantly searching for that perfect fit to call home. Once limited to predatory and scavenging species of intertidal snails, such as mud snails and oyster drills, hermits now have a nearly limitless supply of periwinkle shells to chose from. Since the mid 19th century hermit crab populations have exploded. This is an extremely unusual event: A native organism has actually greatly benefited from the human introduction of a foreign animal.
In the past few weeks I’ve diaried on hermit crabs, with the basics, their reproduction and their relationship with symbionts. I haven’t really gotten into the evolution of how a crab has managed to abandon the typical crustacean exoskeleton and instead use the discarded shell of dead mollusk.
Modern hermit crabs are descended from king crabs, those huge spidery-looking creatures trapped in Alaskan waters in what happens to be considered the most dangerous occupation in North America. They have been around in their current form since the early Cretaceous era over 150 million years ago. Below are some fossilized pagurids (hermit crabs) in ammonite shells. In the photos marked "D" and "E" you can see the claws sticking out of the univalve’s aperture.
Before using univalve shells, there is a good chance that hermit crab evolution occurred when their ancestors used hollow pieces of bamboo to hide in, eventually carrying the plant around with them as they moved. All crabs, being in a group of animals known as decapods, have ten legs (including the claws). Looking at a photo of a hermit it would appear to have only six. The four other legs are smaller and adapted as hooks to hold the snail shell in place. The strength of these back legs is impressive. The grip is so tight that the animal will be torn in half if you tried to forcibly pull it from the shell. See a diagram here.
Removing a crab from its shell is actually pretty simple, if you ever find the need to do so, which I can’t imagine you ever will. All you have to do is light a match and place it at the apex (pointy part) of the snail shell and within seconds the crab will release its grip and hop right out. I don’t recommend doing this.
Other diaries in this series can be found here.
Comments are closed on this story.