03 July 2013

Everything is connected: How a snail in a lake helps a crab in the sea

Many people who have epiphanies often boil their realization down to, “Everything is connected.” Some may have achieved this through meditation, dreamquests, spirit walks, or illicit substances.

They could just study some biology instead.

Perhaps, like van Oosterhout and colleagues, you could study this on the western Atlantic island of Tobago off the coast of Venezuela:

Where van Oosterhout and company found this snail:

This snail, Melanoides tuberculata, is one of a couple of freshwater snail species introduced into the island of Tobago; this one was introduced in the 1970s. Another, which van Oosterhout and colleagues documented for the first time in Tobago, is Tarebia granifera.

The cool part of this story is that although these are freshwater snails, their presence may turn out to be beneficial for saltwater hermit crabs.

Hermit crabs live in snail shells. Tobago has a few species that live offshore, particularly Clibanarius tricolor (pictured) and Clibanarius vitattus. Looking around Tobago, and you will find these sea-dwelling hermits in freshwater snail shells. 

These two new snail species are abundant enough that their shells are common in Tobago rivers. During heavy rains, the increased water flow carries these shells down to the sea, where hermit crabs can pick them up. The further away the crabs are from the mouth of the river, the less likely you are to find one with a freshwater shell.

Not only are the shells just available for the hermit crabs, the hermits actually preferred the freshwater shells over a couple of the shells normally found in the oceans with the hermits. Shells can be a limited resource: there may just not be enough shells to go around for all the hermit crabs. All of these new shells from the introduced snails could actually help these hermit crabs.

Newly introduced species like these freshwater snails are often called “invasive,” which has a negative connotation. But if the invaders are winners, at least some of the bystanders might also be winners.


van Oosterhout C, Mohammed R, Xavier R, Stephenson J, Archard G, Hockley F, Perkins S, Cable J. 2013. Invasive freshwater snails provide resource for native marine hermit crabs. Aquatic Invasions 8(2): 185-191. DOI:

Tobago photo by cheesy42 on Flickr; snail photo by Mean and Pinchy on Flickr; hermit photo by Cephalopodcast.com on Flickr; both under under Creative Commons licenses.

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