Wednesday, January 26, 2011

Snail and hermit mystery

I can never keep enough small snails in the aquarium. The crabs eat them; so do the big trophon snails. I'm always bringing home a few more, and always finding only one or two actually doing their work of keeping the walls and floor clean. The rest are reduced to empty shells lying on the sand, or new outfits for the hermits.

So when we went to Boundary Bay a few days ago, I planned to pick up a good handful of snails. The mud flats around Centennial Beach are loaded with them, starting a few steps below the high tide line. They're mostly the mud-flat snail, Batillaria attramentaria, spread so thickly that at every step I can hear the "crunch!" of a few more.

But this time, for variety, we went to the south end of Beach Grove and walked down to the US border. The tide was coming in, but there was still some flat sand. But no snails. Not one. There were clams in abundance, lying in groups on the sand at the edge of rocky patches. There were worm casts and worm tubes sticking up out of the sand. No crabs, no hermits, no snails.

Just a few metres north of the border, we passed two rotting logs buried deep in the sand. Here there were barnacles and crevasses packed with mussels and mudsnail shells.

Hollowed logs make great hideouts.

Deep in a crack.
I started collecting snails. Each one I picked up, I inspected; I didn't want any more hermit crabs. The aquarium held 11 at that point, more than enough for a small tank.

And each snail, as I turned it over to look for the glossy operculum, waved hermit pincers at me. I put them back on the log, and checked another. And another, and another ... They were all hermits. All around the first log, all around the second one, down the in the innards; hermits, hermits, hermits. Finally, I found 3 very small ones that seemed to have the right dark brown glossy lid on the opening; snails, at last. I dumped them in my bottle, with a few barnacles and mussels for the trophons to nibble on, and brought them home.

And once I'd left them a few minutes in a bowl, out crawled the hermits. Out of 3 "snails", I got 5 hermits. The last two were really tiny, and must have been hitchikers on the barnacle clumps.

The more I think about this, the stranger it seems. There were no snails to be seen in the vicinity of these stumps. And all the snail shells there housed hermits. Where did they get the shells? Why did I find no surviving snails?

Did the hermits invade a colony of snails and wipe out every single one for their shells? But hermits don't usually do that; they use empty shells, or take them away from other hermits. They ignore live snails.

So did something else, crabs maybe, kill the snails and leave them piled into the logs for the hermits? Where are the crabs, then? Why only on the logs?

Or did the hermits immigrate, shells and all, from far down the beach? Why? Why were there none scuttling across the sand, or under the nearby rocks?

Hermit, one of the 11. See how his shell is broken? That's how a crab gets at the tasty snail meat.

I wish hermits could talk. We'll have to take another walk down that way next time the tide is a bit lower, to look for the missing snail population.

And now I have 16 hermits in my tank. At least the newest residents are tiny.

(More about these snails - before the hermits get them - om Dave Ingram's Island Nature blog.)


  1. I belong to Ispot -

    Although it is a site for recording British wildlife, hermit crabs and seashore snails are common here too and I'm sure someone would know the answer to this.

    Do take a look. If you mention your location clearly I can't see that anyone would object. Someone posted fungi from France once.

    I also think Olivia at

    And Tai Haiku at

    Are likely to know too.

    I wish I knew the answer!


  2. What a fascinating post! I wish I knew the answer, as well. But there are some great links here which I can't wait to follow up on! Love the photos.

    good luck with finding snails. It is odd, isn't it?!

  3. Wow, we have a hermit crab in one of our tanks who shell is broken in exactly the same way as the one in your last photo - now I know why!

  4. Really interesting observation and question! A few other questions: could these logs have been carried by wave action from somewhere else? (In other words, could the hermit crabs have been present in an area where there were also some snail-shells and then been brought to this otherwise empty stretch of beach? On the other hand, some studies have shown that, if given no other option and sub-optimal shells, hermit crabs *will* kill live snails in order to get their shells; could there have been some snails originally that met this fate?) Have you seen other logs that have the same sort of demographics to them (i.e. heavy on mussels, barnacles, and hermits, but few to no snails), or is this an isolated incident? So intriguing! Do you think you can increase the sample size so's to run some experiments? :)

  5. Is it possible that the snails' life cycle is timed with the seasons, such that the adult forms are most populous when their main food source (I'm guessing algae) is most abundant (spring through fall)? The shells of course outlast the mollusks, so the hermits get good pickings as the snails die out.
    Just a hypothesis...I really have no clue.

  6. Lucy, thanks for the links. I know tai haku and Olivia (she comments lower down here), but ispot is new to me. Good site!

    Olivia; good points! I think those logs have been there for some time; they're heavily waterlogged and buried deep. I'll go over some old photos tomorrow, and see if I can find them in the background.

    I didn't know that about hermits. That could be, though. They must have appropriate shells, or be eaten.

    I haven't noticed that about other logs and wood I have found on the beach; there are usually more snails than hermits. But I'll investigate a few more, more closely now. And start keeping a record, with numbers, etc.

    Thanks for the tips.

    Tim, The mudsnails eat diatoms. They seem to be on the beach year round. I was stepping on them just a couple of weeks ago, closer to Centennial Beach. But that could be a factor; I'll have to keep records for a year or more to rule that out. Will do.


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