Wednesday, October 1, 2014

Stalking Snails with Sarah

Sarah and her snails.
There's nothing better than getting paid to be at the coast.  Last week Sarah Kingston's Marine Molecular Ecology class asked for an assist in finding two types of intertidal snails -- Littorina obtusata (the smooth periwinkle), and Littorina saxatilis (the rough periwinkle).  So off to Giant Steps I went with them, back in my boots.

Intertidal snails clockwise from top left:  Littorina littorea, Nucella lapillus, Littorina obtusata, Littorina saxatilis.  Common names, respectively:  common periwinkle, dogwhelk, smooth periwinkle, rough perwinkle.

There are actually three Littorine species in Maine's rocky intertidal -- the third one being Littorina littorea, the common periwinkle.  Luckily, they have different habitats and morphologies, so telling them apart is relatively easy.

Littorina littorea is ubiquitous in the rocky intertidal, and is found from the Fucus zone down to the bottom of the intertidal zone, including in tidepools.  The common periwinkle is an invasive species that is thought to have arrived on the ballast stones of European trading vessels early in the colonial period.  These ravenous grazers feed on seaweeds, and probably have a huge effect on the rocky intertidal zone.  Interestingly, they have clear preferences for certain seaweeds, so they have changed the species composition of the habitat quite a bit.  You can identify these by their size -- they become large, perhaps an inch across, although they start as small snails so you can be confused if not careful.  The spire of their shell is pointy, and do not exhibit clear sutures between the "layers" of this whirled part of the shell.

Littorina saxatilis (the rough perwinkle) is probably my favorite intertidal snail.  It can be the hardest to find, since it is very small, and is found high above the intertidal, in the splash zone.  These are only common where there are crashing waves -- so exposed headlands are a common place to find them.  These little guys can go many days without being wet, and have almost lung-like organs.  They feed on the black Calothrix (the slick blue-green algae you can find at the top of the intertidal zone).

Hundreds of rough periwinkles hiding in a crack at Pemaquid Point.

"Halos" of Calothrix algae around the cracks where rough perwinkles hide.  They come out of the cracks when it's wet and cool to feed, then return to the cracks when it's dry and hot (to get some relief from the heat).
Littorina obtusata (the smooth periwinkle) is the jewel of the intertidal.  Relying exclusively on knotted wrack (Ascophyllum nodosum) for habitat and food, this snail  mimics the colors of this beautiful algae -- yellow, orange, green, brown, and even red.  They can be tough to find because they are so well camouflaged.

Typical habitat of the smooth periwinkle.
Our mission was to find about 40 individuals of both rough and smooth periwinkles, so Sarah's class can study their genetics.  The first place we looked was in the knotted wrack, and it wasn't long before the first "I found one!" was heard.  Once people got the search image in their heads, we found our quota pretty quickly.  Then it was on to the exposed rocks at the southern end of Giant Steps (where we were very careful).  We had a great look around, then got to work looking in the crevices at the top of the intertidal zone.  Thousands of tiny rough periwinkles were hiding there, so we quickly hauled in our snails.  Not a bad way to spend a Friday afternoon, and I didn't even have to go to the probability lecture that the students sat through after our intertidal adventure!

Found one!
Maybe I got a bit distracted by the anemones in the tidepools at Giant Steps . . . .
A box full of snails.
Aha!  They thought they could hide from me.  No way.
These are dogwhelks -- predatory snails also found in the intertidal zone.  They're eating these barnacles.
Hunting for rough periwinkles.
Checking out our haul.
A photo for Facebook?


  1. We always struggle with differentiating the three periwinkles. Thanks for this guidance!

    1. No prob! This was a fun post to write -- and research.