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?

A Tale of Two Tidepools

Every once in a while I like to remember that summer's just a few months away.  Sitting by the wood stove with a dog at my feet, I've been thinking about tidepools in the rocky intertidal zone, remembering sunny warm days by the seaside.  My poor students were forced to spend a perfect fall day at Giant Steps researching tidepools, and it was a fascinating lab.  It turns out tidepools vary greatly depending on their tidal height (or elevation), and it's easy to see the differences.

The tidal height of a tidepool affects how long it's separated from the ocean.  Pools near the top of the tidal range are rarely submerged by the tides, whereas those at the bottom are submerged most of the time.  This affects the water chemistry in the pool.  Some of this is obvious, like temperature.  On a hot summer day, a tidepool at the top of the rocky intertidal warms up rapidly, and is downright hot if you're an animal who's used to being in the cold ocean of Maine.  This works the same way in winter -- tidepools commonly get very cold in winter compared to surrounding oceans.  So tidepools at the top of the intertidal zone get hotter and colder than those at the bottom.  Other things are different too, though we can't see them.  Oxygen and carbon dioxide will differ, and pH will too.  As seaweeds photosynthesize during the day, oxygen increases and CO2 decreases; this increases pH (making it less acidic).  On the other hand, don't forget plants have to respire as well as photosynthesize, so at night this is all reversed.

A tidepool at the top of Pemaquid Point, frozen over.  This is rare lower in the intertidal.

Although we can't see differences in water chemistry, it's easy to see their effects.  Visit any rocky intertidal ecosystem with tidepools and here's what you're likely to see:

Tidepools at Giant Steps.  The one at the top is full of Enteromorpha intestinalis.

Pools near the top of the rocky intertidal will be chock-full of a brilliant green seaweed called Enteromorpha intestinalis -- and little else.  Move down the intertidal zone and tidepools will look very different.  There won't be a speck of Enteromorpha to be seen.  Instead the pool is likely to have a greater diversity of seaweeds, and those seaweeds are likely to be dominated by Chondrus crispus -- Irish moss.  You'll probably see a lot more animals in those lower tidepools too -- common periwinkles, slipper shells, and mussels.

Common periwinkle; Littorina littorea.  Always hungry.

Much of the difference in tidepools can be attributed to the innocent-looking common periwinkle.  Periwinkles are ravenous, and it turns out they have very particular tastes.  Enteromorpha?  Delicious.  Chondrus?  Not so much.  Wherever common periwinkles are found, Enteromorpha is not -- it's just too darn tasty to stick around long.  Those lower tidepools turn out to be very comfortable homes for periwinkles, but the ones at the top have water chemistry that excludes snails.  Enteromorpha has a grand time up there -- so much so that it excludes other seaweeds, and animals for that matter.  It's competitively dominant.  Down lower, Enteromorpha gets gobbled up, allowing many other species to grow without competition.  Hence the high diversity.

An Enteromorpha-dominated tidepool at a high tidal height

A high-diversity tidepool at a low tidal height

Jane Lubchenco laid much of the ground work for our understanding the relationship between snails, seaweed, and diversity.  Looking out on intertidal ecosystems, it's hard for me to imagine common periwinkles are an invasive species -- they are ubiquitous.  It's literally impossible to walk around without stepping on them.  Given their major influence on just these tidepools, I have to wonder what rocky intertidal ecosystems looked like 300 years ago.  Would every tidepool have looked like those up at the top of the ecosystem?  Were organisms like slipper shells rare?  It's doubtful I'll be out in the tidepools until it warms up.  Until then, the Enteromorpha filled pools will be frozen, and the periwinkles will be munching away at whatever they can find in lower pools.

A very very low tidepool; dominated by coralline algae (Corallina officinalis).  What's up with that?