Clams that swim

Plum Island Sound – low tide

Atlantic razor clam, Ensis directus, Plum Island Sound, Massachusetts
Atlantic razor clam posing as a barber’s straight razor.

Sharknadoes are fake, but an ocean full of swimming razors is very real.

I was walking the shallow-water tidal flats at low tide when it happened.  The sand shrimp were scuttling  as I shuffled my feet and then a straight razor swam by.  You heard me, a clam, a razor clam, swam by.  Who knew bivalves – who are renowned for their sedentary ways and ability to win staring contests – could move!  Oh, but they can (one day I’ll tell you the story of the glochidia that hitchhiked on a fish to move upstream).  Scallops are the most famous mobile bivalves, using jet propulsion as an escape response.

Today’s mobile bivalve is the Atlantic razor clam ( or known to his close friends as Ensis directus the bestest), a long thin bivalve that resembles a straight razor.  Sometimes called Jackknife clams.  Razor clams can move because of their remarkable foot (what looks like a tongue in the picture above).  Its foot, in relation to it’s body length, is one of the longest in bivalves.  

Digging
A razor clam can slice through the sand up to a meter (3.3 feet) down.  Ever reach down to pluck a razor clam from the sand to only have it get slurped up by the sand?  This happens because the clam is extending its muscular foot, flaring the end as anchor and quickly retracting the foot to pull itself down.  At the same time, it squirts water into the sand to liquefy it – like quicksand – making penetration easier.   That is a clever clam.  But it’s not done yet.  

Swimming
A clam has no arms to swim with (or to hold you with), but it finds it foot to be versatile.  If a clam is laying on the sediment surface and there’s overlying water, the clam can swim short distances by quickly retracting it’s foot and squirting water out of its shell by quickly closing the two valves.  These are motions similar to digging, but here the clam swims as a result of jet propulsion (thrust produced by passing a jet of a fluid in the opposite direction to the direction of motion).  Know who else uses jet propulsion?  Squids.  Scallops.  And, well, jets.  But molluscs have been using jet propulsion millions of years before Tom Cruise ever did.  By the way, squids, scallops, snails, chitons, and clams are all in the Plylum Mollusca (because you need to know this).    

Below is a short video demonstrating the swimming behavior of the Atlantic razor clam as described above.  

The Peruvians apparently have way cooler clams than we do here in the States.  This one is using its foot as a propeller! 

Jumping
A razor clam on the sand without overlying water is not necessarily stranded.  Here the clam uses it’s foot by curling it under its body and rapidly retracting it (no jet propulsion).  The video below shows the jumping behavior from my hand.  It jumps after ‘licking’ my hand with its foot.  It’s trying to dig into the sand, but finding no purchase in my hand, it jumps.  Once it finds sand, it digs in!

Why would a razor clam be so mobile?  Digging: to escape predators such as birds and New York City chefs (razor clams are a trending new treat!).  A side note, razor clams are a small shellfishery in New England and is being studied as a potential aquaculture species.

Why would a clam jump or swim? At least three reasons.

1) Predators again.  Despite the Lisa-Frank name of ‘ribbon worms‘, these not-technically-worms-because-they’re-not-in-the-Phylum-Annelida-worms are wonderfully streamline predators that roam the sands for their own version of razor clam ceviche (minus the ceviche).  

2) Curb appeal.  Razor clams are persnickety about their dirt.  They prefer sandy to muddy sediment that must be clean.  You heard me, their dirt must be clean or they’re moving out.  What is dirty mud?  Mud with too many organics such as tiny bits of dead plant material.  

 3) Water flow.  Being a bivalve means it’s a filter feeding and needs adequate water flow to feed itself properly.

How common is this movement from place to place?  It happens quite frequently when their young, and it’s not uncommon in the adults.  It’s a wonderful adaptation for a bivalve living in the shifting sands of the intertidal zone.  

Useful resource:

http://nrac.umd.edu/files/Factsheets/NRAC%20217-2010.pdf

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