Naturespeak A naturalist's view of the world

October 7, 2008

Stars and Slippers

Filed under: Uncategorized — wykes @ 8:20 pm

  It’s nice to be back in Michigan, but there are a number of odds and ends from the Atlantic seashore that still deserve attention.  When that attention is directed at starfish and slipper shells the subject quickly drifts over to the odd.  All that you take for granted in the natural world is basically rent asunder when these creatures are considered.

 

With 1,000 feet you would think

Starfishes would be faster.

But with all those feet on just five legs

Speed’s just too hard to master.

 

  At low tide, the seaweed covered rocks lining the Cape Cod Canal were full of starfish. These five armed echinoderms are so familiar to us that they defy any need for an introduction. I was able to grab a few for a closer top view (see above) and a look at the flip side (see here). The diming light made it difficult to see much more, so I took one back to my campsite within an impromptu aquarium (Yes, this is a McDonald’s coffee cup).

  This species is called the Northern Starfish – Asterias vulgaris – a technical name which literally means “Common Star.” Equipped as they are with five legs and no head, any given arm can serve as a head as long as the creature is moving in the direction that appendage happens to be pointing. There are light sensitive eye spots at the end of each leg so that the star can detect an obstacle before it bangs into it.  Of course starfish can’t travel at unsafe speeds because they move by means of their tiny feet rather than their legs. Getting all those feet to march in a coordinated manner is difficult. But since Starfish feed on clams and mussels, beasts that ain’t exactly speed demons themselves, glacial serves in place of gallop.

  A close look at the underside of a submerged starfish reveals hundreds of these tiny tube feet (see here). They operate by means of a pressurized water system. If you’ve ever filled a surgical glove with water you’ve noticed how you can manipulate the fingers by squeezing the contents. This is basically how these tube feet work. Take a look at the picture above and you will notice a bright red freckle on the creatures topside which functions as the intake valve for this water vascular system.

  Although I won’t get into it here, Starfish feed by everting their stomach out through their mouth (if you can call it that) to digest their meal externally. Oh, and by the way, they have the choice to reproduce sexually or asexually depending on the mood.

 

 

Slipper Shells are boys when young

With names like Ted or Bill.

As older each they do become,

They change their names to Jill.

 

  As odd as starfish reproductive biology is, Atlantic Slipper Shells do them one better. Named for the slipper-like look of the empty shell, these seashells are familiar beachcombing fare on the bay side of the Cape. They attach themselves to any hard surface and are often found attached to whelks and horseshoe crabs and always to each other. Large “family” clusters of slipper shells can contain a dozen or more individuals (see how many you can count here or here).

 The amazing, if not slightly unnatural, thing about their lifestyle is that all the smaller shells are male.  The large bottom shell of each stack belongs to a female. All slipper shells are born male and they undergo a sex change as soon as another male clamps onto them. It takes about 60 days for a he to become a she. If the main female dies, the next one up the stack will then undergo the gender switch and on ad infinitum.

  This type of lifestyle is called “sequential hermaphrodism” in the dry terminology of the text books. The scientific name spells it out in a bit spicier manner, however. Crepidula fornicata means basically what you think it does (to fornicate means having “relations” outside of marriage). They mate with others in their stack and never have to “pop the question.”

  Slipper shells spend their entire life in one spot, so who can blame them. Once they attach to a surface they remain there until the day they die. About the only unanswered question in their life is what name they will have when they enter the afterlife.

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