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Northwest Wildlife Online
by John M. Regan
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SEA STARS
A Lesson in Simple Complexity
John M. Regan
How can a creature that moves just slightly faster than a snail,
lacks a brain or blood, is unable to control its body temperature, finds
food only because it happens to accidentally slide over the top of it, and
spends most of its life clinging to rocks teach us anything about the beauty
and complexity of life? Well
stay tuned. The animal in
question is one of the true stars of our Northwest wildlife.
In fact, our shore line harbors more of these fascinating creatures
than any other shoreline on earth.
About 2,000 species of sea stars inhabit our planet.
At slightly less than three quarters of an inch,
Leptychaster propinquus is the
smallest. The largest reported
sea star is the yard long Giant Star,
Evasterias echinosoma, found in deep waters of the far north Pacific.
The racehorse of the echinoderms is are very own Sun Star,
Pycnopodia helinathoides.
Barreling along at three tenths of a mile per hour they actually are
quite speedy in the sea star realm.
Should you ever have the pleasure of watching one of these big stars
slide along the ocean floor it really does seem to move quickly.
Cannon Beach Oregon is a premier location for to see Ochre Sea Stars at low tide. The voracious sea star appetite for bivalves is clearly on display in the photograph in the middle Notice how they efficiently they have scoured a portion of the rock clear of California mussels. To get there, however, each and every sea star has to control each and every one of its little tube feet.
Sea
stars belong to a phylum of invertebrates called the Echinoderms which
includes sea urchins, sand dollars, and the sea cucumbers.
The most obvious characteristic of the echinoderms is their “radial
symmetry,” meaning, if you slice one down the middle you end up with two
halves that are mirror images of each other.
To be more exact most exhibit
pentamerous radial symmetry: a body divided into five parts structured
around a central core.
Echinoderms have an internal skeleton composed of calcerous ossicles.
These tightly articulating ossicles serve basically the same function
as our skeleton but also extend to the outer dermis of the animal, hence the
meaning of the name echinoderm – “spiny skin.”
Sea
stars make up the class Asteroidea.
Although found throughout the world the highest density of them are
found from Puget Sound in Washington state to the Aleutian Islands of
Alaska. As noted before most
have five radial arms but there are a number of exceptions, most notably the
Sun Star that can boast up to 40 arms.
With the
possible exception of orcas, sea stars are our most famous northwest
wildlife residents. Their vivid
colors, magical shape, and relatively easy access for observation have made
them a sea shore favorite for all ages.
Yet despite the fact that they are instantly recognized and quite
common these invertebrate creatures have a number of unusual and unknown
characteristics.
From tiny and plain to big and beautiful color. The little fellow appears to be a juvenile Ochre Sea Star that will some day blossom into the beautiful array of color on the far right. In the middle is our big speedster - the Sunflower Star. Catch one of these guys in just the right light and they appear to be on fire.
One of the more interesting facts about coyotes is their well documented association with badgers. The speed and superb sense of smell possessed by the coyote coupled with the immense digging ability of the badger has spelled doom for many a rodent.
There is
no actual sea star “brain.” Instead
they possess a nerve ring with extensions that branch out into each arm.
This is a remarkable arrangement considering the thousands of tube
feet that it must control via a complex water vascular system.
Unique to all echinoderms this dense network of canals is especially
well developed in the Asteroidea.
Turn one of these guys over sometime and you have to marvel at the
thousands of tiny appendages covering the bottom of the animal.
Now consider that each and every one of those tiny feet has a balloon
like structure attached to it.
When this “balloon” is squeezed the tube foot moves.
This is hydraulics on the micro level; and all this controlled by a
couple of simple nerve rings.
Sea
stars are blind by our standards but they do have eyes.
This comes as a surprise to most.
The eyes are found at the very end of their arm.
of Their precise function is unknown but they are believed to serve
mainly as light sensors. The
eye spot is found on the end of each arm.
The
regeneration ability of sea star arms is well known.
But sea stars can regenerate themselves completely.
Cut one in half and the result is two sea stars.
Even more amazing is that cutting the animal in fifths may result in
five sea stars!
Studies have proven that as long as a certain portion of the central
disk remains attached to an arm the entire animal can regenerate.
This ability is very useful, of course, during asexual reproduction.
Although the majority reproduce sexually some species divide
themselves in half to reproduce.
One fascinating representative of these animals can actually cast off
its own arms. The self-severed
arm then regenerates an entirely new sea star!
Now all
this might be odd enough for any one animal but the sea star has another
trick up its tube feet.
Depending on which direction the echinoderm is headed in the particular arm
leading the way takes over as the dominant “brain” of the animal.
Each arm, you see, has a nerve center of its own.
So what would cause a sea star to activate its many thousands of feet
in a certain direction to begin with?
Food, of course. Sea
stars are predators and scavengers.
These guys will eat just about anything they can overwhelm.
They detect food by scent in the water and considering the un-cheetah
like locomotion of sea stars it is not surprising that many prefer bivalves
– clams, oysters, etc. Their
method of attack is to grasp the bivalve shell between its arms and then
apply force to open the shell.
The attack is relatively swift; a surprisingly tiny opening does the trick.
As soon as this is accomplished the sea star extends its own stomach
into the now exposed, fleshy part of its prey.
As you
may well imagine this part of sea star biology does not endear it to oyster
farmers. And if you need proof of
their considerable appetite for clams take a trip to Cannon Beach, Oregon.
There you can witness first hand just how voracious these echinoderms
are as they scour huge boulders free of California mussels.
These
beautiful animals do not start out life in the form we are so familiar with.
Larvae are tiny, almost octopus shaped creatures with a sucker mouth
and small larval arms.
Microscopic sized cilia direct food into its mouth.
Slowly the familiar adult body begins to take shape as its larval
mouth and other body parts either degenerate or transform.
The average life span is about ten years but some species may
approach the ripe old age of forty.
Sea stars – a simple animal of enormous complexity.
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