Posted by: Rob Viens | June 15, 2012

Crinoids Most Graceful

It was mid June (almost solstice) and Darwin returned to the bay to collect some marine invertebrates:

“Collected some beautiful Corallines on the rocks at Botofogo bay.—Mr Earl does not return to his lodgings here, but remains in town till the Beagle sails.” (June 15)

Specifically (according to his Zoological Notebook) it was crinoids that he was collecting on this day in 1832 (which he calls Comatula).  He describes them as such:

“Botofogo Bay. 15th.— Ventral surface “deep reddish browne” arms & with their pinnæ banded with white.— dorsal plate & cirrhi pale.— Suckers on the pinnæ minute, numerous; on inferior surface of arms a fine canal, bending alternately to each pinnæ, meets on the ventral disk with the other canal from the brother arm: (proving that the number 5 is normal, although here apparently there are 10).— The junctions of these canals irregular; meeting in the irregular central mouth.— Lamarck seems to deny this mouth. Cuvier states there to be one.— it certainly is by no means so apparent as in Asterias.  Anus submarginal, tubular, ejecting fæces.— The pinnæ on the lower half of arms are at their base, fleshy & not banded with white. The animal was found adhering on the over hanging project ledge of rocks.— its dorsal cirrhi were firmly fixed in an encrusting sponge.— & the arms widely extended, so as much to resemble an enormous Polypus.— irritable. Motion passing down the body as in a sensitive plant.— arms have considerable power of motion, can curl themselves into a perfect spire.— When placed in fresh water emitted a strong odour & stained the water with a brownish yellow tint.— The animal had a most graceful appearance” (Zoological Notebook)

Darwin’s description (and the fact that this was shallow ocean water) suggests that he was looking at a type of crinoid known as a feather star (or cormatulid).

Passion Flower feather star (Ptilometra australis) from Australia (from Wikipedia Commons)

Passion Flower feather star

Crinoids (class Crinoidea) are members of the Echinoderm phylum (as are the more familiar sea stars, sea urchins, sea cucumbers and sand dollars). One of the distinctive features of echinoderms (which is derived from the Greek for “spiny skin”) is that they have what is called “five-fold symmetry”. Effectively, this means that they have five segments that are all symmetrical with one another. (Imagine a pie cut into five identical “slices”.)  So your typical sea star will have 5 arms (or maybe a multiple of 5).  It is a story for another day, but interestingly echinoderms are the phylum of animals that is thought to be most closely related to our phylum – the chordates.

Many crinoids are attached to the ocean flow by a stalk (or stem), which makes them look like a plant (in fact, they are even called “sea lilies”).  Some crinoids, however, are not attached to the rocks – these are the feather stars. Interestingly, the feather stars start their lives attached by a stalk, but as an adult they detach from this stem and become mobile.

Notice that if you take a slice through the stalk of a fossil crinoid stem, you can see the five-fold symmetry (from GeoKansas):

crinoid cross sections

In both cases the crinoids’ “feathery” arms gather food and pull it into the creature’s mouth (located on the top-center of the star).  Right next to the mouth (as Darwin describes) is the anus. (Notice that this is not exactly the same as the cnidarians (see The Immortal Hydrozoa).  In the corals, the mouth and anus are the same opening – in the echinoderms the digestive tract is more of a u-shape with two separate openings that just happen to be next to one another.)

Although crinoids still inhabit the sea, their heyday was back in the Paleozoic Era. They first appear in the fossil record almost 500 million years ago in the Ordovician time period, and quickly diversified. They are a common occurrence in shallow marine rocks from this time period, and can be very beautiful fossils.

Modern crinoid and fossil crinoids from the Paleozoic (from Wikipedia Commons):

modern crinoid

Paleozoic crinoid

Crinoid biodiversity was greatly impacted by the major extinction event that occurred at the end of the Paleozoic (about 250 million years ago).  The class did survive, but due t o predation the shallow water, sea lilies were never quite able to get a foothold.  Today they only live in the deep oceans (in depths as much as 6000 meters).  As Darwin found in Botafogo Bay, feathered stars do still inhabit the shallow oceans. (RJV)

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Responses

  1. gorgeous

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