Ophiuroid Ossicles: An Indicator for Ocean Acidification?

Molly Miller is part of the scientific research team for the USGS Arctic Cruise 2011. Ophiuroids,  or brittlestars,  have “hard parts” that are are part of an ocean acidification study on the Healy. Here is some information from her on ophiuroid research.

Ophionotus victoriae, brittlestar, lives on the sea floor all around Antarctica, from water just a few feet deep to over 3,000 feet deep.  Its mouth is on the bottom of the central disc.  It’s skeleton is internal and made of ossicles that are linked by cartilage and muscles.  Each ossicle is made of the mineral calcite, a mineral that dissolves in acid.  One of the effects of ocean acidification might be that ossicles of ophiuroids dissolve after the animal dies and the ossicles are scattered; the same might happen to ossicles of starfish, sea urchins, sand dollars and sea cucumbers, all of which are in the same group as ophiuroids – the Phylum Echinodermata. Another effect might be that it is more difficult for the brittlestar to secrete the ossicles, consuming more of their overall energy.  This potentially could be difficult for Ophionotus victoriae because in some places where it lives there is very little food available and the brittlestar does not have extra energy to dedicate to secreting ossicles.


Opiuriod or brittlestar on the seafloor off the coast of Antarctica. Photo courtesy of Shawn Harper.

The brittlestar above was living on the seafloor at a depth of about 75 feet just off the coast of Antarctica at 77 degrees south when diver Shawn Harper took its picture.  Notice all the marks on the sand that the arms have made as the brittlestar moved across the sediment, probably searching for food.  It eats almost anything: sediment, algae, one-celled organisms, krill, dead animals, other ophiuroids, and even seal poop.

Each arm of the brittlestar is composed of a long stack of ossicles that, surrounded by thin tissue and some other smaller ossicles.  The ossicles are held together by muscles and connective tissue. The next picture shows a close up of one the ossicles in the center of the arm of the brittle star.  Each arm has ~400 of these ossicles.

SEM of Ossicle.

A close up view of one of the ossicles taken by a scanning electron microscope (SEM) at Vanderbilt University. The white line is approximately 1 millimeter.


Vanderbilt geoscientist Molly Miller asked the question:  Do ophiuroid ossicles dissolve on the seafloor?  She wondered if, even before Antarctic ocean water became much more acidic, the calcite in the ossicles would begin to dissolve very soon after the brittlestar died and its muscle and other soft tissue decayed.  She and M.S. student Bev Walker thought this might be happening because skeletal pieces like this are rare in the recent fossil record of Antarctica retrieved from cores drilled on the Antarctic continental shelf.

To answer the questions Miller and Walker did two experiments: 1) they put the ossicles on the seafloor in Explorers Cove for 2 years and then evaluated how much they have dissolved (if at all) during the two years; and 2) they carefully weighed ossicles, put them in porous bags, and suspended them just above the ocean floor in Explorers Cove for one month.

Results:  the ossicles left for two years in the ocean dissolved significantly.  The ossicles left in seawater for only one month lost a measurable and significant amount of weight – ~0.06% of their initial weight.

Will the ophiuroids dissolve as much in the Arctic Ocean water in the Healy’s aquaria than they did in Antarctic Ocean water?  More? To answer this, Vanderbilt senior Zach Wright has prepared ossicles that will be submerged in Arctic Ocean water in aquaria on the Healy for the duration of the cruise.

No information about the ocean chemistry was recovered while the ossicles were submerged in Explorers Cove in Antarctica.  It will be exciting to see how the ossicles respond during the cruise to the Arctic Ocean water whose chemistry will be monitored so carefully onboard the Healy.




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