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The Life of a Sea Urchin

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Urchin sand dollar
A pentagon shape can be seen on the underside of an urchin test Five sided symmetry is obvious on the top of a Eccentric sand dollar (Dendraster excentricus).
blood star orange sea cucumber
In sea stars, pentamerous symmetry is obvious in the fact that many have five arms, as in this blood star Henricia leviuscula In the orange sea cucumber (Cucumaria miniata) five rows of tube feet show off their pentamerous symmetry.

The life of an urchin

Although red urchins (Strongylocentrotus franciscanus) are known to mature in their first few years of life, they are long lived and become more fecund with age.  The red sea urchin is among the longest lived organisms in the world, the largest specimens are often found to be over 100 years old. Sexes are separate in urchins, they reproduce by broadcast spawning. Male and Female gametes are simultaneously broadcast into the water, fertilize, and begin life as free swimming larvae. After a short period of time these larvae settle out into the best refuge they can find. It is fairly common for them to seek refuge within the spines of adult urchins. Urchins eat mainly bull kelp in Oregon. They can crawl on the kelp, but often rely on drifting kelp to eat more safely out of surge and surf.

Sea urchins are echinoderms (meaning spiny-skinned), closely related to sand dollars, sea stars, and sea cucumbers.

Pentamerous symmetry

Like the aforementioned echinoderms, urchins have pentamerous symmetry, meaning “divided in five parts”, which is easily seen when looking at a urchin shell or “test”.

This symmetry can be seen in these echinoderm pictures.



Scott Groth- Marine Resources Program, Charleston
Phone:(541) 888-5515


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