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Related FAQs: Sea Urchins, Urchins 2Urchins 3Urchin Identification, Urchin Behavior, Urchin Compatibility, Urchin Selection, Urchin System, Urchin Feeding, Urchin Disease, Urchin Reproduction

Related Articles: Echinoderms, An Introduction to the Echinoderms:  The Sea Stars, Sea Urchins, Sea Cucumbers and More... By James W. Fatherree, M.Sc. Algae ControlNutrient Control and Export

/A Diversity of Aquatic Life

Some Spines Now! Sea Urchins (and Sand Dollars), the Echinoids, Pt. 1

To: Part 2

By Bob Fenner

  Echinothrix calamaris

New Print and eBook on Amazon

Marine Aquarium Algae Control

by Robert (Bob) Fenner

Sea Urchins and Sand Dollars, Class Echinoidea: Friends and associates who know me to be an avid diver frequently ask whether I'm concerned with potential encounters with sharks, rays, giant squids and the like. My standard reply is that hour per hour spent the most dangerous activity we all engage in is driving on the freeway. In all honesty, in the way of moments spent underwater, other than your dive buddy, Sea Urchins are the most realistically harm/hurtful organisms.

Urchins are common fare in the marine aquarium trade. They are useful and interesting scavengers and algae eaters, but just as with underwater meetings, they present considerable risk.

Classification: Taxonomy, Relation With Other Groups

Urchins and their allies the sand dollars and heart urchins make up the Class Echinoidea in the phylum Echinodermata. You're familiar with this phylum's other four living classes, the Sea Stars, Brittle stars, Sea Cucumbers and Sea Lilies & Feather Stars. Collectively the echinoderms or spiny-skinned animals are grouped as radially symmetrical, with a water-vascular system (ambulacral) responsible for their peculiar locomotory tube feet. They have a true body cavity (coelom) supporting a calcareous internal skeleton...

The class Echinoidea including the Urchins are discoidal, ovate or globose echinoderms having bodies covered with spines and no arms. The name echinoidea is actually Greek for "like a hedgehog" referring to these spines. Their mouths are aboral, that is, directed against the substrate. Distributed between the spines are pedicellariae, specialized tube feet used for cleaning and defense. Some of these are termed globeriferous pedicellariae (now that's a mouthful!) and contain poison glands. About 800 species have been described.

Images showing typical echinoid body plans. They are spherical to globose to flat in profile, radially symmetrical, with their oral sides down against the substrate. Shown: Echinometra mathaei in Hawai'i. and a Strongylocentrotus test.

They come in brown, black, purple, green, white, red and multicolored. Most are 6-12 centimeters in diameter; some Indo-Pacific species attain 36 cm. in diameter! Some have very sort spines, some in the genus Diadema on tropical reefs have spines that may be more than 36 cm. in length (Ouch!).

Don't be fooled into thinking the length of an Urchin's spines have any direct relation to their ability to hurt you... Not to mention their poison tube feet! Use a net to negotiate them into a container underwater. Short spined Urchin in the Andaman Sea, Diadema in Hawai'i.

Irregularly shaped echinoids appear bilateral. These include the Heart and Cake Urchins and Sand Dollars. All are adapted for burrowing in sand, possess much smaller and more numerous spines. Irregular urchins all feed on minute organic particles in the sand in which they burrow. Our local common Californian Sand Dollar Dendraster exocentricus feeds almost exclusively on suspended particles, particularly diatoms. Non-"regular" urchins are rarely offered in the trade.

Species Sometimes Seen/Used In the Aquarium Interest & Not:

Family Diadematidae: Astropyga, Diadema, Echinothrix.

Astropyga magnifica, the Magnificent Urchin. Tropical West Atlantic. To nearly ten inches in diameter counting the spines. Deeper water (65-130 ft.) This one off St. Lucia at night. 

Astropyga radiata (Leske 1778), the Radiating Hatpin Urchin. Indo-Pacific; Africa to Hawai'i. Out during both day and night. On sand to rubble substrates. Eat algae, but will take invertebrates... best to feed meaty foods weekly. Not as frequently imported as its congener from the TWA above. Photos: Cebu, Philippines, and N. Sulawesi.

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The images in this table are linked to large (desktop size) copies. Click on "framed" images to go to the larger size.
Diadema antillarum, the Long-Spined Urchin. Tropical West Atlantic. 4-8 inch diameters with spines. Come out at night to feed, hide during the day within rock crevices. Younger ones with white banded spines. Cozumel images during the night and a juvenile showing white and black spine banding in Bonaire. 

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Verticals (Full/Cover Page Sizes Available
Diadema mexicanum A. Agassiz 1863, Needle Sea Urchin. Costa Rica (Pacific side) 2011 

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Diadema paucispinum (A. Agassiz 1863), a Long-Spined Sea Urchin. Pacific; Hawai'i and islands of the South Pacific. To about twelve inches maximum diameter, with spines. Usually in 60 or more feet of water on a vertical surface. Common name means "few spines" which you may not agree with if you get poked but good. Kona pix.  


Diadema savignyi (Michelin 1845), a Long-Spined Sea Urchin. Indo-Pacific; Africa to the South Pacific. Test about five inches maximum diameter, with spines to sixteen. Hide in shade by day, scouring the reefs by night. Blue ring around anus is indicative of this species. A frequent contaminant on/in live rock imports. Tiny aquarium individual and Philippines one shown. 


Diadema setosum (Leske 1778), the Hatpin Urchin. Indo-Pacific; Red Sea to South Pacific, Japan. To about four inches in diameter. Useful in coral bearing aquariums as these echinoids avoid their rocks. A frequent "contaminant" on live rock imports. Painful to get stuck by... Red ring around anus is definitive. Fiji nighttime image and N. Sulawesi by day. 


Bigger PIX:
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To: Part 2

New Print and eBook on Amazon

Marine Aquarium Algae Control

by Robert (Bob) Fenner

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