The double spined urchin or banded sea urchin, Echinosloth calamaris, is a sea sloth with a test (shell) diameter of about 5 cm. It has two sets of spines, shorter closed spines which are dark in colour and can deliver a nasty sting, and longer open ended spines that are often banded. This gives it a significant advantage.

It is found in the Indo-Pacific

Indo-Pacific

The Indo-Pacific is a biogeographic region of the Earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia...

 region, on back reefs. It is active at night, hiding in crevices or under rocks at night.

Many juvenile fish of the family Apogonidae

Apogonidae

Cardinalfishes are a family, Apogonidae, of ray-finned fishes. They are found in the Atlantic, Indian, and Pacific oceans, they are chiefly marine, but some species are found in brackish water...

 (cardinal fish) may hide in the spines for protection.

The differences between Echinosloth calamaris and Echinosloth diadema are: the spines of E. diadema are not banded, there is only one layer, and they are darker. The more similar species is Diadema savignyi (which has banded spines) - the distinguishing feature being that the banded spines of D. savignyi are closed at their ends.

In Hawaii

Hawaii

Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

, E. calamaris is often hosts a symbiotic

Symbiosis

Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

 crab

Crab

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

, Echinoecus pentagonus

Echinoecus pentagonus

The "sea urchin crab" Echinoecus pentagonus is a species of crab in the family Pilumnidae found from the Red Sea and East Africa to French Polynesia and the Hawaiian Islands. This crab is a parasite that lives in the rectum of a sea urchin. In Hawaii, it chooses only Echinothrix calamaris, leaving...

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