White Sea Urchin & Reef Urchin

Sea urchins or urchins, archaically called sea hedgehogs, are small, spiny, globular animals that, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. About 950 species of echinoids inhabit all oceans from the intertidal to 5000 m deep. The shell, or "test", of sea urchins is round and spiny, typically from 3 to 10 cm (1.2 to 3.9 in) across. Common colors include black and dull shades of green, olive, brown, purple, blue, and red. Sea urchins move slowly, and feed on mostly algae. Sea otters, starfish, wolf eels, triggerfish, and other predators hunt and feed on sea urchins. Their roe is a delicacy in many cuisines. The name "urchin" is an old word for hedgehog, which sea urchins resemble.

Tripneustes ventricosus

Tripneustes ventricosus, commonly called the West Indian sea egg or white sea urchin, is a species of sea urchin. It is common in the Caribbean Sea, the Bahamas andFlorida and may be found at depths of less than 10 metres (33 ft).

The test of the West Indian sea egg is dark in colour, usually black, dark purple or reddish brown, with white spines 1 to 2 centimetres (0.4 to 0.8 inches) long. The test can reach 10 to 15 centimetres (3.9 to 5.9 in) in diameter. It is often covered with pieces of seagrass, fragments of shell and other debris in a manner similar to the closely relatedTripneustes gratilla. These decorations are held in place by tube feet among the spines and are believed to provide protection from the intense sunlight that penetrates the shallow water.

Echinometra viridis

Echinometra viridis, the reef urchin, is a species of sea urchin in the family Echinometridae. It is found on reefs in very shallow parts of the western Atlantic Ocean and the Caribbean Sea.

The reef urchin has an elliptical reddish brown test (shell) covered with medium length spines. These are greenish in colour with paler bases and darker, often violet, tips. This urchin grows to a diameter of 5 centimetres (2.0 in) with the longest spines being 3 centimetres (1.2 in). It looks very similar to the rock-boring urchin Echinometra lucunter, but the dark tips and the greater length of the spines are distinctive.

Image Credit & Copyright: Nick Hobgood
Explanation from: https://en.wikipedia.org/wiki/Tripneustes_ventricosus and https://en.wikipedia.org/wiki/Echinometra_viridis