Echinometridae Family of Sea Urchins
Van Brunt’s Sea Urchin, Echinometra vanbrunti. A representative of the Echinometridae Family of Sea Urchins.
Phylogeny: Sea Urchin of the Echinometridae Family, like Starfish and Sea Cucumbers are in the Phylum Echinodermata. They are in the Class Echinoidae and the Order Camarodonta. Differences in the construction of the test and lantern distinguish this Order of Sea Urchins from other Orders.
Distribution: The Echinometridae Sea Urchins are distributed worldwide in tropical and sub-tropical seas only in marine environments. There are twenty-six species in the Echinometridae Family of which two are found along Mexico’s Pacific Coast. They are normally found on coral or rock substrate, but also may be found on mud, rubble or sand substrate. Most family members are found at depths less than 30 m (98 feet), but some exceed 140 m (460 feet).
Morphology: These Sea Urchins that they have an internal skeleton (test) made of overlapping plates (ossicles). The test is disk shaped for irregular urchins (sand dollars), and globular for regular urchins (sea urchins). They may be disc, star, sphere, or cucumber-shaped and are unsegmented. They lack the arms of sea stars, but are covered by moveable spines. There are tiny pincers (pedicellariae) located between the spines. The spines may be pointed, flattened, or club-like. Less noticeable are the imperforate tubercles and compound ambulacral plates. Their mouth is located on the flat, or concave, underside. Inside the mouth is a complex chewing apparatus, made of five jaws, that is known as an Aristotle’s Lantern. They move by the use of tube feet and by moving their spines with the spines being as long as the test diameter. These urchins generally have an asymmetric lateral tooth on the blade of their globiferous pedicellariae. They have a water vascular system, tube feet, and a complete digestive system, but they lack a head, eyes, nervous system, or excretory system. They have a larval stage with bilateral symmetry and an adult stage with 5-rayed radial symmetry. They have test diameters that can reach around 8.0 cm (3.1 inches). Sea Urchins of the Echinometridae Family are found in a variety of colors including black, green, purple and white.
Ecosystem Roles: Echinometridae Sea Urchins are primarily nocturnal. They may be herbivorous grazers or depositional omnivores. They feed on algae, detritus and sea grasses. In turn they are preyed upon by brittle stars, fish, gastropods and sea stars. Some species in this Family are Rock-boring Urchins. Rock-boring Urchins use their teeth to dig a depression into the rock surface to help protect them from certain predators as well as extreme water movement. They usually leave their hole at night to feed. In some cases they dig such a deep hole that they can no longer leave. When that happens, they feed on algae that grows in the hole and larger algae that are moved into the hole by currents. These holes can become microhabitats, and are often shared by other species.
Reproduction: Like other Sea Urchins, Echinometridae Sea Urchins reproduce sexually, with fertilization taking place externally.
A Word of Caution! Sea Urchins of the Echinometridae Family are not venomous, but should be handled with care because the spines are sharp, will break off in the skin, can be very painful to humans and are exceedingly difficult to remove.