Obese Heart Urchin, Brissus obesus

Obese Heart Urchin, Brissus obesus. Size: 3.8 cm (1.5 inches) x 2.8 cm (1.1 inches) x 1.8 cm (0.7 inches). Collected off the beach in the greater Los Cabos area, Baja California Sur, April 2020.

Obese Heart Urchin, Brissus obesus.  Collected off the beach of Aqua Verde, Baja California Sur, March 2020. Collection, photographs and identification courtesy of Barry Mastro, Escondido, California.

Phylogeny:  The Obese Heart Urchin, Brissus obesus (Verrill, 1867), is a member of the Brissidae Family of Heart Urchins. The genus Brissus is one of thirteen genera in the Brissidae Family, and there are six species in the Brissus genus.  They are known in Mexico as Erizo de Corazòn. Heart Urchins derive their name from their heart-shaped test (skeleton). The genus name Brissus was first used by Aristotle to describe these urchins but its meaning is unknown. The species name obesus comes from the Latin word meaning “fat” or “plump”.

Morphology: Obese Heart Urchins are bilaterally symmetrical. They have a somewhat elongated oval outline. Their mouth is placed towards one end of the animal, and the anus towards the other. Their tests are flat on the oral surface (bottom) and rounded on top. Obese Heart Urchins have spines covering their tests.  The spines are used for movement, including helping the urchin burrow into the sand. They have tube feet that extend from petal-shaped grooves in their test called ambulacral grooves. The tube feet are used for respiration, movement, and feeding.  Obese Heart Urchins also have pedicellariae (minute pincers) to help deter predators. Heart Urchins lack the complex mouth parts (Aristotle’s Lantern) found in regular urchins. Living specimens may be white, pinkish, or brownish in color. Obese Heart Urchins reach a maximum of 10 cm (3.9 inches) in length.

Habitat and Distribution: Obese Heart Urchins are found in  sand. They are usually buried with their front end pointing downward. They range in depth from 1 m (3 feet) to 240 m (787 feet). Since these are burrowing animals, heart urchins are not often seen alive, but their tests may wash ashore. Obese Heart Urchins are residents of the tropical Eastern Pacific. The Obese Heart Urchin is found in all Mexican waters of the Pacific Ocean with the exception that they are absent from north of Todos Santos, Baja California Sur, along the central and northwest coasts of the Baja Peninsula.

Diet: Obese Heart Urchins are detritivores that use their tube feet to gather organic particles, from the sediment and the water around them, and to pass the food to their mouth.

Predators: Obese Heart Urchins are a poorly studied and understood species. There is no documentation regarding their predators. Other species in the Brissidae Family are preyed upon by fish and starfish.

Reproduction: Obese Heart Urchins are gonochoric remaining as females or males for life. They reproduce sexually through broadcast spawning, with external fertilization. The fertilized eggs hatch into planktonic larva. These larva eventually settle to the sea floor and develop into their adult form.

Ecosystem Interactions: Obese Heart Urchins are a poorly studied and understood species. Their their commensal, parasitic and symbiotic relationships have not been formally documented.

Human Interactions: Obese Heart Urchins have no direct impact on human activities.   They are seldom seen by human. From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.

Synonyms:   None