Red Heart Urchin, Meoma ventricosa grandis

Red Heart Urchin test, Meoma ventricosa grandis. Collected from within Zihuantanejo Bay, Guerrero, March 2022. Photographs courtesy of Ron Woheau, Zihuantanejo. Size: 4.8 cm (1.9 inches) x 4.2 cm (1.6 inches) x 3.4 cm (1.3 inches).

Phylogeny: The Red Heart Urchin, Meoma ventricosa grandis (Gray, 1851), is a member of the Brissidae Family of Heart Urchins. The genus Meoma is one of thirteen genera in the Brossodae Family, and there are four species in the Meoma genus. The subspecies Meoma ventricosa grandis is one of two subspecies. This subspecies  previously had species status and was known as Meoma grandis. Meoma ventricosa is a Western Atlantic species, while this subspecies lives in the Eastern Pacific. They are also  known as the Cake Urchin and in Mexico as Erizo de Corazòn. Heart Urchins derive their common name from their heart-shaped test (skeleton). The species name ventricosa comes from the Latin word meaning “pot-bellied” or “inflated”, and refers to the somewhat globose shape of this urchin.

Morphology: Red Heart Urchins are bilaterally symmetrical. They have a somewhat elongated pentagonal 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. Red Heart Urchins have short spines covering their tests. The spines are used for movement, including helping the urchin burrow into the sand. Heart urchins 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. They also have pedicellariae. Heart Urchins lack the complex mouth parts (Aristotle’s Lantern) found in regular urchins. Living specimens may be rust, reddish-brown, or reddish orange in color. Red Heart Urchins reach a maximum of 16.5 cm (6.4 inches) in length.

Habitat and Distribution: Red Heart Urchins are found on  sand, mud, stones, shell hash, and vegetation. They are nocturnal and remain buried most of the day. They live at depths between 2 m (6 feet) and 200 m (656 feet). Since these are burrowing animals, heart urchins are not often seen alive, but their tests may wash ashore. Red Heart Urchins are a tropical Eastern Pacific species. They are found in all Mexican waters of the Pacific Ocean, including the Revillagigedos Islands, with the exception that they are absent from north of Magdalena Bay, Baja California Sur, along the central and northwest coasts of the Baja Peninsula and north of Bahía de Los Ángeles, Baja California, in the Gulf of California.

Diet: Red Heart Urchins feed on the film of algae and bacteria that covers sand grains. To do this they use specialized tube feet to gather the sand and move it to their mouth. They swallow the sand and the organic matter is extracted from the sand as it passes through the urchin’s gut. They can also deposit feed when large amounts on organic matter are on the surface of the substrate, such as during algae blooms.

Predators: Red Heart Urchins  are preyed on by fish, including stingrays, gastropods, sea stars, and sea turtles. As a defense mechanism, these urchins can emit a toxic yellow substance that deters, and may kill, fish.

Reproduction: Red Heart Urchins are gonochoric (female or male 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: Red Heart Urchins are a poorly studied and understood species. The Western Atlantic Red Heart Urchin, Meoma ventricosa, is known to host pea crabs in either a parasitic or commensal relationship. It is unknown if this Eastern Pacific subspecies hosts pea crabs. Their commensal, parasitic, or symbiotic relationships have not been formally documented.

Human Interactions: Red Heart Urchins have no direct impact on human activities.   They are seldom seen by humans. 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: Kleinia nigra and Meoma grandis