Low-lying Sea Urchin, Tripneustes depressus

Low-lying Sea Urchin, Tripneustes depressus. Urchin collected from a tidal pool in the greater Los Cabos area, Baja California Sur, January 2016.  Diameter: 15 cm (5.9 inches).

Low-lying Sea Urchin, Tripneustes depressus. Underwater photograph taken in coastal waters off Puerto Vallarta, Jalisco, September 2021. Photograph courtesy of Kevin Erwin, Seattle, Washington. Identification courtesy of Identification courtesy of Dr. Francisco A. Solís-Marín, Mexico City.  Kevin reports that the photograph does not do justice to the color of the urchin, it being yellow and green with black. Dr. Solís-Marín indicated that these urchins vary significantly in color during their growth stages.

Low-lying Sea Urchin, Tripneustes depressus. Underwater photograph taken in Zihuantanejo Bay, Guerrero, December 2019. Photograph and identification courtesy of Maude Jette, Dive Zihuantanejo,   www.Divezihuantanejo.com.

Phylogeny:  The Low-lying Sea Urchin, Tripneustes depressus (A. Agassiz, 1863), is a member of the Toxopneustidae Family of Globular Sea Urchins. The Tripneustes genus is one of eight genera in the Toxopneustidae Family, and there are four species in the Tripneustidae genus. They are also known as the Brown Urchin, the Sea Egg and the White Urchin. In Mexico they are called Erizo de Bajo Perfil. The genus name Tripneustes comes from the Greek words meaning “three breather” or “three blown”. Perhaps this somehow refers to their water vascular system. The species name depressus is Latin and means “pressed down” or “flattened”, and refers to the low profile of this species.

Morphology:  Low-lying Sea Urchins are large dark brown to purple globes that are covered with short thin pale brown spines. The spines are arranged in three vertical series with the internal and external being in a regular pattern and the middle series being irregularly spaced. They have tiny pedicellariae (pincers) between their spines. Their globiferous pedicellariae have a head with three moveable jaws. Each jaw ends in a sharp tooth and has a venom sac at its base. When they pinch their victim, the tooth penetrates the skin and injects the venom. They have an internal skeleton (test) made of overlapping plates (ossicles). The underside or ambulacral area is wide and flattened. The mouth is located in the center of the ambulacral area. 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. Low-lying Sea Urchins reach a maximum of 15 cm (5.9 inches) in diameter.

Habitat and Distribution: Low-lying Sea Urchins are found in shallow coastal waters at depths of 3 m (10 feet) to 73 m (240 feet). They live on rock, coral and sand. The Low-lying Sea Urchin is an Eastern Pacific species. They are found in all Mexican waters of the Pacific Ocean with the exception that they are absent north of Todos Santos, 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:  Low-lying Sea Urchins are opportunistic omnivores. Their diet primarily consists of brown, green or red algae, depending on availability. On occasion they are found with pieces of sponge and other invertebrates in their stomachs.

Predators: Despite their venomous  pedicellariae, Low-lying Sea Urchins are preyed upon  by crustaceans, fish and sea stars.

Reproduction:  Low-lying Sea Urchins are gonochoric (male or female for life). Fertilization is external, through broadcast spawning. The fertilized eggs develop, after about one month, into  planktotrophic larvae (echinoplateus) and live for several months before they sink to the bottom and metamorphose into young urchins.

Ecosystem Interactions:  Low-lying Sea Urchins are subject to endo- and ectoparasitism at the micro organism level. Very little has been documented regarding other parasitic, commensal, or symbiotic relationships. The venomous pedicellariae probably keep away the epibionts that are usually associated with other sea urchins.

Human Interactions:  Low-lying Sea Urchins face some fishing pressure because they are harvested for their roe. They are currently being farmed in small quantities as an experiment for future aquaculture. In medical/pharmaceutical research, this species is of interest because the coelomic fluid in the body cavity contains peptides that act as antivirals against the pseudorabies virus and the rabies virus. The pedicellariae of this species, unlike most sea urchins, are strong enough to penetrate human skin if left in contact for a few seconds. They are known to deliver painful stings when stepped on. From a conservation perspective the Low-lying Sea Urchins have not been evaluated. They are common with a wide distribution and should be considered as a species of Least Concern.

Synonyms: Hipponoe depressa.