Thin-spined Sea Star, Echinaster tenuispina

Thin-spined Sea Star, Echinaster tenuispina. Sea Star collected from coastal water of Bahía Concepción Baja California Sur, March 2020. Pictured stars wingspan: 18 cm (7.1 inches). Dorsal side is dark gray; ventral side is orange. Collection and photograph courtesy of Barry Mastro, Escondido, California. Identification courtesy of Bob Hillis, Ivins, Utah.

Thin-spined Sea Star, Echinaster tenuispina. Underwater photograph taken in the coastal waters of Magdalena Bay, Baja California Sur, October 2019. Photograph and identification courtesy of Bob Hillis, Ivins, Utah.

Phylogeny: The Thin-spined Sea Star, Echinaster tenuispina (Verrill, 1871), is a member of the Echinasteridae Family of Starfish. The genus Echinaster is one of eight genera in this family, and there are thirty species in this genus. This species is also known as the Orange Sea Star. In Mexico it is called estrella de mar finamente salpicada.

Morphology:  The Thin-spined Sea Star has a flattened small central disc and 5 blunt thin tapering arms. They have robust dorsal plates with small isolated dorsal spines, which are arranged in contiguous circles. The dorsal surface is covered with numerous small bumps. Adambulacral spines are arranged 3 or 4 on each plate together with 1 or 2 small spines within the margin of the groove. These stars vary in color from a yellowish brown to blue-gray or deep orangish brown. They have a dark purple spot on the tip of each arm.  Thin-spined Sea Stars reach a maximum of 25 cm (10 inches) in diameter.

Habitat and Distribution:  Thin-spined Sea Stars are found on rocks and on, or just under the surface of,  sandy substrates. They reside from the intertidal zone to depths up to 73 m (240 feet). The Thin-spined Sea Star is a resident of all Mexican waters of the Pacific Ocean with the exception that they are absent from Guerrero Negro northward along the Central and Northwest coasts of the Baja Peninsula. They are found predominantly in the central Sea of Cortez.

Diet: The diet of Thin-spined Sea Star has not been formally documented, but other members of this genus feed mostly on biofilm, microalgae,  and encrusting invertebrates, such as sponges. Some sources also include bivalves, detritus, and micro invertebrates.

Predators:  Predation of the Thin-spined Star has not been formally documented but other members of this genus are eaten by crabs, fish, and other sea stars. These sea stars are at greatest risk in their larval and juvenile stages. Adult Thin-spined Sea Stars appear to have few predators.

Reproduction: Thin-spined Sea Stars are gonochoric (male or female for life). Reproduction can be asexual, through fission, or sexual. Sexual reproduction occurs through broadcast spawning, with external fertilization. The eggs hatch into planktonic larva, which eventually settle to the bottom and grow into their adult phase.

Ecosystem Interactions: The available literature does not specifically discuss the commensal, parasitic, or symbiotic relationships of Thin-spined Sea Stars, but other members of this genus host gastropods, scale worms and shrimp.

Human Interactions:  Thin-spined Sea Stars have no direct impact on human activities. 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.

Synonym:  Echinaster tenuispinus