Sand Anemone, Actinostella bradleyi
Sand Anemone, Actinostella bradleyi. Underwater photographs taken in Zihuantanejo Bay, Guerrero. Photographs and identifications courtesy of Ron Woheau, Zihuantanejo, January 2020.
Phylogeny: The Sand Anemone, Actinostella bradleyi (Verrill, 1869), is a hexacoral in the Actiniidae Family of True Sea Anemones. The Actinostella Genus is one of fifty-five genera in the Actiniidae Family, and it contains six species. The genus name Actinostella is a hybrid of Greek and Latin words meaning “star-ray” or “ray-star”. This refers to the radial ray-like tentacles of these anemones. This species was named in honor of the 19-century American geologist and naturalist, Frank Howe Bradley, who collected specimens throughout the Panamic province. This species is also known as the Bradley’s Sand Anemone and the Collared Anemone and in Mexico as Anémona Cuello.
Morphology: Sand Anemones have a well developed pedal disk that anchors them to hard surfaces. The pedal disk reaches up to 2.3 cm (0.9 inches) in diameter. The column is long, reaching 5.5 cm (2.2 inches) in length and 4.2 cm (1.6 inches) in diameter. The column exterior is sculpted with 96 longitudinal rows of verrucae (warts). The column is topped by a ruff (frill or collar) and an oral disk. The ruff lies under the tentacles, on the margin of the oral disk. The ruff is covered with many filiform (filamentous or frond-like) appendages, and harbors photosynthetic zooxanthellae. The ruff can be inflated with water to expand or contract. The oral disk is covered with 5 concentric rows of smooth tentacles. There can be up to 96 tentacles, all of similar length, reaching up to 1.8 cm (0.7 inches) in length. The oral disk measures up to 3.4 cm (1.3 inches) in diameter. The pedal base and column are pinkish to orangish in color, with darker verrucae. The oral disk is olive green with beige radial lines. The marginal ruff is generally olive green with radial brown and beige stripes. Specimens from Panama have a brownish ruff. The tentacles are brownish or reddish, with a brown base, lighter spots, and an elongate brown dot running along the tentacle. Sand Anemones have historically been treated the same as California Sand Anemones Actinostella californica, which means that much of the documentation for both species may not be valid. Both species overlap in part of their ranges, but Actinostella californica extends much higher north. These easiest field mark to distinguish the two species is that the Sand Anemone has only up to 48 tentacles.
Habitat and Distribution: Sand Anemones are found attached to rock, shells, and other hard surfaces, while the pedal disk and column are buried under sand or mud. The ruff and oral disk remain above sand. They normally reside from the intertidal zone to depths of 10 m (33 feet), but can extend depths up to 63 m (207 feet). Sand Anemones are a subtropical to tropical Eastern Pacific Species that are found in all Mexican waters of the Pacific Ocean, with the exception that they are absent from the west coast of the Baja Peninsula, north of Magdalena Bay, Baja California Sur. Citings further north are most likely the California Sand Anemone.
Diet: Sand Anemones are zooxanthellate, carnivorous, benthic feeders. They use their stinging tentacles to capture zooplankton from the surrounding water, and to capture passing small benthic or pelagic prey such as tiny crabs, shrimp, or marine worms that come too close to their hidden column. They feed primarily at night because during the day they inflate their ruff to capture more sunlight so that the photosynthetic, symbiotic, zooxanthellae can produce energy.
Predation: Predation of Sand Anemones is poorly studied and documented. By keeping the column buried, and by use of the venomous tentacles, they can defend themselves from most predators. While documentation of their specific predators is lacking, we know that other anemones in are prey for nudibranchs, sea stars, and fish.
Reproduction: There is no mention in popular scientific literature regarding the reproduction of Sand Anemones. Species within the Actiniidae Family may be either gonochoric or hermaphroditic. Most species reproduce sexually, through broadcast spawning, with external fertilization. Other species have direct insemination with internal fertilization or brooding.
Ecosystem Interactions: Other than their mutualistic relationship with zooxanthellae, no commensal, mutulistic or parasitic relationships have been documented for the Sand Anemone.
Human Interactions: Sand Anemones have no direct on human activities. Their venomous tentacles pose no threat to humans. From a conservation perspective, Sand Anemones have not been formally evaluated. They are fairly common and have a wide distribution and should be considered to be of Least Concern.