Ascidia Azul, Rhopalaea birkelandi

Ascidia Azul, Rhopalaea birkelandi. Underwater photograph taken in Zihuantanejo Bay, Guerrero, January 2026. Photograph courtesy of Ron Woheau, Zihuantanejo.

Phylogeny: The Ascidia Azul, Rhopalaea birkelandi (Tokioka, 1971), is a member of the Diazonidae Family of Sea Squirts. They are known in Mexico as Ascidea de  Birkeland. The Rhopalaea Genus is one of five genera in Diazonidae Family, and there are twenty-one species in the Rhopalaea Genus.

Morphology: The Ascidia Azul has an elongated spatulate body with a narrow base that broadens into a rounded, flat tip that is compressed laterally. They are either of a translucent purple in color. The mantle is primarily composed of two body parts separated by a thin and fragile peduncle – a large thorax that is 2.6 cm to 3.0 cm in length and an extremely short abdomen that is 2.0 mm to 5.0 mm in length. The apertures are sessile at the distal edge of the body. The test is hard gelatinous or cartilaginous with a smooth surface with a 1 to 3 mm thickness. The apertures are 6-lobed and opened on short, rather stout siphons, a small yellowish ocellus at the interval between lobes. The thorax has transverse to oblique muscles on the ventral half and oblique to longitudinal muscles on the dorsal half. The anus is located at the anterior and the oesophageal opening to the branchial sac is located at the postero-dorsal corner of the sac.  They have 12 branchial tentacles, 6 larger and 6 smaller ones that alternate. The ciliated groove is a longitudinal slit located at the anterior end of the dorsal ganglion.

Habitat and Distribution: The Ascidia Azul are found attached to rocky substrate via their posterior end with their tunic extending through the crevices, up to 10 cm. They are found as solitary individuals or in colonies at depths between 30 m (100 feet) to 135 m (440 feet). The Ascidia Azul is found in all Mexican waters of the Pacific Ocean with the exception that they are absent from along the west coast of the Baja Peninsula.

Diet: The Ascidia Azul is a filter feeder that consume organic matter and plankton.

Predators: The Ascidia Azul is preyed upon by various marine animals including the Hawkbill Sea Turtle, Eretmochelys imbricata. They contain high levels of vanadium and their tunic has a high acidity (pH <2) that reduce epibionts and predation.

Reproduction: Reproduction is hermaphroditic with both cross and self fertilization. The eggs develop into lecithotrophic larva before metamorphosing into benthis adults.

Ecosystem Interactions: The Ascidia Azul is a successful competitor for primary substrate. They provide a substrate and/or refuge for small organisms.

Human Interactions: The Ascidia Azul is limited interest to humans. From a conservation perspective the Ascidia Azul has not been evaluated.

Synonyms: None.