Red Sea Squirt, Halocythnia papillosa

Red Sea Squirt, Halocynthnia papillosa. Underwater photographs taken in Bahía de La Ventana, La Paz, Baja California Sur, December 2025. Photographs courtesy of Aaron Woodrow, Juneau, Alaska.

Phylogeny: The Red Sea Squirt, Halocynthnia papillosa (Linnaeus, 1767), is a member of the Pyuridae Family of Tunicates. The Halocynthnia Genus is one of fifteen genera in the Pyuridae Family, and there are sixteen species in the Halocythnia Genus. The genus name Halocythnia comes from the Greek word Halos, meaning sea, and Cynthia, meaning “from Mount Cynthus”. The species name “papilla”, meaning nipple, and the suffix “osa”, meaning plenty, refers to its bumpy texture. They are known in Mexico as Rojo Ascidia.

Morphology: The Red Sea Squirt has a sac-like ovoid body with a tunic that contains cellulose-protein fibril arranged in a complex helicoidal and supramolecular architecture creating a rigid structure. They have two siphons that have surrounding sensory rigid bristles, an oral for intake and an atrial for expulsion. They are a deep red to orange color with a rough and bumpy granulose exterior. These sponges reach a maximum of 20 cm (7.9 inches) in height. Size can vary significantly depending on habitat and depth. They have the ability to contract and close the siphons when disturbed.

Habitat and Distribution: The Red Sea Squirt is a solitary benthic species that are found in rocky substrate avoid luminous area, to which they attach themselves, at depths between 2 m (7 feet) and 100 m (330 feet). The Red Sea Squirt is native to the Atlantic Ocean and Mediterranean Sea and has recently been introduced to Mexican waters of the Pacific quickly becoming highly invasive. Growth and distribution are affected by abiotic factors including temperature, salinity and hydrodynamic conditions.

Diet: The Red Sea Squirt is a suspension feeder that filters organic particles and planktons from the surrounding sea water. Feeding rates will vary seasonally with more activity during colder months.

Predators:  The Red Sea Squirt has no known predators.

Reproduction: The Red Sea Squirt are simultaneous hermaphrodites (having both male and female sex organs) with seasonal reproductive cycles. Reproduction is sexual and is done via spermcast spawning. Spermcast spawning involves releasing sperm into water, while retaining the eggs. The eggs are then fertilized internally. The embryos develop in brood chambers until they reach a certain size, then disperse as planktonic parenchymella larvae. During this larval stage, they have a strong preference for darkness.

Ecosystem Interactions:  Ecosystem interactions specifically for the Red Sea Squirt have not been well documented. They are known to coexist with other sessile invertebrates, increasing the biodiversity of the benthic systems which includes areas with high hydrothermal activities. Other sponges of the Halocynthnia Genus are known to provide shelter for fish, amphipods, decapods, isopods, and other small invertebrate life.

Human Interactions: The Red Sea Squirt is native to the Atlantic Ocean and Mediterranean Sea and suddenly appeared in the La Paz Marina, Baja California Sur, in 2016. It’s introduction to Mexican waters was attributed to ballast water release. It quickly spread and became highly invasive suffocating the Axe Callus Scallop Pen Shell or Callo de Hacha, Atrina maura, causing massive mortality resulting in a high loss of revenues for traditional fishermen. It has also been found with the Bay of La Paz where it is reported to be less aggressive. Increased oceanic water temperature and dredging activities were believed to contribute to the explosive growth of this tunicate. Costly, time-consuming and generally ineffective strong conservation efforts were made in efforts to remove the Red Sea Squirt in an effort to try to recover the Axe Callus Scallops. From a conservation perspective they have not been formally evaluated. They have a wide global distribution and are common and should be considered to be of Least Concern.

Synonyms: Ascidea rustica, Ascidia rustica, Cynthia papillata, Cynthia papillosa, Tethyum coriaceum, and Tethyum papillosum.