Black Colonial Ascidian, Botrylloides niger
Black Colonial Ascidian, Botrylloides niger. Underwater photograph taken in Bahía de La Ventana, El Sargento, Baja California Sur, March 2025. Photograph courtesy of Aaron Woodrow, Juneau, Alaska. Identification courtesy of Greg Timms, youtube.com/@CoralmasterdivesPuerto-Vallarta/videos.
Phylogeny: The Black Colonial Ascidian, Botrylloides niger (Herdman, 1886), is an encrusting colonial marine tunicate belonging to the Styelidae Family of Colonial and Solitary Sea Squirts. The Botrylloides Genus is one of thirty-eight genera in the Styelidae Family, and there are twenty-four species in the Botrylloidae Genus. Members of the Botrylloides Genus are distinguished from closely related genera (such as Botryllus) by having their zooids (individual organisms that make up the colony) systematically arranged in continuous, winding tracks or elongated chains rather than circular, star-like clusters. The genus name Botrylloides comes from the Greek and means “resembling grapes”. The species name niger comes from the Latin and means “dark” or “black”. In Mexico, this organism is referred to as Ascidia Colonial Negra and Tunidacis de Cadena Oscura.
Morphology: Black Colonial Ascidian colonies grow as a thin, irregular, flat-spreading sheet over a diverse array of natural and artificial underwater substrates. The individual microscopic animals making up the colony, known as zooids, measure roughly 1.5 mm to 1.85 mm in length and align themselves side-by-side. Their bodies produce a distinctive structural coloration consisting of highly contrasting, horseshoe-shaped loops of bright yellow or gold pigmentation set against an otherwise dark purple or charcoal-black background. Each tiny golden rim outlines an individual incurrent branchial siphon. These siphons constantly pull down surrounding seawater through an interior mucous-lined gill basket to extract suspended plankton and loose organic matter. The prominent, elongated tube-like cavities visible in the center of the looping tracks function as shared excurrent cloacal pathways. Each zooid has 8 larger tentacles with smaller rudimentary ones in between, a funnel-shaped stomach with 9 to 10 glandular folds, and longitudinal muscle bands on the mantle. Coloration varies widely for Black Colonial Ascidians, ranging from dark orange-red in the Jamaican mangrove areas of the Caribbean to vibrant orange, orange-brown, or brown-striped morphs in other regions. Black Colonial Ascidian colonies reach a maxium of 20 cm (7.9 inches) in width.
Habitat and Distribution: Black Colonial Ascidians are commonly found attached to artificial and natural hard substrates like marina ropes, moorings, rocks, mangroves, debris, oyster banks, mussel farms, and as epiphytes on seaweeds or fouling other invertebrates. In the Gulf of California, they commonly overgrow shallow rocky reef systems, vertical walls, macroalgae, coral rubbles, and man-made structures like dock pilings and artisanal oyster culture beds. They are found at depths up to 20 m (66 feet) but may extend to 40 m (131 feet). They are native to the West Atlantic region (including the Gulf of Mexico and Caribbean), but have been introduced globally and are now found throughout the subtropical and tropical Atlantic, Pacific, and Indian Oceans. They are found in all Mexican waters of the Atlantic Ocean and Pacific Ocean with the exception that they are absent from the west coast of the Baja Peninsula.
Diet: Black Colonial Ascidians feed by pumping water into the oral siphon and through finely meshed ciliated gills on the pharynx. The gills filter out phytoplankton and detritus, which are then passed on mucus strings to the stomach and intestines. Once the zooids extract nutrients from the water, they channel the filtered waste streams into large, communal central chambers. The wastewater is then forcefully pumped clear of the colony, ensuring the microscopic filter-feeders do not continuously re-ingest their own expelled water.
Predation: The predation of Black Colonial Ascidians is a poorly studied and documented subject. This is in due in part to their wide distribution and wide range of habitats with their predators present in one place are absent in another. Colonial Ascidians are preyed upon by crabs, fishes, flatworms, nudibranchs, sea stars, and sea urchins.
Reproduction: Black Colonial Ascidians reproduce both asexually, by budding, and sexually. The zooids are simultaneous hermaphrodites (having both male and female reproductive organs). It seems to be unclear for this species if the eggs are self-fertilized or fertilized by sperm from nearby animals. The eggs are internally fertilized and the embryos are incubated in a brood pouch. The fertilized eggs hatch into larvae with a muscular tail, notochord, eyespots, and a set of adhesive papillae. The larvae swim briefly, usually less than a day, before settlement. Once they mature sufficiently and find the proper habitat, they settle to the substrate, the tail is absorbed, the gill basket expands, and the tunicate begins to feed by filtering water.
Ecosystem Interactions: Black Colonial Ascidians are sometimes epibionts on bivalves and other hard shelled invertebrates. These relationships is commensal with no harm coming to the host organism. Otherwise, there are no documented parasitic, commensal, or mutualistic relationships for this species. While they have been introduced through much of the world, they are generally not considered to be invasive. However, in areas where they exist in high population density they will smother native species or displace them from critical habitat.
Human Interactions: Black Colonial Ascidians are considered to be fouling organisms. They attach to buoys, chains, lines, pilings, vessel hulls and other infrastructure, adding weight and slowing vessels. In sufficient numbers, they can smother aquaculture farms, however, their economic impact has not been significant. From a conservation perspective, Black Colonial Ascidians have not been formally evaluated, however, based on their rapid dispersal they appear to be thriving and should be considered to be of Least Concern.
Synonyms: Botrylloides chazaliei, Botrylloides nigrum, Botryllus niger, Botryllus nigrum, and Metrocarpa nigrum.