Kingdom: Animalia

Phylum: Chordata

Class: Ascidiacea

Order: Pleurogona

Family: Styelidae

Genus: Botryllus

Species: B. schlosseri


These colonial sea squirts form 2-4 mm thick sheets. The zooids (animals) are positioned
in star shaped formations. In the center of each star there is a common cloak siphon
(opening). Each formation includes between 6 and 12 individuals. Colonies grow in flat
sheets or lobes on hard substrata and are composed of many (hundreds to thousands)
clonal, asexually produced zooids arranged in approximately star- or flower-shaped
clusters called systems. Each system is comprised of approximately 20 oval- to
tear-shaped zooids. Color is variable but all zooids within a colony are the same color.
Zooids are often orange, yellow, black, red, gray-green, or white and are embedded in a
firm fleshy matrix (the test) that is often purple or brown to colorless. Each zooid
possesses its own incurrent siphon while all zooids within a colony share a single, large
common excurrent siphon (excurrent siphons are the tubular structures that burrowing
bivalve molluscs, such as clams, use to feed, respire, reproduce and expel waste into the
water, while the body of the bivalve remains buried beneath the surface of the sediment,
where it is hidden from predators).


All sea-squirts filter suspended particles from the water. They maintain a current that
passes through their body by beating tiny hair-like structures known as ‘cilia’; the water
enters through an opening called the ‘inhalant siphon’, and passes through the pharynx,
where plankton and detritus become trapped in mucus and are passed to the stomach.
The water then passes out through a second opening called the ‘exhalent siphon’. In the
star ascidian, the exhalent siphons of all the members of the colony open into a shared
chamber, known as the cloaca, water then passes out of a shared exhalent siphon at the
center of the colony.

In the star ascidian, the zooids are hermaphroditic, after fertilisation, eggs are retained
until the tadpole larvae have formed. The larvae are then released through the exhalent
siphon, and live in the water column for about 36 hours, before settling and forming new
colonies. Asexual reproduction can also take place through budding. Colonies may live
for up to one and a half years. Star ascidians are predated upon by cowries (members of
the genus Trivia), which are gastropod molluscs, cowries also lay their eggs into holes
made in the star-ascidian test.


Reproduction in B. schlosseri, a sessile hermaphrodite, includes a sexual and an asexual
component. Sexual reproduction involves the release of male gametes into the water
followed by uptake in the incurrent siphons of nearby colonies and internal fertilization
of eggs. Asexual reproduction involves a synchronized budding process that occurs on an
approximately weekly basis to increase the size of the clonal colony until it becomes
large enough to reproduce sexually. Closely genetically related colonies may fuse with
one another when they come into contact, resulting in larger “chimera” (made up of
genetically distinct individuals) colonies that may reach sexual maturity more rapidly
than smaller unfused colonies. Sexual maturity in field populations in Monterey, CA was
attained in 49 days, corresponding to 7 asexual replication cycles. Reproductive
seasonality appears quite variable across the broad distribution range of the species.


Very common and widespread around the coasts of Britain. It also occurs around Ireland
and in mainland Europe from the Faeroe Islands and Norway, reaching as far south as the
Mediterranean. It is also known from the western Atlantic along parts of the coast of
North America, where it is thought to have been introduced on the hulls of ships