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You are here:   animal list > Botryllus schlosseri




Botryllus schlosseri (Pallus 1766)

 The Star Ascidian

Dylan Moffitt (2011)




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Botryllus schlosseri (Pallas 1766) is a sessile, compound, cosmopolitan ascidian commonly found on solid substrata in the subtidal zone. Colonies are comprised of numerous stellate systems of clonal zooids, or buds, embedded in a common, gelatinous matrix. Zooids share a vascular system made of a network of blood vessels that terminate in pigmented ampullae on the periphery of the colony. Individual zooids possess their own buccal siphon but share an atrial siphon with all the other zooids in their system.


B. schlosseri is a filter-feeder extracting phytoplankton, zooplankton and organic matter from suspension (NIMPIS 2011). Water is drawn through the buccal siphon into the branchial basket of each zooid. A ciliary current draws water through the pharyngeal slits and food particles become trapped on a diffuse mucous net secreted by the endostyle (Carver et al. 2006). This net is then conveyed, by ciliary movement, towards the esophagus and stomach. Waste products and larva are expelled via a common exhalent siphon.

Reproduction and Growth
B. schlosseri is a cyclical hermaphrodite (Carver et al. 2006). Testes only mature several days after ovulation thus preventing self-fertilization in nature (Grosberg 1987). Fertilised eggs give rise to large tadpole larva which are brooded in the atrium until mature, and then released via the exhalent canal into the water column . Larvae bear all the prototypical chordate characteristics - the notochord, dorsal hollow nerve chord, postanal tail, pharyngeal slits and endostyle (Satoh & Jeffery, 1995). Following settlement, the larva metamophose into a sessile, juvenile ascidian, losing the postanal tail and the notochord (Hickman et al. 2008).

Asexual reproduction

Figure 2: A schematic diagram of the entire 7 day palleal budding cycle for B. schlosseri. Abbreviations: bs,  buccal siphon; end, endostyle; fz, functional zooid; pb, primary bud; sb, secondary bud 

A colony of B. schlosseri is a clone that arises from a single founder zooid. The founder zooid, or oozooid, forms during the settlement and metamorphosis of a tadpole-like larvae (Lauzon et al. 2007). The oozooid proliferates by asexual reproduction producing a colony of identical zooids (Cima et al. 2010). Zooids are arranged into star-shaped systems that share a common atrial siphon. At any point, systems within the colony possess three generations of zooids: filter-feeding adult zooids, primary buds and secondary buds. Adult zooids produce primary buds asexually by a process known as palleal budding (Kürn et al. 2011). These primary buds, in turn, produce their own secondary buds. At weekly intervals (for temperatures of 20°C) colonies undergo a generation change called 'takeover' (Cima et al. 2010). It begins with mass apoptosis and resorption of adult zooids mediated by phagocytic cells in the blood (Lauzon et al. 2007). The primary buds migrate into the vacant spaces left by the resorbed adult zooids and open their buccal siphons, effectively becoming the new functional zooids. Similarly, the secondary buds move into the place of the primary buds and produce a new generation of secondary buds by palleal budding (Cima et al. 2010). Therefore, one complete asexual cycle, from bud to adult zooid, takes 14 days (Kürn et al. 2011).

Sexual Reproduction

The colonies reach sexual maturity at 1-2 months age. Sperm mature rapidly in each zooid's testes, but the ova maturing in a zooid are derived from oogonia produced by an earlier individual in the colony (Morris et al. 1980). Fertilization is internal and fertilized eggs are brooded in the atrium until the tadpole-like larvae are mature enough for dispersal. The larvae of botryllus schlosseri exhibit positive phototaxis and negative geotaxis (NIMPIS 2011)


Botryllus, like all tunicates have a biphasic life cycle with a mobile larval phase and a sessile adult phase. Larvae only remain in the plankton for short periods and thus have limited dispersal capabilities (Grosberg 1987). Long distance dispersal events are thought to be facillitated by the settlement of botryllus larvae on mobile artificial substrates such as ships (López-Legentil et al.  2006). In fact, it is thought that this may have allowed its spread throughout the world.


Regeneration of organs or entire organisms from a few cells is uncommon in chordates. Botryllid ascidians are rare exceptions with the ability to generate an entire functional zooid from a fragment of a blood vessel and a few blood cells within 10-14 days in a phenomenon known as whole body regeneration (Rinkevich et al. 2010).