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Botryllus tuberatus (Ritter & Forsyth, 1917)

Pacific Star Tunicate

Adeline Ang (2014)



Fact Sheet

Brief Summary

Physical Description

External Features

Internal Features

Slides Preparation


Life History & Behaviour

Feeding Mechanisms


Internal Transport & Movement

Asexual Reproduction

Sexual Reproduction

Anatomy & Physiology

Evolution & Systematics

Biogeographic Distribution

Biofouling Threats

References & Links

Asexual Reproduction

Asexual reproduction, also known as clonal reproduction, is responsible for modular growth in B. tuberatus (Ruppert, Fox & Barnes 2004).  2 main forms of budding are present in B. tuberatus colonies, namely palleal budding and vascular budding.  
Palleal budding is the process whereby a new blastozooid is formed on the lateral wall of the blastozooid of the parental generation, resulting in an unequal distribution of material and a much smaller progeny than the parent, as seen in figure 1 and 2 below (Akhmadieva et al. 2007).  
Figure 1.  (a) Schematic drawing of the palleal budding process in B. tuberatus.  (b) Movement of the undifferentiated stem cells from the parent zooid to the progeny during initial phases of budding.  Adapted from Akhmadieva et al. (2007)

Figure 2.  Image showing palleal budding process in transverse section of B.tuberatus colony.  It also shows the co-existence of multiple generations of zooids within a single B. tuberatus colony.

The zooids in the colony of B. tuberatus synchronize their palleal budding such that most of the zooids undergo similar stages of budding at any one time (Abbott & Newberry 1980).  There is also continuous replacement of old zooids with new ones, with each zooid serving the colony for less than a month (Kott 1989).  
Past research on B. tuberatus stem cells have shown that the morphology of their blastozooids is typical of the genus Botryllus.  The inner cell layer of the progeny was not uniform and included a thick layer of undifferentiated embryonic stem cells.  These are critical for the development of the new zooids (Akhmadieva et al. 2007).  

Vascular budding, on the other hand, refers to the formation of new zooids via the vascular ampullae, which are the terminal ends of the test vessel systems.  Such zooids are developed solely from the cells of the test-vessel wall and are still able to reach sexual maturity (Abbott & Newberry 1980).  Images of the vascular ampullae in B. tuberatus could be seen in the section of 'Internal Transport & Movement'.  Undifferentiated embryonic stem cells, as mentioned before, was also found in B. tuberatus blood vessels, and is believed to play a part in vascular budding (Akhmadieva et al. 2007).  

The ability of B. tuberatus to undergo asexual reproduction and regenerate itself from the undifferentiated stem cells is the main reason for their success in colonizing new areas and becoming invasive species in some parts of the world (Shenka & Swalla 2011).