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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

Anatomy & Physiology

Due to the relative inability of B. tuberatus colonies to move, morphological adaptations should be in place to ensure its survival within its environment.  The test, as seen in figure 1, is made out of cellulose-like fibres of tunicin, surrounds the zooid and subsequently the entire colony.  These are well connected with test-vessels that has ectodermal origins.  B. tuberatus has a test that is translucent and thin and soft, thus forming thin colonies that are less obvious and provides protection to the organism (Kott 1989).  Even though soft and thin, the test constitutes as the only known form of protection for B. tuberatus from physical damage by waves and sand, as well as from predation.  

Figure 1. Image showing the transverse section of a B. tuberatus colony, with 3 visible living zooids.  X shows the remnants of dead zooids.  The test could be seen surrounding both living and dead zooids.

Also present in figure 1 is the existence of both living and dead zooids within the same colony.  This is due to the continual replacement of zooids within the colony.  Once the new ones have grown to a suitable size via asexual reproduction, the old ones would get replaced and their nutrients would be recycled within the colony.  Hence, at any point in time, there would be 3 generations of zooids present, the budding zooid, adult functional zooid, and the remnants of the dead zooid.  Even though colonies may live for more than a year, the zooids within them persist for less than a month (Abbott & Newberry 1980).  The development over the entire colony tends to be in sync.  
The gut of B. tuberatus forms an almost horizontal loop, as seen in figure 2 below.  The gut may be partly behind the branchial sac, with the rectum curving at right angles to it.  The stomach is also small and almost spherical (Kott 1985).

Figure 2.  Image showing the gut of a B. tuberatus zooid and younger individuals.  The developing rows of stigmata could be seen very clearly in the young developing ascidians as protrusions out of the walls of the atrial cavity.