Evolution & Systematics

Phylum: Chordata
The Phylum Chordata consists of tunicates, cephalochordates and vertebrates.  Traditionally, tunicates are considered to be ancestral whilst cephalochordates are seen as the closest relatives to vertebrates.  However, with research into molecular data, that view has been justified.  Tunicates are now seen as the closest living relatives to the vertebrates (Delsuc et al. 2006).  
Despite the lack of chordate characteristics in adult tunicates, these characteristics are well represented in the tunicate larvae (Ruppert, Fox & Barnes 2004).  

Class: Ascidiacea (Subphylum: Tunicata)
The class Ascidiacea consists of solitary or colonial filter feeding organisms surrounded or within an external test of ectodermal origin.  An atrial cavity envelops the body and sides of the branchial sac/basket.  Water and substances flow through this cavity.  Water current is generated by the cilia lining the pharyngeal perforations/stigmata (Kott 1989).  

Order: Pleurogona
Gonads are situated on the inner surface of the body wall and the epicardium is not present (Ruppert, Fox & Barnes 2004).  

Family: Styelidae
The Family Styelidae consists of stolidobranch ascidians with simple branchial tentacles, 4 branchial folds and a simple dorsal lamina.  Wall of stomach forms longitudinal folds.  Gastric caecum usually present without liver diverticulum.  Usually, more than 1 gonad would lie on each side of the body with variable form (Kott 1985).  There are more than 500 species in this family alone (Shenka & Swalla 2011).  

Genus: Botryllus
Characterised by the endogenous development of the ova, which sticks out into the atrial cavity as it develops.  The ovaries are always anterior to the testes.  Australian species of Botryllus often have a restricted incurrent aperture.  The stomach is short and barrel-shaped, with no less than 8 deep folds.  
All Botryllus species are colonial, like the B. tuberatus.  This is associated with a reduction in zooid size.  Ancestral forms of ascidians are described to be similar to the species Ciona intestinalis, which is a fairly large and solitary ascidian.  Evolutionary selection pressures would have selected for colonial existence in the environments where Botryllus species are found.  Such evolutionary pressures may have selected for the fast replication, and energy conservation traits in the smaller colonial ascidians (Kott 1989).  
The first ever description of an ascidian was that of a Botryllus schlosseri dredged along in the British Island sand, by Schloser in 1756 (Shenka & Swalla 2011).  


The direct ancestor of B. tuberatus is thought to be Botryllus Gaerner, 1774 (WoRMS 2014).  Research on B. tuberatus stem cells have shown that there is evolutionary conservation of the germline and totipotent stem cells within the Metazoans, indicating its evolutionary importance (Akhmadieva et al. 2007).  


Fossil record of ascidian spicules have shown that ascidians had a wide distribution and are well represented during the Eocene period (Lukowiak 2012).  Even though it is not specifically for the ascidian B. tuberatus, it is likely to represent one of the ascidian ancestral forms.  
Even in modern times, ascidians have been known to colonize areas all around the world, ranging from oceanic abyss to antarctica to subtropical waters (Shenka & Swalla 2011).