Adult ascidians are predominantly sessile and have therefore adapted to a sedentary lifestyle. Ascidians are able to filter 24 to 540 ml of water per hour, per gram of wet weight of the individual, making them highly efficient filter feeders (Kott 1989; Goodbody 1974). Volume of food ingested is dependent on food availability, time spent filtering and the filtering capacity of the individual animal (Kott 1989). Ascidians are able to control their food consumption by halting the secretion of mucus by the endostyle, seizing ciliary beating, closing the branchial siphon or by expelling unwanted particles from the atrial siphon (Ruppert et al. 2004; Young 1989; Hetch 1918;McGinitie 1939).

Colonial ascidians mainly feed on phytoplankton and organic matter, however various species also consume zooplankton (Young 1989, Ruppert et al. 2004). Some species have been observed having the capability of retaining particles as small as 0.0005mm through their mucous sheet (Kott 1989; Flood and Fiala-Medioni 1979; 1981). 

Colonial ascidians are hermaphrodites which reproduce sexually and asexually. Sexual reproduction is ovoviviparous whereby eggs are internally fertilized and embryogenesis occurs in the atrium in a brood pouch (Satoh 1994). Asexual reproduction in colonial ascidians occurs through budding strobilation. Strobilation is the most common asexual reproductive method in the family Polyclinidae, however reproductive methods can vary between species within genera (Satoh 1994).

Aplidium have gonoducal openings directly into the communal cloaca system where sperm is released to the exterior or the cloacal cavity. This suggests that Aplidium are not self-fertilising. Fertilization in Aplidium, generally occurs in the atrial cavity or top section of the oviduct (Kott 1992).

Colonial species typically have larger eggs rich in vitellum and undergo lecithotrophic development (Satoh 1994; Ruppert et al. 2004; Núñez-Pons 2012). Colonial species have short ranges of larval dispersal due to limitation in egg abundance (Núñez-Pons 2012). The lecithotrophic larvae are brooded internally until it is released as a tadpole (Núñez-Pons 2012; Lambert 2005; Brusca and Brusca 2003). Reproduction typically coincides with food availability. Maximum phytoplankton productivity has been suggested to correlate with ascidian reproductive seasons with studies finding ascidian tadpoles able to delay settlement if cues do not occur (Lambert 2005; Svane and Young 1989; Havenhand 1991).

Ascidian larvae contain all key chordate traits; notochord, endostyle, dorsal tubular nerve chord, pharyngeal slits and a postanal tail (Ruppert et al. 2004).  Larval complexity is higher in colonial ascidians due to modes of adultation and caudalization (Figure 6) (Satoh 1994).  Precocious development allows a developed tail, cerebral vesicle, adhesive organs and partially developed but non-functional branchial and atrial apertures, branchial sac and gut loop (Kott 1992; Ruppert et al. 2004). In caudalization, where muscles cells are added to the tadpole tail, has been suggested to it increase the swimming ability of the larva (Satoh 1994), and therefore may allow greater dispersal range or predation avoidance. Egg size is roughly correlated to caudalization and therefore it has been suggested that mode of development is maternally controlled (Satoh 1994).

Aplidium larvae are generally small with a trunk measuring often less than 0.8mm. They differ from other genera in Polyclinidae due to the presence of ectodermal vesicles being directly separate from larval ectoderm around the anterior end of trunk. They have a cerebral vesicle which contains a well developed ocellus and otolith. Three anteriorly located adhesive papillae allow larvae to settle and attach itself to the substrate (Kott 1992). This then triggers radical metamorphosis where the tail is retracted and causes the loss of the dorsal tubular nerve chord, notochord and tail musculature (Ruppert et al. 2004)

Intra and inter-specific competition for space and food is a common occurrence with ascidians. Occupying spaces in order to out-compete neighbours and other species with similar niches is an example of interspecific and intraspecific competition between colonial ascidians (Lambert 2005; Lambert 2000; G. Lambert 2001).  Generally, size of an animal or a colony will increase competitive ability, fecundity, resistance to predators and environmental fluctuations (Jackson 1986).  Aplidium sp., however, are limited in their ability for spatial competition due to their limitations in size (Digestive and Excretory System) , therefore different defence mechanisms are required.

Colonial ascidians unpalatable and toxic tunic is usually the first line of protection assumed due to their ability to accumulate heavy metals and acids (Tunic) (Núñez-Pons 2012). Colonial ascidians consisting of genetically identical but independent zooids, generally cultivate more of these inducible bioactive secondary metabolites (Núñez-Pons 2012; Jackson 1986). They also typically maintain unfouled surfaces in order to inhibit bacteriofilms, infections, epibiosis and biofouling. These mechanisms are indicative of anti-fouling properties, often seen with animals that do not self-fertilise (Reproduction) (Núñez-Pons 2012; Wahl 1989).

These secondary metabolites are mostly nitrogen-bearing such as peptides, alkaloids and amino acid derived products. Types of metabolites change among species, and despite little knowledge of their ecological function, some of these are known to be predator deterrents and antifoulants. High concentrations of vanadium found in vanadoctyes (Hemal System) and sulfuric acid within bladder cells play a crucial role in predatory defence, particularly considering the lack of physical defence ability in ascidians (Stoecker 1980).  

The genus Aplidium is known for its diversity in alkaloids and variability of metabolites (Núñez-Pons et al. 2010; Zubía et al. 2005; Arabshahi and Schmitz 1988). Locality of these metabolites is generally expected to be within the tunic. However, and particularly with colonial ascidians, tunics often have little nutritional value and therefore it is suggested to be unappealing to predators.  Furthermore, taking into consideration the importance of larval defence on species survival, it is hypothesised that metabolites may be found within the gonads or visceral mass where protein and lipid content is higher (Núñez-Pons et al. 2010; McClintock et al. 1991; Young and Bingham 1987).

Tunic shrinkage allows relatively fast tunic regeneration and is an important defence mechanism for colonial ascidians (Hirose 1997 et al.). The selection of settlement in habitats such as rock crevices or beneath marine flora where predators have difficulty reaching prey, is another defence strategy that ascidians commonly use (Stocker and Bergquist 1987). Consequently, predators will often prey on larvae prior to settlement and tunic synthesis.  Once metamorphosis occurs, predation usually reduces significantly (Lambert 2005; Osman and Whilach 1998; Pisut and Pawlik 2002). 

Tunicates (or Ascidians) gain their name from a gelatinous outer integumentary tissue called a tunic (Figure 7). Tunics are multifunctional as they harbour multiple cell types, and in some species symbionts (Núñez-Pons 2012; Hirose 2009). Due to the tunics multifunctional capabilities, it is theorized to be one of the explanations for ascidians high adaptability and tolerance to extreme and changing environmental conditions (Hirose 2009). Species lacking tunic vessels have a higher abundance of tunic cells than those with tunic vessels. Phagocyte cells, found in all ascidians, function as scavengers by engulfing remnants of discarded tunic cells. Tunic bladder cells contain sulfuric acid filled vacuoles and are believed to be associated with chemical defence, however, they are absent in Polyclinidae species (Hirose 2001, 2009). Polyclinidae have tunic contracting cells called tunic net cells.  Tunic contractility varies between species and therefore the amount and distribution of these tunic net cells is suggested to differ among species (Hirose 2001). Tunic contraction is believed to be a process of regeneration whereby the tunic is shrunk around wounds, resulting in tunic cuticle closure. Regeneration then completes with the deposition of electron-rich fibers located on the exposed wound surface.  (Hirose et al. 1997). Certain Aplidium species have been found to have 7 types of tunic cells (Hirose 2009). Tunic cell types were not identified in the Aplidium sp. However, the absence of tunic vessels suggests the species may have a high number of tunic cells (Hirose et al. 1997).  

Aplidium ascidians are composed of three body regions; the thorax, abdomen and post-abdomen. Zooids are typically small and thread-like with their posterior abdomen being continuous from the abdomen (Kott 1992). The thorax is narrow with the endostyle situated along the ventral mid-line of the branchial sac (Figure 8) (Kott 1985). The 6-8 lobed branchial siphon is found anterior of the thorax while the small atrial siphon is situated mid-thorax with the small atrial lip arising from the body wall above it. The pharyngeal basket houses the pharynx and rows of stigmata. Aplidium typically have 6 or more rows of stigmata with transverse muscles fibers between them (Moreira de Rocha et al. 2012).  The anus and brooding embryos are found approximately halfway up the atrial cavity (Kott 1992). Longitudinal muscles extend from the thorax down to the posterior abdomen. The oesophagus is vertical and descends into a U-shaped vertical gut-loop. The stomach is vertical and barrelled-shaped with longitudinal folds in its wall (Figure 9) (Kott 1992). Gonads and heart are found in the posterior abdomen (Kott 1990). 

The structural body plan and central nervous system of ascidian larvae demonstrate a close relationship to vertebrates (Zaniolo et al. 2002; Burighel and Cloney 1997). However, during larval metamorphosis, reabsorption of the post-anal tail results in adult ascidians losing the notochord and are left with modified remnants of the neural tube developing into a modified central nervous system (CNS) and a neural gland. The CNS is composed of the cerebral ganglion, the visceral nerve, nerves of the body wall, the dorsal strand plexus and sensory structures of the siphons (Satoh 2014). The cerebral ganglion is located anteriorly between the two siphons with the neural gland posterior to it (Ruppert et al. 2004). The ganglion innervates the caudal viscera, siphons and body wall (Satoh 2014), and holds exclusively motor neurons (Zaniolo et al 2002).  

A peripheral nervous system branches posteriorly from the cerebral ganglion. It is associated with ciliated feeding mechanisms with relative autonomy from the CNS and has been observed to respond to external stimulus in deganglionated species (Zaniolo et al 2002). A ciliated duct and coiled funnel called the dorsal tubercle lie anterior to the neural gland (Figure 10). These create an incurrent flow of water entering the gland, crossing the walls and entering the branchial blood vessels (Ruppert et al. 2004) The neural gland together with the duct, dorsal tubercle function to mediate blood volume and has been theorised as being a precursor for the pituitary gland due to pituitary-like hormones found in neural gland tissue (Ruppert et al. 2004).

Despite the absence of sensory organs, sensory cells are found in abundance on internal and external surfaces of siphons, branchial tentacles and in the atrium cavity. It is suggested that these contribute to water current flow control within the pharynx (Ruppert et al. 2004). 

Ascidians are mostly ciliary-mucus filter feeders that feed on plankton that have been pumped though the branchial siphon into the pharynx (Lambert 2005; Ruppert et al. 2004). Food particles enter the branchial sac, crossing ciliated gill slits and are strained through a mucous sheet secreted by the endostyle (Ali and Tamilselvi 2016; Kott 1985). The secreted food net is rolled into a thread-like cord and is slowly transported from the pharynx to the dorsal lamina and then to the oesophagus (Ruppert et al. 2004). Food is then digested extracellularly in the secretory cell lined stomach. Absorption is suggested to occur in the intestine situated on the ascending arm of the gut loop. The intestine then forms faeces that are discharged through the anus and into the atrium to be expelled by the atrial siphon (Ruppert et al. 2004).

Ascidians have well developed hemal systems with a high diversity in hemocytes. These include, vanadocytes which serve two functions; tunic synthesis through the polymerization of tunicin filaments, and the possible use of its toxicity for antifouling and predation deterrence (Ruppert et al. 2004). The hemal system includes a short curved heart located in the posterior abdomen of Aplidiums, vessels and small sinuses for blood flow. Periodic reversal blood flow found only elsewhere in certain arthropods is suggested to be the reason for organs and systems to be arranged along the blood circuit rather than parallel to it (Ruppert et al. 2004).