This species page aims to describe the ascidian species Cystodytes dellachiajei, with special attention to the physiology of the species. C. dellachiajei is a colonial ascidian first identified by Della Valle in 1877 (Kott, 2010). The species boasts a colourful purple tunic with white zooids. A colonial sessile species, Cystodytes has some interesting adaptations that aids their survival and allows them to compete for limited space. Existing on tropical coasts, Cystodytes dellachiajei has been mainly recorded within Australian waters.

With a purple test in which irregularly distributed small, white globe-like zooids are all completely embedded, this colonial ascidian has a distinctive and objectively beautiful physical appearance.

Specifically to the species, Cystodytes dellachiajei is described in A Coral Reef Handbook as a species with “purple, pink, or whitish colonies”, featuring a “white capsule around zooids” (Mather & Bennett, 1984). The measured length of each zooid in the specimens collected from Heron Island were less that 2mm.

Initial observations of the specimen included around two dozen zooids within a colony. 5 weeks later, only three zooids survived after storage in an indoor aquarium. Despite the reduction in zooid numbers and presumably health, the Cystodytes retained their purple colour and their form.

As ascidians are sessile, they rely on surfaces such as coral reef crests, rocky substrates and sometimes soft sediments to settle as larval forms and to thereafter develop into adults (Shenkar & Swalla, 2011). Most are found in shallow tropical waters; only 100 of 2000 discovered species have been found below a depth of 200 meters (Ruppert et al, 2004). The sample reef crest uncovered from Heron Island where the studied specimen was discovered is depicted in Figure 1.

Ascidians universally have a biphasic life cycle, with a plantonic larval and sessile adult stage (BIOL3211 Lecture material). No coelom is found with the Ascidiaceans (BIOL3211 Lecture material). Colonial species “brood small numbers of large eggs” (BIOL3211 Lecture material). Adults are hermaphroditic (BIOL3211 Lecture material).

Larvae

Larvae are viviparous and have developed sense organs equipped for settlement (Mather & Bennett, 1984).

Reproduction

As each zooid within a colony reproduces by cloning, growth of parent zooids may be limited, which may explain the variance in size between colonial and solitary ascidians (Mather & Bennett, 1984).

The tunic of the ascidian is a thick protective layer that also functions in structural support for the animal. In C. dellachiajei  this can be referred to as the test, due to its colonial nature. The test is utilised by the animal as it attaches to the substratum, sometimes featuring root-like stolons that were observed on the study specimen (Ruppert et al, 2004). The zooids of C. dellachiajei are almost completely embedded in the test, as is common with all members of Family Polycitoridae (Mather & Bennett, 1984). The structural matrix within the test is seen under a microscope after being sectioned and stained in Figure Two.

Watch the following video to witness the tunic being removed during a dissection of a fixed Cystodytes dellachiajei specimen. (Link: https://youtu.be/IkySRZgXZsg)

The buccal (inlet) and atrial (outlet) siphons are visible in a dissected individual zooid with the tunic removed in Figure Three. Utilised for ingestion and and excretion of excess water and waste products, a single ganglion exists in between these two siphons (BIOL3211 Lecture material). Difficult to make out, Figure Four shows a sectioned and stained zooid (as slide 2 of 25 slides).

Although the zooids are unsegmented, the internal structures of animal can be loosely classed into three regions: Thoracic, Abdominal, and Postabdominal regions (BIOL3211 Lecture material). C. dellachiajei specifically has a short abdomen (Mather & Bennett, 1984).

Thoracic Region

Ingestion

The pharynx, located on the buccal siphon (Ruppert et al, 2004), is utilised to draft water currents for the animal to obtain nutrients through suspension feeding (BIOL3211 Lecture material). Beating cilia aid this movement (Mather & Bennett, 1984).

Abdominal Region

Digestion

The gut, gonads and heart of Suborder Plousobranchia are posterior to the thorax (Mather & Bennett, 1984). The digestive tract of the tunicate is U-shaped (Ruppert et al, 2004).

Excretion

Waste is largely removed from the body through the apical siphon (Ruppert et al, 2004). Storage excretion is also a strategy for uric acid and other urates, which are expelled after animal death (Ruppert et al, 2004). There are no excretory organs in ascidians (BIOL3211 Lecture material), but Suborder Aplosobranch (which Cystodytes belongs) does harbour the excretory structure epicardium (Ruppert et al, 2004). The epicardium lies behind the heart and extends to the pharyngeal cavity (Ruppert et al, 2004).

Postabdominal Region

In other species of solitary ascidians not associated to Suborder Aplousobranchia, the postabominal and even abdominal region may be lacking (Ruppert et al, 2004).

In this colonial species, however, the post abdominal region is place for the heart and reproductive organs of the animal.

Circulatory System

The ascidian heart is located at the base of the U-shaped digestive tract (Ruppert et al, 2004). The developed ascidian hemal system may contain hemocytes (Ruppert et al, 2004).

Nervous System

Adult Cystodytes lack a hollow nerve chord characteristic of Chordata (Kott, 1990). This character is exclusively present in the larval stage.

The sessile adult stage of the Cystodytes dellachiajei rely purely on the dispersal of the larval stage to determine biogeographic distribution. Limited to shallow tropical waters and a short larval phase, population dispersal from continent to continent poses a significant challenge. Figure One indicates current records of Cystodytes dellachiajei localities. With a concentration of the species in Australian waters, the appearances in eastern American and northern African coasts indicate in interesting series of events that could be related to the first evolution of the species. Perhaps a lack of research on the species has reduced the number and nature of localities the species has been recorded.

Species Identification

Phylum Chordata

Sub-phylum Tunicata

Class Ascidiacea

Suborder Aplousobranchia

Family Polycitoridae

Species Cystodytes dellachiajei


Identification Metrics

Tunicates are placed within the Chordates for their pharynx, cells in the larval tail that are reminiscent of notochord cells (Mather & Bennett, 1984).

The name Tunicate derives from the presence of the tunic (or test as it is so called for Cystodytes).

Evolution

In the evolution of Ascidiacea, the colonial body form has arisen independently multiple times (Ruppert et al, 2004). Kott (2008) suggests gene flow as the key mechanism for colonial ascidian speciation, in opposition to earlier isolation hypotheses. Although colonial ascideans may have integrated zooids, the species C. dellachiajei has independent zooids.

Climate Change

As a sessile, tropical, shallow water species, Cystodytes would be greatly impacted by climate change and the health of a coral reef environment. Although some 200m deep water ascidians can settle on soft sediment, a majority of species and certainly C. dellachiajei have evolved to settle on solid substrates closer to the surface.

Competition

As sessile organisms, Cystodytes are significantly threatened by other ascidians and sessile filter and suspension feeders.

Parasites

During a dissection of a specimen, a worm-like parasite was discovered inside the tunic of a C. dellachiajei specimen (as shown in Figure 6). Parasites may be a threat to the health of a Cystodytes colony, although it may not result in colony death.

Predators

Depending on where the ascidian settles, predation may be an issue for Cystodytes (Mather & Bennett, 1984).

Conservation

Conservation efforts should focus on coral reef habitat retention and health.

Brown F, and Swalla BJ. (2012) Evolution and development of budding by stem cells: Ascidian coloniality as a case study’, Developmental Biology, vol. 369, no. 2, pp. 151-162.

BIOL3211 Lecture Slides.

Global Biodiversity Information Centre, http://www.gbif.org/species/4355903. Accessed May 2016.

Kott, P. (1990) The Australian Ascidiacea. Part 1, Aplousobranchia, Memoirs of the Australian Museum, viewed 31 May 2016 http://biodiversitylibrary.org/page/40604346#page/175/mode/1up

Kott, P. (2008) Biographic Implications of Ascidiacea (Tunicata) from the Wessel Islands (Arafura Sea), Records of the Museums and Art Galleries f the Northern Territory, 24.

Kott, P. (2010) Ascidiaea (Tunicata) from deep waters of the continental shelf of Western Australia, Journal of Natural History.

Mather P, and Bennett I. (1984) A Coral Reef Handbook: A Guide to the Fauna, Flora and Geology of Heron Island and Adjacent Reegs and Cays (2nd. ed), The Australian Coral Reef Society.

Ruppert E, Fox R, and Barnes R. (2004) Invertebrate Zoology: A Functional Evolutionary Approach (7th. ed), Thompson.

Shenkar N, and Swalla BJ. (2011) Global Diversity of Ascidiacea. PLoS ONE 6 (6).