Diagram

Morphologies, Life History,
and Taxonomy of Tunicates

Home

Observations and Experiment:

Morphologies and Life History

Native vs. Invasive

Tunicates in Immunology

Organisms in the Rocky Intertidal Zone

Photo Galleries:

Pictures of compound ascidians:

-Botrylloides growing around Chondrus crispus, Nahant

-Botryllus growing on rock in Nahant

Pictures of Colonial ascidians, Bermuda:

As with most animals and plants, tunicates come in all shapes and sizes. Benthic ascidians are usually sessile in their adult lives and prefer substrates on rocky shores or in subtidal habitats. Pelagic tunicates, highly efficient filter-feeders, are dispersed throughout plankton, living as motile or floating organisms (Pearse V. et al. 1987). Benthic tunicates make up the majority of the subphylum urochordata and are what struck my interest upon visiting the rocky intertidal zone of Nahant, MA. Below are interactive diagrams of the adult and larval stage of these ultimately sessile ascidians.

Move the mouse over the words to see explanations...

In addition to the organs listed above, tunicates have several other integral entities. The squirting that earns tunicates their nickname is regulated by the nervous system, a large ganglion being the primary component. The nervous system is able to slow or halt feeding due to its control of the beating pharyngeal cilia (Pearse, V. et al. 1987). In addition to the nervous system, tunicates also have a heart capable of reversing the direction of blood flow. This unique characteristic is found in all tunicates, but its importance is not understood. The blood is carried throughout the organism, supplying the pharynx and tunic. Different cells in the blood aid in distribution of nutrients, storage of excretory products, and production of tunic (Pearse, V. et al. 1987).

Certain tunicates produce self-defense mechanisms capable of protecting them from predators such as crabs and fish. Some species contain high concentrations of toxic compounds like sulfuric acid and vanadium compounds in their tunic cells (Pearse, V. et al. 1987). Other tunicates are decorated with spiny projections that help fend off large snails (Pearse, V. et al. 1987).

This is the opening of the tunicate where water enters.

Water is forcefully expelled out of both the oral opening (incurrent siphon) and atrial opening (excurrent siphon). This clears away any gathered debris.

Covers the pharyngeal pouch and opens to the outside, allowing the water that leaves the pharyngeal slits to exit the tunicate.

A sieve-like sac that has cilia lining it, creating a current. Water flows into the mouth and out through the pharyngeal slits. Mucoid sheets that lay against the pharyngeal walls trap food particles, thus allowing the tunicate to filter-feed.

This covering surrounds and protects the inside components of the body. However, it acts as a connective tissue and contains many different cells and sections of the circulatory system.

Water is forcefully expelled out of both the oral opening (incurrent siphon) and atrial opening (excurrent siphon). This clears away any gathered debris.

Digested food exits the gut via the anus, opening into the atrium.

Food is flushed from the mucous sheets into the esophagus and stomach, where food is digested extracellularly.

This key chordate characteristic runs parallel below the nerve cord. The notochord facilitates swimming.

This is a well-developed entity that is anteriorly enlarged. It is one characteristic that places tunicates in the phylum Chordata.

The muscular tail of a tunicate is another chordate characteristic. When it undulates, the tadpole is propelled through the water.

Tunicates may be solitary, compound, or colonial, though little is different in the morphology of the zooid itself. Individual compound and colonial tunicates are smaller than solitary tunicates, and are connected primarily by blood vessels (Pearse V. et al. 1987). When individual zooids are embedded in a common tunic, or test, and share a single atrial opening, the colony is considered to be compound (“Sea squirt”, 2005). This is the highest degree of organization among colonial tunicates (Pearse V. et al. 1987). B. Schlosseri appears as a star or flower, the zooids in circular clusters. B. violaceus zooids are arranged in elongated clusters, often in double rows. These two species are firm and maintain their system structure. However, it is also possible for individuals to be loosely bound by the tunic (see pictures from Bermuda on left).

B. schlosseri and B. violaceus are examples of these colonial organizations that allow for extraordinary growth and reproduction. Individuals in a colony may live for only a couple days as they mature and release their young, then die, all in near synchrony (Pearse V. et al. 1987). It has been recorded that B. schlosseri may live for approximately 3 months when larvae settle in the spring, and 8 months when larvae settle in the fall (Cohen, 2005).

B. schlosseri and B. violaceus are founded by sexual reproduction while the colonies grow through asexual budding. Tunicates, with rare exceptions, are hermaphroditic, producing both the egg and sperm. Once a zygote is formed, a tadpole develops. This tadpole larva can swim for as little as a few minutes or up to a few days, using sensory guides such as chemical and tactile receptors and a light-sensing eye to assist in settlement (Pearse V. et al. 1987). Once the tadpole has attached itself to a firm surface by its anterior end, it begins rapid metamorphosis, loosing its chordate characteristics (dorsal nerve cord, tail, and notochord) and developing other organs such as its heart, digestive system, atrium, and nerve ganglion (Cohen, 2005; Pearse V. et al. 1987). Following metamorphosis, the oozooids produce buds (blastogenesis) simultaneously, just as the organisms died simultaneously (Grosberg, 1988). Blastogenesis leads to colony mats that can encrust and overgrow other organisms. These colonies are able to grow until bud production exceeds adult zooids (Yund and Stires, 2002).

Taxonomy of B. schlosseri:
Phylum: Chordata
Subphylum: Tunicata
Class: Ascidacea
Order: Pleurogona
Suborder: Stolidobranchia
Family: Styelidae
Genus: Botryllus
Species: schlosseri

Taxonomy of B. violaceus:
Phylum: Chordata
Subphylum: Tunicata
Class: Ascidiacea
Order: Pleurogona
Suborder: Stolidobranchia
Family: Styelidae
Genus: Botrylloides
Species: violaceus