Feature Story                                                        November 2011 

All in the Family: The Tunicates
By Christina Simkanin and Monaca Noble

The Marine Invasions Lab at the Smithsonian Environmental Research Center has been studying the distribution of native and non-native tunicate species in the temperate and subtropical waters of North America for several years. As part of this research, the National Exotic Marine and Estuarine Species Information System (NEMESIS) was created. The NEMESIS database includes detailed information on approximately 500 different non-native species that have invaded marine and estuarine environments in the continental U.S, Alaska and Hawaii. In December 2011, the Marine Invasions Lab will be launching the Tunicate section of the NEMESIS website; making information on exotic tunicates accessible to the public at http://invasions.si.edu/nemesis/.

A larval Phallusia nigra, an inky-black solitary tunicate from the Red Sea and western Indian Ocean that has been introduced into several locations including Hawaii and the Gulf of Mexico. Photo by Christina Simkanin 

Tunicates, also known as ascidians or sea-squirts, are marine invertebrates belonging to the phylum Chordata, sub-phylum Urochordata. Chordata is the same phylum that humans are in. Even though tunicates look like sacks of water or sheets of colorful jelly, they are the invertebrate group most closely related to us. During their tadpole-like larval phase they possess critical chordate characteristics such as a notochord, a dorsal tubular nerve cord, and pharyngeal slits. Although not all of these traits are retained in the adult stage, in their larval stage they look very much like other developing chordate embryos, including humans.   

Tunicates are found throughout the world’s oceans, from the tropics to the poles and from shallow water to the deep sea. Although most tunicates are found on hard benthic substrates, some have the ability to live within subtidal soft sediments, and others (salps and larvaceans) drift in the plankton. Most of the introduced tunicates are permanently attached (sessile) filter feeders that grow in either solitary or colonial forms. Solitary tunicates are self-contained as one individual, and have typically been called sea grapes or sea-squirts. Colonial tunicates like Rock-Vomit (Didemnum vexillum) grow in sheets and are a collective of individuals, called zooids that share a protective nonliving cellulose layer called a tunic. Star Trek’s Borg might well have been inspired by colonial tunicates, as the Borg collective and colonial tunicates have a lot in common. Each zooid is an individual, but they are such an intricate part of the colony that they can’t be separated from it. In fact, the zooids are connected to one another by a network of blood vessels and work together to make the colony act like a single animal. In some species of colonial tunicates each zooid can perform both feeding and reproductive tasks, but in other species the zooids are specialized, some are feeding zooids, others are reproductive. But unlike the Borg colonial tunicates don’t assimilate other species, although they can overgrow and smoother them.

The solitary tunicate Halocynthia aurantium, is native to the Western Coast of North America from California to Alaska. Photo by Christina Simkanin The invasive colonial tunicate Didemnum vexillum, populary know as Rock-vomit, covering subtidal boulders in Sitka, Alaska. Photo by Ian C. Davidson

Each tunicate feeds by sucking water, phytoplankton and other nutrients into the body cavity through an oral siphon, filtering it, and then expelling water and waste through an exhalent or atrial siphon. The zooids of colonial tunicates each have an incurrent siphon to collect food, but the colony shares a common exhalent siphon. All tunicates are hermaphroditic, meaning individuals produce both eggs and sperm, but many species lack the ability to self-fertilize. Once fertilized, an egg hatches into a larva with a vertebrate-like notochord. The larva stay in the water colony for only a brief time before finding a suitable place to settle and begin its life as a sessile invertebrate. Tunicates are diverse and abundant members of benthic marine invertebrate communities and studies on their biology and ecology are providing critical insights into evolution and development, natural products chemistry, physiology, biogeography, the impacts of climate change and species invasions.

There are approximately 3000 described species of tunicates worldwide and a recent assessment suggests that around 64 of these species have been introduced to locations beyond their native range (Shenkar and Swalla, 2011). Tunicates are most often transported though hull fouling on commercial and recreational boats, as well as accidental transfers on oyster shipments. While some may be recent world hitchhiking travelers, many of these invading species have been moved around for centuries. The impacts of exotic tunicates vary across time, place and species, but some of the most detrimental invaders are those that have had negative effects on shellfish aquaculture. Exotic tunicates can overgrow shellfish stock and foul infrastructure causing large scale shellfish mortality reducing shellfish growth and dramatically increasing processing costs.

The Tunicate database, when released in December 2011, will contain geographic and ecological information for 28 species introduced to North America, many of which have also been introduced to multiple locations around the globe. One example is the colonial tunicate Rock-vomit (Didemnum vexillum). This tunicate is native to the Northwest Pacific (Asia) and has been introduced to many locations around the world including both coasts of North America, Europe and New Zealand. It grows quickly and overgrows shellfish and other marine invertebrates and algae, causing both economic and ecological impacts in areas where it has been introduced. The information provided in NEMESIS for Rock-vomit and other tunicate invaders will greatly increase our knowledge of marine invasions in North America by providing up to date distribution data and ecological information for researchers, managers, and the public.

Reference: Shenkar, N., Swalla, B.J. 2011. Global Diversity of Ascidiacea. PLoS ONE 6(6): e20657. doi:10.1371/journal.pone.0020657