Sea squirts, also called tunicates, are marine invertebrates that are abundant worldwide. Here in North America, they are widely distributed along the Pacific and Atlantic coasts. As adults, they are immobile or sessile (permanently attached to the substrate) and they capture their food by filter feeding. They can be found living alone (solitary) or in colonies. Sea squirts tend to be cylindrical or globular in shape, much like water balloons. The name "sea squirt" comes from the feeding and excretion process where the organism ‘squirts’ water out of its body. The name "tunicate" comes from their outer covering which resembles a ‘tunic’ and may be either gelatinous or leathery in texture. Despite their seemingly simplistic appearance as adults, the larval stage of sea squirts show characteristics of an idealized chordate, and there is some evidence to suggest that they may be the closest living relative of vertebrates.
Habitat Sea squirts thrive in sheltered waters with good flow and attach themselves to hard substrates. They may reside on rocks, vegetation, or other benthic organisms like oysters or mussels. They are nuisance species when they foul ships, ropes and docks. Because they are filter feeders and don't move around once they have settled, the only thing these invaders really need is a parking space!
Predators Small, juvenile sea squirts are eaten by some marine snails such as Hydrobia species and Littorina species. Some fish such as sticklebacks (Gasterosteus species) and cunner (Tautogolabrus adspersus) also eat juvenile sea squirts. Jellyfish are known to feed on larvae and eggs of sea squirt species when they are suspended in the water column. Invasion History There are five alien species of sea squirts which are of concern here in Nova Scotia: the golden star, sea vase, violet and club tunicates, as well as Didemnum species. Of these, at least two of them (golden star and sea vase) have been in Nova Scotia since the 1900s but have recently taken on characteristics of invasive species. The alien (non-native) club and violet tunicates have been collected recently in Nova Scotia (please click here for more information). Didemnum species have not been found within Nova Scotia or Cape Breton, but scientists and resource managers are keeping a close watch on a population that has recently been found on the US side of George's Bank. For additional information about each species of sea squirt, please choose from the menu on the right.
Sea squirts spread their ranges naturally, however, like most invasive organisms their spread can be enhanced through human activities. Sea squirts can be transported via ballast water in ocean-going vessels which spend a long time away from their home ports, such as navy ships. In addition, aquaculture activities can promote the regional spread of sea squirts because shellfish (e.g. mussels, oysters) may be transported from one area to another with invasive sea squirts on their shells. However, once an invasive species of sea squirt is introduced it is virtually impossible to eradicate it form its newly colonized waters. Surprisingly, since 1970, a new invading sea squirt species has been reported every five years in the Atlantic and/or Gulf waters.
According to scientists at the Department of Fisheries and Oceans, one significant challenge with managing these aquatic invaders is knowing which sea squirts are native and which species have been introduced. Local information for sea squirts is lacking along with qualified individuals who can positively identify sea squirt species.
Potential Impacts As fouling organisms, sea squirts grow over a variety of surfaces, altering marine habitats and threatening to interfere with fishing, aquaculture, and other coastal and offshore activities. When sea squirts aggressively colonize new areas, they reduce the habitat available for other organisms. Spatial crowding is not the only concern, sea squirts in large numbers can diminish food supplies for many other organisms, especially other filter feeders. Sea squirts have had very significant effects on the Prince Edward Island aquaculture industry. In addition to the negative biological effects on the cultured species (e.g. mussels), dealing with sea squirt colonies on aquaculture equipment (e.g. mussel socks, boat hulls and ropes) is a very time-consuming and costly process. In general, sea squirts lack natural predators, so they are free to invade new areas with ease.
Additional websites about Sea Squirts in the Maritime Provinces and around the world
- Invasion of the Sea Squirts - from Department of Fisheries and Oceans Canada
- Tunicates Research 'A case study of tunicates in Maritime Canada' - from Department of Fisheries and Oceans Canada
- Identification information for tunicates - Salem Sound Coastwatch
- Invasive Tunicates in Washington State - Pictures, Information and Links
- BC shellfish growers association - Aquatic Invasive Species - Interpretive Guide (general sea squirts, clubbed, violet, golden star, Didemnum sp.
- Washington ascidians homepage - Ascidian basic biology and other useful information
- Mussel harvesting increasingly risky, say workers - CBC news
- Sea squirts being used in brain implant studies! - BBC Science news