Nonindigenous Ascidians in Tropical Waters

Abstract

Ascidians (invertebrate chordates) are abundant in many ports around the world. Most of them are nonindigenous species that tolerate wide fluctuations in temperature, salinity, and even pollution. These sessile suspension feeders have a rapid growth rate, usually a short life span of a few months, reach sexual maturity when only a few weeks old, and produce large numbers of shortlived nonfeeding planktonic larvae. They thrive on marina floats, pilings, buoys, and boat bottoms in protected harbors where there is reduced wave action and enhanced nutrients from anthropogenic activities. Nonindigenous ascidians frequently overgrow oysters and mussels, which are often cultivated in or near busy harbors. Adult ascidians on ship or barge hulls may survive transport over thousands of kilometers to harbors with conditions similar to those they left; occasionally live larvae have also been recovered from ships' ballast water. U.S. Navy dry dock movements between major Pacific ports have transported large masses of fouling nonindigenous taxa, including ascidians. Transfer between culture sites of oysters, mussels, and associated lines and nets may provide an additional mode of transport. Once nonindigenous ascidians become established, they provide large local sources of larvae for further possible invasions into additional harbors and nearby natural marine communities. Invasive species include both solitary and colonial forms, with a preponderance of large solitary species that thrive in highly disturbed habitats. In Guam, for example, most nonindigenous ascidians are confined to harbor structures and have not as yet significantly colonized natural reefs. In contrast, healthy natural benthic regions both inside and outside the harbors of Guam are usually stable coral reef communities containing a high diversity, but very low biomass, of native colonial ascidian species. However, in several areas of the Caribbean a native colonial didemnid has recently begun overgrowing coral reefs. In the Gulf of Mexico a nonindigenous didemnid now covers many offshore oil rigs and may become a threat to neighboring natural reefs. Additional data on nonindigenous ascidians in Australia, Palau, Hawai'i, and the Mediterranean are included. Although serious invasion of coral reefs has not yet been reported, more studies and regular monitoring are needed.