- Nonindigenous Ascidians in Tropical Waters1
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 short-lived 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.
Introductions of nonindigenous ascidians into harbors in both tropical and temperate waters are now commonplace, with the tempo of introductions increasing (for reviews see Monniot et al. 1991, Lambert and Lambert 1998, Coles et al. 1999). Ascidians are invertebrate members of the phylum Chordata, with a sessile suspension-feeding adult and a short-lived nonfeeding motile larva. Colonial species brood their young and release mature larvae into the water, but most others spawn eggs and sperm that develop in about 24 hr into swimming larvae with an average larval duration of 12-24 hr. Because the larval stage is so short (perhaps only a few minutes between release and settlement in colonial forms), their primary mode of anthropogenic transport is probably boat hulls or other [End Page 291] fouled surfaces. Movement between culture sites of fouled nets and shells used in mari-culture, for example, may inadvertently transport nonindigenous ascidians (unpubl. data), as were burrowing polychaetes on oysters imported from Maine into Hawai'i in 1990 (Bailey-Brock 2000). Although ascidian larvae are known to survive transport in ballast water (Carlton and Geller 1993), this may not be their major mode of human-influenced transport. Sampling of U.S. Navy dry dock hulls both before and after their movement across thousands of kilometers in the Pacific (including Honolulu to Guam in July 1999 and San Diego to Honolulu in December 1999) has shown that many adult ascidians arrive alive (G. Paulay and S. Godwin, pers. comm.). When a bridge collapsed in Palau in 1996, a floating pontoon bridge was towed there from China the following year; it arrived carrying many live fouling organisms including ascidians (P. Colin, pers. comm.). By whatever means they achieve transport, the same nonindigenous ascidians are often recorded from widely separated localities (Boyd et al. 1990, Monniot et al. 1991, Monniot and Monniot 1994, 1996, 2001, Lambert and Lambert 1998).
Many nonindigenous ascidians reach sexual maturity in just a few weeks and have long breeding seasons (Lambert and Lambert 1998). They tolerate wide fluctuations in temperature and...