The ascidean tunicate Styela clava is a fast-growing, subtidal fouling species ("fouling" in this context is a somewhat anthropocentric term used to describe marine organisms that settle and develop on submerged structures, including structures on which humans would rather they didn't settle, such as ship hulls and underwater pipes and shellfish being grown in aquaculture). It is a prolific breeder and a highly efficient suspension feeder. It is native to the northwest Pacific from the Sea of Okhotsk through southern Siberia, Japan, Korea, and northern China south to Shanghai. However, it was discovered in western North America, on the coast of California, in 1932 and is now found from Baja California (Mexico) north to Washington State (U.S.A.) and into southern British Columbia (Canada). It was found in the Atlantic, off southern England, in 1953. In the northeast Atlantic, it is now found from northern Denmark to Portugal. It is present in the northeastern United States and on both the Atlantic and Pacific coasts of Canada, although it has not been detected in Arctic waters. It is widespread and well established in New Zealand. The geographic spread of this invasive species in both the northern and southern hemispheres is reviewed by Clarke and Therriault (2007) [available here], who also review its general biology and economic significance to humans. (Clarke and Therriault 2007 and references therein; Dupont et al. 2009; Goldstien et al. 2010)
The population structure of introduced populations as inferred from genetic studies, as well as the speed with which it has spread, suggest a major role for human-mediated dispersal in the spread of this species, e.g., by adults attached to the hulls (or other areas with less water flow) of commercial ships or pleasure boats or juveniles inadvertently transferred with oysters in the course of aquaculture operations (Clarke and Therriault 2007; Dupont et al. 2009; Goldstien et al. 2010).
Styla clava are oviparous (egg-laying) hermaphrodites. A mature adult is around 70 to 200 mm in length and produces about 5000 eggs, which hatch after 12 to 15 hours. The larvae, which are around 0.85 mm long, are lecithotrophic (i.e., they do not feed until after they settle to the bottom and become non-planktonic juveniles). Larvae are able to swim just a few millimetres in short bursts of activity. After approximately 12 hours, they settle on hard substrata to develop into sessile juveniles. Since S. clava eggs hatch 12 to 15 hours after spawning/fertilization, the total pelagic dispersal time for the developing egg/embryo and larva is on the order of 24 hours. (Davis and Davis 2007)
