San Francisco Bay is the most invaded aquatic region on Earth, with more than half its fish and most of its bottom-dwelling organisms representing non-native species, according to a new report released by the Nature Conservancy.

Asian clams, Chinese mitten crabs, Amur River clams, New Zealand carnivorous sea slugs, Black Sea jellyfish and Japanese gobies are just a few of the exotics that have ended up in San Francisco Bay, usually transported in ships' ballast water.

The study looked at the globe, with researchers combing through more than 350 databases and other sources for information. The study showed 85 invasive species in the bay, with 56 considered "harmful" - meaning they have an ecological impact, spread easily and are difficult to manage. Overall, 84 percent of the world's coasts are being affected by foreign aquatic species.

"The scale of this problem is vast," said Jennifer Molnar, conservation scientist at the Nature Conservancy and lead author of the study. "Every day, thousands of vessels cross our oceans with invasive species hitchhiking on their hulls. Because of this, as many as ten thousand species are estimated to be in transit at any one time."

The problem is acute in San Francisco Bay because it is a vibrant economic hub that has several ports.

"You not only have the Port of Oakland, but also San Francisco, Sacramento and even Stockton," said Sausalito resident Steve Moore, who has served on the federal Aquatic Nuisance Species Task Force. "There is a lot of ballast water from around the globe that ends up here."

Ballast water is sea water taken on by ships to balance a boat, allowing it to float properly.

"When a ship gets to, say, the Port of Oakland, it may only be half filled and its carrying a lot of balance water," Moore said. "Once it is loaded up at the port with goods, less ballast water is needed and it is flushed into the bay."

Inside that ballast water are species from the original port, from faraway places such as the Black Sea. Such invaders as the green crab - a voracious eater of shellfish - hitch rides in the ballast water as larvae or plankton. Once released, they begin their life cycles, and, with no natural predators, their populations balloon.

"We tend to see more invasives in areas like San Pablo Bay, where there is a mix for saltwater and freshwater coming from the Delta," Moore said. "When you get north of the Richmond-San Rafael Bridge you begin to see more invasives."

For example, in San Pablo Bay, the Amur River clam - native to Russia - has taken root. The clam gorges on phytoplankton to the detriment of other species.

Phytoplankton boost the aquatic environment and are responsible for up to 60 percent of all photosynthetically-produced organic matter on Earth, but the Amur River clam hog much of the material.

"You not only have the invasives, but then they begin to affect native species," Moore said.

The clams also absorb large amounts of selenium, a trace mineral that can be highly toxic and carcinogenic to humans and wildlife. The clams are eaten by ducks and other wildlife and there is concern they could trigger health or reproductive issues in the birds, scientists say.

"Everyone in the world depends on healthy oceans and coasts for survival," said Stephanie Meeks, chief executive of the Nature Conservancy. "Invasive species are severely impacting native plants and animals, and are causing significant economic damage at the same time. By understanding the scale and scope of these invaders, we are better equipped to stop them."

Ship hulls can be inspected for invasives, but ballast water is more difficult, Moore said. There have been calls for filtering or disinfecting water to kill larvae.

"Once alien species become established in marine habitats, it can be nearly impossible to remove them," Molnar said. "The best way to address these invaders is to prevent their arrival or introduction in the first place."

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