"Aliens From Beyond." "Exotic Invaders."

It is easy to get a bit carried away when discussing the introduction of non-native species into an ecosystem. Yet, in reality, these sometimes tongue-in-cheek descriptions are not necessarily all in fun. For recently, throughout the world, non-native species are wreaking terrifying havoc on the environment.

The zebra mussel has become notorious for the disaster it has brought to the Great Lakes. Sadly, though, its introduction is not an isolated case.

California has had its (un)fair share of these troubles. And underwater California has also suffered from the assault of "aliens from the beyond". The San Francisco Bay alone has been called the "most invaded aquatic ecosystem in the world" with a least 230 non-native species having been found there - so far - where they comprise an astonishing 90% by weight of all the biota.

Some of the worst offenders have been a couple of crabs. Now, many people off-hand would consider crabs to be rather desirable. After all, they are typically good eating and easy to catch, and these two species are no exception in that regard. But these particular crabs are capable of doing immeasurably more harm than good.

The Green Crab (Carcinus maenas) was first discovered in the San Francisco Bay in 1989. Native to European water, it had already made horrific inroads into the East Coast of North America in the early nineteenth century, where it is said to have ravaged and destroyed that area's soft shell clam industry. In some parts of that range, it has now become the dominant crab.

From the Bay area, the Green Crab has already spread like underwater wildfire. Normally, a non-native invasive animal will expand its range at a rate of a little over a mile a year. The Green Crab, however, over ran 700 miles of the West Coast in only 6 years! Furthermore, wherever it goes it eats just about any shellfish it can get its pincers on. And considering that it is bigger, faster and meaner than many of California's native marine invertebrates, that translates into a lot! For example, just one adult Green Crab can consume 40 clams in a single day. No wonder the Green Crab has made the Nature Conservancy's "Dirty Dozen - America's Least Wanted" list.

California's other recent crab invader is not any more wanted, though. The Chinese Mitten Crab (Eriocheir sinesis) first appeared in the San Francisco Bay in 1992, and like the Green Crab, it brought with it an already notorious reputation. Named because of its woolen mitten looking claws, the Chinese Mitten Crab reproduces - prolifically - in the sea and in brackish areas, but then, the juveniles migrate into freshwater. If there happens to be any obstacle in its patch, it will simply get out of the water and walk around, so it has often been found hundreds of miles upstream. And as with the unwanted alien from Europe, this more recent arrival from the Orient has the all too real potential of destroying valuable fishing industries - and whole ecosystems.

Both the Green Crab and the Chinese Mitten Crab - and scores of other invasive marine species - probably snuck into California in ship ballast water. Or maybe in a shipment of live bait. Either way, no problem. Yet, now that they are here, trying to get rid of them - or at least control their further spread - has become a major problem. Along the East Coast, attempts at fencing, trapping, and poisoning the Green Crab didn't work very well.

The introduction of a predatory barnacle (Sacculina carcini) is being considered which potentially would attack the Green Crab and render it sterile. But any introduction of a non-native predator is highly risky business.

Oh, sometimes it works extremely well. It was in California where the now almost legendary introduction of a heroic predator occurred. In the late 1800s, when the Cottony-cushion Scale (Icerya purchai) inadvertently arrived, the State's valuable citrus industry was on the verge of disaster. But, in 1888, a ladybug, Vedalia (Rodolia carinalis) was purposely brought in from Australia and the citrus industry was saved!

Other times, the introduction of a predator not only doesn't work, but it disastrously backfires. When the Giant African Snail (Achatina fulica) got out of hand in Polynesia, the predatory snail Euglandina rosea was introduced. Unfortunately, that predator decided that native snails were more appetizing than the Giant African, and as a result, it caused the extinction of many beautiful endemic snails. So, obviously, as bad as the Green Crab is, this type of introduction isn't what California wants.

As far as the Chinese Mitten Crab, talk is of control and management. Eradication is acknowledged as not being feasible. They aren't bad eating, in fact they are considered to be delicacies in some parts of their natural range. There is a daily limit of 35 crabs per person when taken in the ocean or the San Francisco Bay with no limit on those taken from inland waters. There is no minimum size limit. But Fish and Game also very clearly stipulates - and for good reason - that what is caught is to be killed immediately. The last thing anyone wants is to have the Chinese Mitten Crab transported into a new area thus given "a hand" in expanding its range!

Because really, the use of scary sounding descriptions to describe the introduction of non-native species is not all that far from the true situation. Figures that run into the multi-millions of dollars are often sited for the damage that just these two aggressive exotic crabs are threatening to do to California's fishing industries. But how can any price tag be placed on the loss of just one of California's unique underwater environments?