A signal crayfish found in Alaska's Buskin Lake, where the species is invasive Lisa Hupp/USFWS

Crayfish probably don’t cross your mind too often, but they should if you care about freshwater ecosystems. These tiny lobster look-alikes are important players in their streams and lakes, but we’ve knocked their populations way out of balance.

The United States is home to almost 400 species of crayfish, about half of which are considered endangered, threatened, or vulnerable to extinction. One study of these crustaceans in the Ozarks found that more than 200 species of fish, birds, and other animals rely on crayfish for food.

Now, you may be thinking, “A crayfish is a crayfish is a crayfish—what’s the big deal if some go extinct? Others can take their place, right?”

Wrong. Let me introduce you to the signal crayfish, a Pacific Northwest native that is wreaking aquatic havoc from Alaska to Japan to Finland to Greece. At up to seven inches long (impressive for a crayfish) and with claws like whoa, signal crayfish are capable of taking down frogs, fish, and importantly, other crayfish. Peek under a submerged rock or tree root anywhere these crustaceans dwell, and a pair of beady eyes may stare back at you as their owner debones and devours its latest victim with its terrifyingly complex mouth parts.

Engineering Chaos

A signal crayfish’s rapacious appetite is only one way it influences its environment. These predators are also prolific burrowers, tunneling through mud, sand, and gravel to construct subterranean lairs. In fact, scientists refer to the species as ecosystem engineers, since their burrows change the consistency of a riverbank, the way water flows through it, and the amount of sediment suspended in the water. In a 2013 study, researchers even found that when signals are around you can watch a stream’s water quality worsen in real time after dark. That’s when these nocturnal crayfish start digging and kicking up sediment.

Signal crayfish also chew up a lot of leaf litter, which increases the amount of nutrients available in its ecosystem. By eating leaves and other detritus, the crayfish turn waste into energy for themselves; what’s more, all that shredding and chewing and digesting and, yes, pooping unlocks nutrients for other aquatic organisms—from insects to fungi to bacteria.

Scientists know a lot about signals—but most of their knowledge stems from the fact that these crustaceans are considered one of the worst invasive species ever to hit freshwater. Indeed, all the things that make signal crayfish a keystone species in their native streams cause them to be a menace in new waters.

In California, for instance, signal crayfish have driven the sooty crayfish into extinction and are well on their way to doing the same to the Shasta crayfish, whose population now hovers in the mere hundreds. Signals are much bigger than Shastas, which grow to two inches, tops, and they’re much more aggressive, says Josh Hull, division chief for listing and recovery at the U.S. Fish and Wildlife Service’s Sacramento office. And, of course, signals love a Shasta lunch.

A female signal crayfish with eggs Lisa Hupp/USFWS

Making matters worse, signals reach sexual maturity in two years, while Shastas require about five. And when the invaders lay eggs, they tend to produce twice as many as the locals. A signal mommy also protects her brood for their first three sheddings—one more than Shasta mommies do. The home team doesn’t stand a chance.

The World’s Gone Cray-Cray

Today you can find signal crayfish in Austria, Belgium, Cyprus, Denmark, Finland, France, Germany, Greece, Italy, Latvia, Lithuania, Luxembourg, the Netherlands, Poland, Portugal, Russia, Spain, Sweden, Switzerland, the United Kingdom, and even Japan.

Signals are wanderers. They “crawl considerable distances” out of water and “climb substantial heights,” writes Peter Coates in Invasive and Introduced Plants and Animals. This allows them to colonize new stretches of water at a rate of more than half a mile per year.

Impressive, but these crayfish didn’t crawl their way across the globe from Oregon and Washington. They were transported there by us. Not a lot of great documentation exists on this, but Hull says signals first popped up in California watersheds sometime after the Gold Rush. Why? Because they taste great—especially when boiled up with some potatoes, corn on the cob, and Old Bay seasoning.

And when a plague knocked down Sweden’s and Finland’s native crayfish populations, the mudbug-hungry countries willingly imported signal crayfish because, they reasoned, a crayfish is a crayfish is a crayfish.

Again, wrong. In addition to their aforementioned traits, signals are unique in their tolerance for greater temperature and salinity extremes and for their resistance to crayfish plague. Unfortunately, what wasn’t understood at the time was that signals can still transmit the plague. So they actually helped to spread the disease that was killing off European species.

Northern Exposure

Signal crayfish have also made their way to Buskin Lake on Alaska’s Kodiak Island, as either released pets, bait, or food. Wildlife officials have been monitoring this population out of concern for the mayhem the invasion could create for salmon and the Native Alaskans who have relied on these fish for thousands of years.

The ominous claw of a signal crayfish David Castor via Wikimedia Commons

“Alaska has no native crayfish species,” says Aaron Martin, a fish biologist working on invasive species for the USFWS. Because the island’s native fish did not coevolve with these crustaceans, they are unlikely to have any defenses against those snappy claws.

Whether the signals are affecting the lake’s salmonid species is yet to be seen, but Martin worries that the crayfish could have a serious effect on salmon eggs and fry that emerge from the lake bottom. And due to the signal’s proclivity for burrowing, he’s on the lookout for bank destabilization and an increase in sediment. Such conditions could hinder juvenile sockeye salmon as they search for zooplankton in what should be clear water. Drinking water quality for the island’s U.S. Coast Guard base could also take a hit.

“This isn’t to say it’s going to happen,” says Martin, “but if you follow the pattern of events that has occurred elsewhere . . .”

Here to Stay?

Of course, singling out signals is a little unfair. They are hardly the only invasive crayfish out there. Rusty crayfish, natives to Indiana and Ohio, are currently clawing their way up watersheds from Minnesota to Maine. Meanwhile, Mississippi’s red swamp crayfish are crawling over land and through lakes between California and New York. There’s even a population of red swamps scuttling around and waving their chelicerae at tourists in Berlin.

Once established, invasive crayfish are nearly impossible to remove. Catching them all is unrealistic since the critters live under rocks and deep within the mud. (Not that Alaskans aren’t trying their darnedest, donning goggles and snorkels and bringing up signals by the bucket.) Poisoning them off—without killing native wildlife in the process—is also not an option.

So, next time you think of crayfish—if you think of them at all—remember, if you’ve seen one crayfish, you haven’t seen them all. All crayfish are unique. They are important. And they are destructive when thrown into places they just don’t belong.

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