By Chris Bowman

Many of the alien species invading California’s lakes and streams would make for wickedly good Halloween costumes.

Take the Shokihaze goby, Tridentiger barbatus (above and right), a native of Asian now common in Suisun Bay and the lower Sacramento River. Its spiky stubble of whisker-like barbels about the mouth and cheeks defines “ugly.” And its eyes, ringed with heavy mascara and seemingly misplaced near the top of its head, are downright spooky.

The goby’s barbels, however, are not nearly as charmless as those of the channel catfish (Ictalurus punctatus), shown below.

Then there’s the red swamp crayfish (Procambarus clarkii), aka Louisiana crawdad. Never mind the pinchers. The eyes are not only beady; they’re freakishly mounted on moveable stalks that slide independently along the side of the head. The dagger-like snout also makes this clawed carpetbagger especially hard to cuddle.

Beware Halloween night of  trick-or-treaters masquerading as freshwater aliens. If true to form, they’ll trick you even if you treat them.

Although highly prized in Cajun cuisine, the red swamp crayfish wreaks havoc in rice fields of the Central Valley by burrowing several feet into rice checks and levees, weakening the earthen banks and causing erosion.

Another alien trickster is this microscopic algae, or diatom, to the right that looks innocently whimsical with its bottle-shaped body.

But Didymosphenia geminata (“Didymo” for short) can pull an underwater prank akin to toilet-papering your trees. It coats riverbeds with thick and slimy brown mats, choking off bottom-dwelling organisms that fish eat.

No wonder this invasive species is nicknamed “rock snot.” In the photo below, the South Fork Yuba River looks as though it has a serious sinus infection.

Rivers with stable, dam-regulated flows are particularly susceptible to infestation. Peek, who has snorkeled many California streams as a biologist with the UC Davis Center for Watershed Sciences, calls the South Fork Yuba “rock-snot Armageddon.”

He says the river sometimes looks like it is polluted with toilet paper as pieces of Didymo mats slough off and drift downstream.

A section of the South Fork Yuba River infested with Didymo, or rock snot. Photo by Ryan Peek/UC Davis

The Uruguay water-primrose (Ludwigia hexapetala) boasts bright yellow flowers and may be viewed as an attractive addition to California wetlands. But under the right conditions, the species grows explosively to clog waterways. Among other problems, the aquatic plant can harbor mosquitos carrying West Nile virus, as the dense patches shield the insects from pesticide spraying.

Amber Manfree, a scientist now with the UC Davis Center for Watershed Sciences, wades through a jungle of Uruguay water-primrose in the state Laguna Wildlife Area of Sonoma County. Photo by Julian Meisler

Microscopic view of a freshwater jellyfish collected by an angler at Lake Clementine in early October. Photo by Ryan Peek/UC Davis

The drought seems to have invited a ghostly invader to some California lakes. Earlier this month, boaters began reporting blossoms of freshwater jellyfish (Craspedacusta sowerbii) in the much depleted Lake Oroville and other diminishing reservoirs in the northern Sierra foothills.

Jana Frazier, a Department of Water Resources spokeswoman, told KNVN news of Chico that the warming low water levels “seems to trigger a good bloom of the jellyfish.” The translucent, quarter-size creature is a friendly Halloween ghost with a stinger too small to penetrate human skin.

A southern watersnake eats many different freshwater fish species such as this sunfish. The non-native snakes are invading California waters, posing a threat to native fish, amphibians and reptiles. Photo by J.D. Willson/University of Arkansas

Invasive water snakes are slithering into the Sacramento area. An estimated 300 snakes of two species — the northern or common watersnake (Nerodia sipedon) and the southern watersnake (Nerodia fasciata) — have been found in Roseville and Folsom, and at least 150 have been seen in Long Beach, according to a recent UC Davis study. Though nonvenomous, the snakes are not picky eaters. Biologists are concerned they will spread and prey on the giant garter snake and the California tiger salamander — both on the federal endangered species list — and the foothill yellow-legged frog, an amphibian of special conservation concern.

Whether slithering or slimy, ghastly or ghostly, several of California’s alien water species can be an ecological nightmare. If only they were as scared of us as we are of them.

A Shokihaze goby recently caught in Suisun Marsh. Photo by Amber Manfree/UC Davis

Chris Bowman is communications director at the UC Davis Center for Watershed Sciences. He occasionally invites freshwater aliens over for dinner, once they are cooked.