Cape Cod researchers plumb the world of slugs

It’s not often a slug stops someone in their tracks but Owen Nichols, director of the Marine Fisheries Initiative, was halted one night by a gray slug garishly spotted with black.

“I came across them one night walking back to my office in Provincetown working late and found one on the sidewalk at 11 or 12 o’clock at night. I’d never seen anything like it. I went back, hunted online and e-mailed the (Cape Cod Cooperative) extension office and they put me in touch with a retired entomologist.”

That entomologist and slug specialist was Gary Bernon of Sandwich. The slug was a leopard slug, a European import first discovered in Philadelphia in the 1860s.

“It is Linex maximus, this very animal was named by Linnaeus (the Swedish biologist who devised the binomial naming system),” Bernon explained. “Now it is found all over the world.”

“The first Massachusetts record was 1881 in Springfield,” noted Nichols, a presenter at Wellfleet Bay Wildlife Sanctuary’s Cape Cod Natural History Conference.

The leopard slug is now on Cape Cod, along with many other non-native slugs. Ten of the dozen slugs known to occur on the Cape are not from North America.

“Most indigenous slugs are found in natural habitats like forests,” Bernon noted, “so they don’t come into contact with invasive slugs because those are in disturbed areas.”

As slugs go, the leopard slug, named by its spots, is pretty interesting. It’s 4 to 8 inches long when all stretched out.

“It has really strange mating behavior. When two slugs mate they chase each other (not very rapidly) and then climb up on a tree branch and wrap themselves around each other and drop down several feet on a ropelike thread of slime hanging in midair,” Bernon explained. “A French naturalist said the couple passes its honeymoon between Heaven and Earth.”

There are more intimate details we won’t go into. But slugs are certainly on the cutting edge. They are both male and female and can fertilize themselves if they can’t find a partner.

“Owen has got a good population we can work with,” Bernon said.

All the whale identification work at the center gave Nichols the idea that slugs could be individually identified by their spots just as humpback whales are identified by the markings on their tails. By photographing them each time he encounters one, he may be able to track their home range and foraging activity patterns.

“I find in a few areas they are abundant, like right around the lab in Provincetown,” Nichols said. “They’re often associated with human dwellings. People find them in their basement.”

Little is known about their habits. The leopard slug is an opportunistic feeder, eating mostly decaying matter, leaves, dead animals, other slugs, but also fresh plant material. Bernon has raised them successfully on lettuce.

“They are gregarious,” Bernon pointed out,” that means they live in groups. And the neat thing, and Owen has seen this, when they go out at night and forage looking for food they come back to where they started.”

That makes them true homebodies.

“Both Owen and I feel this is a neglected group of animals on Cape Cod,” Bernon said. “We have extremely little idea what is going on with slugs.”

Some may feel that lack of interest is understandable; there are less creepy creatures to observe. But slugs can do a lot of damage, in greenhouses, to hostas, lettuce, peppers, zinnias and other garden plants.

“Slugs may be a great group to use as an indicator species of the impact of global warming in the ecosystem,” Bernon pointed out.

“Someone said slugs are snails with a housing problem because slugs are snails without a shell,” Bernon said with a chuckle. “As slugs evolved, the shell became much smaller, vestigial. Basically by losing the shell the slugs gained mobility and can get into habitats snails can’t. But the cost was they’re more vulnerable to predators.”

Bernon lamented that there is no American field guide to slugs, but the Canadian government has recently published a guide that covers just about any slug you’d find on the Cape. Still, the study of land mollusks is fading.

“It’s a lost art,” he said. “We’re behind the eight ball when it comes to studying terrestrial mollusks.”