What Is This Slug Doing On This Cat?

What’s going on with this slug? I saw it in my Twitter feed on Monday, and can’t stop thinking about the slimy protuberance.

Entomologist Suzanne Wainwright-Evans discovered the intrepid invert glued to her cat’s fur. “It was emerging out of casing,” she tweeted. Wainwright-Evans told me her cat—a “love monster” whose name is Adjunct—”comes in every day to get brushed and that’s when we noticed the clump in his fur and decided to cut it out.”

Apparently this was weird enough for other bug experts to chime in, though none seemed sure of why a slug would encapsulate itself like that.

Now infected with this nagging question, I decided to email a few biologists, mainly those specializing in gastropods. The first to respond was Ronald Chase, a professor emeritus at McGill University who told me I sent him “an odd request.” Despite this, or perhaps because of it, he was kind enough to share his thoughts.

“It definitely looks like a snail, not a slug. As for the ‘casing,’ no they don’t make them. The animal seems to be emerging from its own shell. Depending on the species (I don’t recognize it), the shell could be normal or, from the appearance, it could be regenerating. In the latter case, it would be soft to the touch. The yellowish stuff at the back is dried up mucus,” Chase wrote.

A snail! What a twist. Chase said its sculpted “neck” and reared-up posture are two characteristics that betray the animal’s true identity. He also suggested that it’s actually regenerating its damaged shell. (“If they have no shell, they die, because their lung collapses,” he added.) Snails can sometimes repair cracks in their shells by secreting calcium carbonate from their bodily tissue, but they can’t replace them entirely.

Still, others disagreed. “Apparently slugs will indeed encapsulate themselves when necessary,” wrote Lindsey Groves, a collections manager at the Natural History Museum of Los Angeles County’s Malacology Department. I couldn’t find any scientific literature on this, but did come across several studies about slug larvae who go through an encapsulated period.

One expert told me the species of slug could be Arion subfuscus or Arion fuscus, both of which are widespread in North America. (The two are only distinguishable when dissected.)

“This species might possibly seal itself in soil cavities with a bit of dried slime, but nothing at all substantial like shown on the picture,” wrote John Hutchinson, who works in the Department of Malacology at Germany’s Senckenberg Natural History Museum.

But Marla Coppolino of Cornell University told Wainwright-Evans that Arion subfuscus doesn’t make “cases” like this. “It could be that the slug wasn’t able to work its way out of the cat fur, so it exuded extra mucus, which dried, and then the slug crawled out of that,” Coppolino said.

Hutchinson’s alternate theory, however, is my favorite. I’m afraid that paraphrasing won’t do it justice, so here it is in its entirety:

But how come the slug was inside the fur ball? I can think of several possibilities. To trigger the regurgitation, cats often eat some grass and maybe the slug was on the grass and got licked off and enveloped in the ball. Or the slug was on the cat’s food (they love it; cat pellets are part of the diet we keep them on in the lab) and was swallowed, but was lucky enough to be regurgitated with a fur ball. The slime on a slug may protect it briefly from stomach acids. Or the stickiness of the slime may have lodged the slug in the oesophagus and that physical obstruction, or perhaps some unpleasant taste from the slime, may have triggered the retching. If it was really inside the “casing” perhaps the most plausible of these alternatives is that the slug actually did make it down to the stomach before being regurgitated.

He also said the animal is “absolutely definitely not a snail.”

Here’s what we do know: The majority of my admittedly tiny survey think this is a slug, and not a snail. It likely didn’t mean to get stuck in cat fur. And since slugs can only crawl (slither?) forward, it’s certainly coming out of the casing, and not going inside of it. Wainwright-Evans told me she’s sent a sample of the mucus to a slug specialist at Oregon State University for analysis.

Adjunct enjoying some yard time. Photo: Suzanne Wainwright-Evans

I’m sorry to end this journey on an inconclusive note. But if you have any smart thoughts, I’d love to hear them! You can email me at sarah.emerson@vice.com