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An Unwelcome Snot Blob - Hakai Institute

An Unwelcome Snot Blob

Scientists and community members track the spread of two non-indigenous tunicates.

Amorphous slime is unlikely to be the first thing you go searching for when exploring coastal British Columbia. To most people, the ocean creatures known as colonial tunicates, or sea squirts, are barely recognizable as animals. But despite their alien appearance, scientists want you to keep an eye out for them. They’ve hitched a ride with humans around the world, and for many local critters, these particular tunicates mean trouble.

In British Columbia, two of the uninvited guests are tunicates commonly called botryllids. The golden star tunicate comes from the Mediterranean region, while the violet tunicate likely originates from northwestern Asia. Their presence is of particular interest to shellfish aquaculture, as botryllids can grow over and smother bivalves, such as mussels and oysters. They also love to attach to human-made structures, stifling the creatures that live there.

“They can be extremely abundant on docks for years and years, and yet not show up on natural areas nearby,” says Gretchen Lambert, one of the region’s foremost tunicate experts.

Botryllids would make an exemplary addition to any invasive species starter kit. That’s because they have a few tricks up their (proverbial) sleeves when it comes to reproduction. For long distance dispersal, a chunk of the adult can bud off asexually, and the resulting clone can attach to floating debris, a ship, or aquaculture equipment. For shorter distances, botryllids can reproduce sexually, resulting in a short-lived larva that rides the currents before landing on a suitable spot nearby.

Larval form of the violet tunicate (B. violaceus) found during a plankton survey off Quadra Island in July 2020. Visible is the notochord, the precursor of a backbone, which is lost after they settle as adults. While their adult forms arent what you’d expect from a relative, tunicates are in the same phylum as humans. Photo by Matt Whalen
A settlement plate that spent most of 2018 underwater on the Central Coast of British Columbia. The plate, roughly the size of a large bathroom tile, is covered in two non-indigenous tunicate species, which are growing over the indigenous brooding transparent tunicate (Corella inflata). Photo by Matt Whalen

Regardless of how it gets there, botryllids grow quickly after settling down, and can do so under a wide range of water temperatures and salinities. Once established, colonies are a challenge to eradicate for a few reasons. They release chemicals to dissuade potential competitors from settling nearby. Although experiments have shown that certain urchins, sea slugs, and sea stars can eat botryllids, alas, like a freezer burnt burrito of unknown age, scientists from Fisheries and Oceans Canada found these would-be predators only resort to munching on tunicates when their preferred food is scarce.

While botryllids were first spotted in BC in the early 1990s, their spread hasn’t followed a linear path up the coast. By 2012, botryllids had been found in the southern part of the province, including most of Vancouver Island, as well as on Haida Gwaii, but not on the Central or North Coasts. By 2012, botryllids were also found on the Central Coast near Bella Bella. However, an extensive survey by the Hakai Institute and its partners 50 kilometers south of Bella Bella, around Calvert Island, yielded 36 tunicate species, but no botryllids—at least not until the following year. That’s when botryllids made their first known appearance on Calvert Island.

“I’m curious if our timing on Calvert is impeccable—and we caught the beginning of an invasion—or whether botryllids were already present and we missed them,” says Matt Whalen, a Hakai postdoctoral researcher at the University of British Columbia, who added that the 2017 surveys mostly focused on natural areas and fewer collections were done on human-made structures. Whalen subsequently found both botryllid species on Calvert Island every year from 2018 to 2020.

It isn’t just scientists who can be on the lookout for botryllids. Community members can help by uploading photos of tunicates to the iNaturalist website or app. Photos and observations uploaded to the site greatly improve the ability of scientists to monitor more of the vast British Columbia coastline and track the spread of botryllids. And that’s permission to go out to your nearest dock and scavenge for slime blobs—you might be surprised what you find.

A timeline of the spread of botryllid tunicates in North America. Graphic by Josh Silberg with input from Heidi Gardner (Royal BC Museum)