A Tiny Invader
Japanese mud snails are taking over Padilla Bay but no action has been taken to remove them.
A mud snail wedged into the tread of a boot May 1, 2024. // Photo by Ian Ferguson
Story by Piper McKibbin // Photos by Ian Ferguson
June 13, 2024
It is nearly impossible to stroll along the Padilla Bay beach without stepping on invisible invaders, and even unknowingly aiding them in their takeover.
Nestled on the picturesque shores of Skagit County lies Padilla Bay, renowned for its expansive tidal flats and wildlife habitat.
The bay, a sanctuary for native creatures, is facing an ambush. An estimated 12 to 15 billion snails now live in Padilla Bay. These intruders are not just consuming vital nutrients, but are also eroding the sediments of the tidal flats.
Japanese mud snails, Batillaria attramentaria, hitched a ride to the bay with Pacific oyster shipments across the ocean in 1924. This cargo was intended to boost the oyster economy but instead contributed to an invasion. Within the past century, the snail population has skyrocketed.
These snails, native to Japan, are characterized by their slightly marbled mud-tone and corkscrew-shaped shell. Their tiny bodies can measure up to 3.5 centimeters, about half the length of an average adult’s thumb.
Other marine snails hatch as larvae, which allows them to take advantage of ocean currents for dispersal. Japanese mud snails, however, hatch directly into miniature versions of adults, skipping the larval stage and allowing them to reproduce sooner.
Humans are the primary means of dispersal for these snails. Roger Fuller, the Natural Resource Restoration and Stewardship Coordinator at the Padilla Bay Reserve, expressed concern about the lack of local education and policies surrounding the snail invasion. Fuller also acts as a Project Manager for Western Washington University's College of the Environment.
“I think at a minimum, it would be great to see some statewide education programs around not transporting organisms from bay to bay,” Fuller said.
The snail’s sheer numbers and ceaseless burrowing into the mud are shaking things up beneath the surface, according to members working at the Padilla Bay Reserve. The sediment, once stable, is now under attack, with each snail acting like a tiny but relentless excavator.
Research on the long-term impacts of these snails on sediment is scarce, and pinpointing the precise contribution of the snails to sediment erosion presents a significant challenge.
An image of the shore of Padilla Bay covered by hundreds of Japanese Mud Snails May 1, 2024. // Photo by Ian Ferguson
A walk along the mudflats of the Bay at low tide is accompanied by an earthy aroma, rich with the scent of damp soil, decaying organic matter, and the subtle tang of shallow, salty waters.
Low tide offers optimal visibility of the snails. Cory Gardner, a Padilla Bay Reserve team member, sees these snails as a serious threat.
"I think they’ve already caused irreversible changes to the bay,” Gardner said.
Regular monthly observational work conducted by the Padilla Bay Reserve has revealed the population trends of Japanese mud snails in the intertidal zones. They record the population density in monitoring plots established by Emily Grason, a Western alum.
The data the team gathered showed a notable decline in the average population, from a bustling community of 600 Japanese mud snails per square meter in 2020, to 400 by 2023. This period coincides with a La Niña phenomenon, characterized by the emergence of cooler water from deeper ocean layers to the surface.
A study conducted at the Swire Institute of Marine Science in Hong Kong, China, shows a correlation between water temperature anomalies, such as El Niño and La Niña events, and snail reproduction. As water temperatures rise, the metabolism of the snails increases, allowing them to reproduce at higher rates. The opposite occurs with cooler temperatures, which could explain the population decrease in past years.
There are concerns that Japanese mud snail populations will spike from the warm trade winds blowing west during this year’s El Niño event.
Cory Gardner standing on the mud flats of Padilla Bay, Washington, May 1, 2024. // Photo by Ian Ferguson
“We’ve also seen a spike in the younger snails this winter and fall,” Spencer Johnson of the Padilla Bay Reserve, said.
He noted that this spike could be the beginning of the snail’s population rebound.
As these invasive snails multiply, they are engaged in competition for the vital nutrients within the mudflats: dead plant matter and high-protein microscopic algae that live on the surface of the mud.
“They’ve completely changed all of the habitat here for all of the native species,” Gardner said.
Japanese mud snails lack a natural predator in Padilla Bay, but the invasive parasite Cercaria batillariae, which also arrived on the west coast via oyster shipments, is a threat to the snails. The parasite causes infertility in the snail and increases their size up to 50%. These larger, host snails then migrate deeper into the intertidal zone.
Native snail populations in Padilla Bay have decreased as a result of this parasite, in addition to competition with Japanese mud snails.
Japanese mud snails have infiltrated California at the Loch Lomond Marina in San Rafael. To prevent the snails from spreading throughout San Francisco Bay, dozens of volunteers hand picked them out of the mud over five days across July, Aug. and Sept.
Similar approaches would not be practical at Padilla Bay, according to Gardner.
"It would probably have negative unintended consequences on our native species, just because Batillaria aren’t the only thing living in the mud here,” Gardner said.
The data available to researchers is limited, which is why there is currently no eradication plan in place for the Japanese mud snails of Padilla Bay. Grason also emphasized that while economic impacts haven't been observed yet, controlling the spread of Japanese mud snails remains important.
An area of mud at Padilla Bay decorated by the winding trails of the mud snails May 1, 2024. // Photo by Ian Ferguson
Educating the local public about the potential consequences of transporting these snails to other areas is crucial to safeguarding their beloved beaches, and their furry friends.
Beachgoers Tammy and Jerry O’Rourke had no idea that the snails beneath their feet were invasive. However, they noticed the overwhelming numbers and changed their walking path to avoid pointy snails from pricking their dogs’ paws.
Research into the impacts of Japanese mud snails on the local wildlife of Padilla Bay continues, and some of the results aren’t all bad. Hermit crabs, for example, often call empty snail shells home.
For now, there is no plan in place to manage Japanese mud snails in Padilla Bay, whose tiny bodies, but large population, could determine the future for countless other species native to the area.
Piper McKibbin is an environmental studies student studying education and eco-social justice. She likes to examine grassroots solutions for environmental issues that center local voices and community strength.
Ian Ferguson is studying environmental studies to further his passion for the outdoors and to learn more about our world.