Much attention has been paid to invasive species on land – barberry plants, for example. There are impressive efforts to remove them from parks and other public properties and programs to encourage people to plant native species. These are easy to see and identify and pose a problem for the native growth that they often outcompete.

Less easy to see, however, are the invasive species that are underneath the water. According to a report put together by the Maine Department of Marine Resources, there are more than 33 invasive marine and aquatic species. This includes those in fresh water as well as salt. While they may vary from weedy plant to spikey crab, they all share the ignominious designation as invasive and therefore problematic.

The best known of the marine species is probably the European Green Crab (Carcinus maenus). It is a nasty predator that was introduced in the early 1900s. With no native predators and remarkable ability to survive in just about any condition, they multiplied like crazy and then devoured much of Maine’s native shellfish population. The green crab is not the only culprit, however. Other problem critters include slimy encrusting tunicates like Didemnum spp. and drippy colonial sea squirts that clog up nets and traps with its rubbery flesh.

While it too is invasive and competes with what is naturally in our tidepool ecosystems, I have a somewhat affectionate feeling for Codium fragile. Also known as dead man’s fingers, green sea fingers, felty fingers, forked felt-alga, stag seaweed, sponge seaweed, green sponge, green fleece and even oyster thief, its green fuzzy branches often trap tiny bubbles of oxygen along the surface that shimmer in the sun as it waves beneath the surface.

Dead man’s fingers are native to the Pacific Ocean near Japan. It is thought to have been introduced to Maine in the 1960s through oyster aquaculture. As is the case with many invasive species It might not seem to be a problem at first consideration, but it competes with native kelps and can compete with shellfish as well as overgrow aquaculture cages.

While these species are problematic, there are several initiatives designed to combat them. The Department of Marine Resources has formed several partnerships with both local and national groups. They’ve created programs like the Maine????s Action Plan for Managing Invasive Aquatic Species, which was adopted in 2002. One of the efforts includes the Department of Marine Resources coordinating with the Coast Guard to manage ballast water and make sure that it does not contain unwanted species.

There is also a Maine Marine Invasive Species Working Group that convenes scientists and policy makers together to come up with solutions. The guide that is co-produced by several of these organizations and agencies also provides some simple things that people can do to help on their own. These include making sure to properly clean the hull of your boat before heading into new waters and not releasing unwanted pets from other waters into local areas.

The oceans are an amazing mixing pot for species from around the world – some that naturally travel in migratory routes to swap locations at different times of the year and others that hitch rides on man-made vessels to arrive in new places. It is important as a coastal resident to get to know what belongs here and what doesn’t. You can be a part of the solution just by helping to report what you see. This helps scientists to track the spread of invasives and create strategies to respond to them. You can join a monitoring group like the Maine Shore Stewards Program, for example.

For more information on which species are native and which are not, you can check out the guide at and sign up to be a part of several community science initiatives that help to control these pests. One thing few people think of is how to dispose of shells after eating a shellfish dinner. While you may think that these are perfectly fine to return to the sea, they can carry both bacteria and other species used in packing or storage materials that should not be put back into the water. There are some local shell composting programs that are best for this. Or you can let them dry out and sanitize in the sun to kill anything unwanted.

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