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Marine Invasive Species

What are Marine Invasive Species and Why are they a Problem?

Asian shore crab
Photo taken of an Asian shore crab in Sandwich, Massachusetts
in 2006. Photo by: Jenn Forman Orth (License CC BY-NC-SA 2.0)

Marine invasive species are live marine plants and animals (including their seeds, eggs, spores or other biological structures) that cause harm when intentionally or unintentionally introduced into a marine, estuarine or brackish ecosystem where they are not native. Not all introduced species survive in their new habitat, but those that thrive are termed invasive. There are certain characteristics that allow a species to flourish in the new environment and, as a result, negatively impact the ecosystem, local economy and, potentially, human health.

Some invasive species are capable of being extremely destructive in their new environments. These species tend to mature quickly and reproduce rapidly, which makes controlling their populations difficult if not detected early. Many invaders can adjust to new environments due to their broad diet and tolerance for varying environmental conditions (such as salinity, temperature and dissolved oxygen). Because they did not evolve with the local flora and fauna, they may have few to no predators in the new location, and native species may have no defense against their predation or no chance in a competition for food or shelter. Ultimately, an invasive species can reduce native populations and habitat, leading to a less diverse ecosystem that is more susceptible to further disturbances such as disease and natural disasters.

The way invasive species alter marine habitats and ecosystems can impact the local economy and recreational activities. They can clog waterways, interfering with recreational activities such as boating and swimming. Some marine invasive species attach to substrates such as vessels and fishing gear, reducing their efficiency. As mentioned, the presence of invasive species can reduce biodiversity, which in turn could negatively impact tourism and fisheries important to the region's economy. The impacts of invasive species can be costly, and eradicating them or restoring habitats can be expensive too.

Although most of the impacts caused by invasive species are to the environment and economy, in some cases human health can also be impacted. Some invasive species can be carriers of diseases that could potentially be transmitted to humans. Additionally, some invasive organisms could be poisonous to humans if encountered.

How are Marine Invasive Species transported?

Invasive organisms are transported from one part of the world to another by various pathways. Humans most often play a role, whether intentionally or inadvertently. Though the means of introduction is not always known, here are a few different pathways in which invasive species can make their way to our New York waters:

  • Ballast water: Ballast water is used to stabilize vessels during voyages by taking in water in one region and discharging it in another, allowing an aquatic invasive species to travel hundreds of miles in a short period. Most ships are required to use water treatment systems to remove or kill any organisms in their ballast, which has slowed the rate of species transport, but it is difficult to ensure the effectiveness of these expensive systems. A majority of goods are transported by shipping, which creates many opportunities for introduction of non-native species to slip past our safeguards.

  • Hull fouling: Some organisms can attach themselves to the bottom of vessels and travel wherever they go. This is called "biofouling." Marine organisms will attach to both commercial and recreational vessels, as well as any fishing gear they may use. This pathway does not only transport invasive species, but can diminish the performance of the vessel, reduce the vessel speed and requires time and money to eliminate the unwanted organisms.

  • Live seafood/aquarium releases: Live seafood, which refers to any fish, invertebrates and algae, is imported regularly into the United States, whether to sell as food or for aquarium purposes. Organisms can be transported in tanks of water or on ice, which, after use, can be discarded into nearby bodies of water, accidentally releasing viable biological structures of non-native species.

    There are also incidents when invasive species are released intentionally. Individuals who believe they are "saving" aquarium or food species will release animals into their local marine habitat. This behavior can lead to the demise of the animal released or have extremely negative consequences on that environment.

  • Marine bait dumping: Marine worms are commonly used as bait for sport fishing. They are harvested from tidal mud flats, packed with seaweed to keep them moist, and transported to locations around the world. The seaweed, if not disposed of properly, can introduce live organisms, such as small crustaceans, juvenile shellfish, snails and other worms, to a non-native waterbody.

To learn more about these and other pathways, visit the Northeast Marine Introduced Species webpage.

What I can do to help reduce the spread of invasive species?

There are many marine habitats where invasive species can be spotted, from marinas and fishing docks, to sandy beaches and salt marshes. Whether you are a beach goer or an active fisherman, here are a few simple things you can do to help protect your marine environments from invasive species.

Help Detect Marine Invasive Species Early
Because marine invasive species can grow and reproduce quickly and frequently, it is extremely important to identify unwanted species in your area early. If you come across a species you believe to be invasive, take a photo and record the location and the date you found it.

Use the New York "iMap Invasives" Reporting System
Help New York State document invasive species by using your smartphone or computer! Visit the New York iMapInvasives webpage or download the iMapInvasives Mobile App! If you've found a species that isn't yet listed, contact the iMapInvasives team.

Visit the Long Island Invasive Species Management Area (LIISMA) website
Learn more about some of the terrestrial, aquatic and marine invasive species in New York's geographical area that includes Staten Island (Richmond County), Long Island (Kings [Brooklyn], Queens, Nassau and Suffolk counties), and eleven additional coastal plain islands. Visit the LIISMA webpage for more information. Additional invasive species resources can be found to the right of this page under, "Offsite Links."

Always Clean your Boat and Gear
Help prevent the spread of aquatic and marine invasive species by thoroughly cleaning your boat and recreational gear after use, and especially before introducing it into a new waterbody. Learn more by visiting the Prevent the Spread of Aquatic Invasive Species webpage.

Some of New York's Marine Invasive Species

Chinese Mitten Crab, Eriocheir sinensis

Chinese Mitten Crab
Chinese mitten crab caught in a crab pot in 2018 in the Hudson
River near Yonkers, New York

Originating from Eastern Asia, the Chinese mitten crab is a well-known invasive species in New York's waters. This crab species can be easily recognized by their white-tipped claws covered in hair. The carapace is about three inches wide, roughly the size of a human palm, and the legs are long (about two times the carapace width) and hairy in adults. They're light brown to olive-green brown in color and their claws are roughly the same size.

The Chinese mitten crab is thought to have been transported to the United States through ballast water or introduced through live trade. Mitten crabs are found in both freshwater and marine environments because they breed in brackish-to-marine waters and move into freshwater environments as a juvenile to mature. Their burrows, causing holes about 3 cm in diameter, can be identified on the sides of river banks.

The Chinese mitten crab is a big problem for many reasons. They feed on both plants and animals and will consume whatever is readily available. Mitten crabs will actively compete with native species, such as blue crabs and other economically important crustaceans, for food and shelter. Not only will this invasive crab disturb ecosystems, but it could also impact the local economy if they outcompete native species.

Please report any sightings of a Chinese mitten crab immediately to DEC. Visit the DEC Chinese Mitten Crab webpage for more information.

European Green Crab, Carcinus maenas

European Green Crab
Photograph of a European green crab taken on the beach near
Great Peconic Bay in Long Island. Photo by: Vincent Cavaleri

Another common crustacean in New York is the European green crab, also known as the "shore crab" or "Joe rocker". The green crab is native to Europe and is believed to have been brought to North America through ballast water, hull fouling or intentional release. The green crab, as its name suggests, is green to dark brown in color, with green walking legs speckled with black. An easy way to identify a green crab is by the pentagon shaped carapace and 5 spines on each side of the eye. The carapace of a green crab can grow up to 3.9 inches wide.

The green crab is most frequently found in sheltered intertidal and shallow subtidal habitats, usually near the low-tide line and on mud, sand or pebbles. The green crab can tolerate a wide range of salinities and can be found in estuaries and water with salinities as low as 6 parts per thousand (ppt).

The green crab has been listed as one of the "100 worst invasive species." In addition to being a rapid reproducer, the green crab is a dominant predator in regions it has invaded, feeding on clams, oysters, scallops, mollusks, crustaceans and marine invertebrates. Because it feeds on a variety of commercially important species, it is often blamed for the reduction and collapse of economically important soft-shell clams and other bivalves.

Asian Shore Crab, Hemigrapsus sanguineus

Asian Shore Crab
Asian shore crab, image from Wikimedia Commons, 2007.
(License CC BY-SA 3.0) Photographer unknown.

The Asian shore crab is indigenous to the rocky shores of the Western Pacific Ocean and was first recorded in United States in 1988. Because this species can tolerate a wide range of environmental conditions, it has expanded its range and is now abundant along the Atlantic intertidal coastline, from Maine to North Carolina. Though the means of introduction are not clear, many speculate the larva or adults were introduced through ballast water.

Adult Asian shore crabs have a square-shaped carapace that ranges from 35 to 42 mm in width. They have 3 spines on each side of their carapace, and their color ranges from green to purple to orange-brown to red. They can typically be found within natural and artificial intertidal rocky habitats, but have also been collected in salt marshes and subtidal habitats.

The Asian shore crab has caused major ecological change on the rocky and intertidal communities along the East coast of North America, including Long Island. This species reproduces rapidly, and recent trends demonstrate populations of Asian shore crabs increasing while native crab populations are decreasing. Because it predates and competes with several native species, it could directly impact the populations of several important animals in New York including blue crab, rock crab, and lobster.

Asian Sea Squirt, Styela clava

Asian Sea Squirt
Asian sea squirt, image from Wikimedia Commons.
(License CC BY-SA 3.0)
Photo by: Matthieu Sontag, 2010.

The Asian sea squirt, also known as the leathery or club tunicate, is native to the Northwest Pacific and has become widely distributed in coastal waters of North America, Europe, Australia and New Zealand. It is thought to have been accidentally transported by ships, through hull fouling, or with imported oysters.

The body of the Asian sea squirt is cylindrical or club-shaped, has a thin, leathery and bumpy tunic, and attaches to substrates by a disc-shaped holdfast. This invasive species can grow up to 150 mm in length, nearly as long as a dollar bill, and its colors can range from yellowish to reddish to brownish.

The Asian sea squirt has been known to compete with and displace native species and adversely affects aquaculture production. Even though it is a solitary species, it can grow in dense clumps that foul cages used for fish aquaculture, and ropes, rocks, and other gear used in mussel aquaculture. In some regions, the Asian sea squirt has negatively impacted the economy by reducing mussel harvest. Dense aggregations of the Asian sea squirt can outcompete other fouling species, further damaging the local ecosystem.

Carpet Tunicate, Didemnum vexillum

Carpet Tunicate
Carpet tunicate off of Sandwich, Massachusetts, image from
Wikimedia Commons. (This image is in the public domain)
Photo by: Dann Blackwood, USGS, 2012.

The carpet tunicate is an aggressive and fast spreading invasive species whose first known occurrence was in Maine in 1982. It is now common between Eastport, Maine to the South shore of Long Island. It was most likely introduced by means of ship hull fouling.

Colonies of the carpet tunicate are blob-like and yellowish-cream in color and have a firm, leathery texture and a veined or marbled appearance. Their large, sponge-like masses often have long, flexible leaf or flag-like projections that are cylindrical and branched. Because they are filter feeders, numerous small pores can be identified on the surface of the colony and appear as tiny, white colored spots when closed.

This invasive species grows rapidly and forms dense colonies that can cause both ecological and economic damage. Overgrowth of this species can smother and reduce the abundance of previously established, native benthic organisms. The carpet tunicate can attach and encrust nearly any substrate it encounters, altering benthic environments and completely changing the habitat structure. The carpet tunicate has the potential to cause economic damage to various fisheries and aquacultures that include bottom fish, scallops, lobsters and mussels.

Veined Rapa Whelk, Rapana venosa

Veined Rapa Whelk
Photo of a veined rapa whelk near the Black Sea, image from
Wikimedia Commons. (This image is in the public domain).
Photo by: George Chernilevsky, 2008.

Veined rapa whelk is a large predatory gastropod native to the Northwest Pacific thought to be introduced by ballast water. It is known from a wide range of habitats including rocky shores, oyster reefs, mussel beds, sand, silt, eelgrass beds and pilings. It can tolerate low salinities, temperatures between 4 and 35 degrees Celsius and is usually found at depths between 0.5 and 25 m.

The veined rapa whelk is heavy and rounded with a large body whorl and short spire, broad and somewhat concave columella and deep umbilicus. The external shell presents distinctive longitudinal ribs and blunt knobs at each shoulder of the body whorl. The colors of the external part of the shell can vary with older adults, but typically their color ranges from gray to brown, with dark brown dashes on the spiral ribs. In soft sediments, this species will burrow into the sand for long periods of time with only their siphon will be exposed. Their shells can reach up to 168.5 mm or more in size.

As a predator, the veined rapa whelk has been shown to decrease populations of many economically and ecologically important bivalves. This invasive species will directly attack clams, mussels, oysters and scallops, though the extent of this predation is difficult to determine in the wild. Additionally, they actively compete with other whelk species for food and space, which can negatively impact populations of native channeled and knobbed whelk.

Devil's Tongue Weed, Grateloupia turuturu

Devil's Tongue Weed
Photograph of devil's tongue weed found in the Northeast Atlantic, image
from Wikimedia Commons.(License CC BY-SA 4.0). Photo by: Jymm, 2010.

The devil's tongue weed is one of the largest known red algae. Native to the Northwest Pacific, it was first reported in Rhode Island in 1996 and is typically found in a wide range of coastal habitats. This species survives in both cold and warm temperate regions, native and artificial habitats, and grows on a variety of substrates including bedrock, cobbles, boulders, shells and even ship hulls. It is believed to have been introduced by ballast water or hull fouling.

The devil's tongue weed is a large seaweed that has a deep red, burgundy or maroon color, but can appear yellowish when dying. Blades of this plant vary in size and shape and can be irregularly divided, ranging from one to eight blades that can reach up to 15 cm in width and over 1 meter in length. The blades transition into a short, narrow, cylindrical stem that attaches to a substrate with a small, disk-shaped holdfast. The plant is thick and firm and the blades can be slippery and have a gelatinous texture.

This invasive species outcompetes native algae species and can alter benthic biodiversity. Additionally, observations of devil's tongue weed on the East Coast, in regions including Long Island Sound, suggest that this species supports fewer biological organisms, such as epiphytes and invertebrates, compared to the local red alga, Chondrus crispus.

Learn more about these and other marine invasive species at the National Exotic Marine and Estuarine Species Information System (NEMESIS) website.

Additional Invaders to Consider

When exploring New York's natural marine environment, it is also important to consider other non-native species. Species such as the Asian horseshoe crab and dungeness crab are regularly and intentionally imported live and sometimes introduced into NY's marine environment. Other species are invasive in coastal environments similar to New York, and biologists expect their introduction here is inevitable. Additionally, some organisms, although perhaps unlikely to become invasive themselves, can be possible carriers of invasive parasites or disease causing organisms. Here are some additional species to keep an eye out for in New York's marine waters:

Dungeness Crab, Metacarcinus magister

Dungeness Crab
Dungeness crab (Photo by: NOAA)

Similar in appearance to the native rock crab (Cancer irroratus), and Jonah crab (C. borealis), the Dungeness crab is native to the West coast of the United States. There have been few reports of this crab in New York waters, however, because they are heavily fished and widely shipped, it is possible that the Dungeness crab may be introduced to the East coast in the future. Learn more about this deep water invasive species on the Dungeness Crab NEMESIS webpage.

European Rock Shrimp
, Palaemon elegans (image Wikicommons, 2009, Photographer unknown.

European Rock Shrimp
European rock shrimp, image from Wikimedia
Commons, 2009.
(License GNU Free Documentation License).
Photographer unknown.

Native to Northeast Atlantic, the European rock shrimp is a recent invader of the Long Island Sound. This species is an extremely competitive and abundant populations have led to the displacement and reduction of native, coastal species along Northwest Atlantic regions it has invaded. Learn more about this invasive shrimp on the European Rock Shrimp NEMESIS webpage.

Dead Man's Fingers/Green Fleece
, Codium fragile

Dead Man's Fingers
Dead mans finger's, image from Wikimedia
Commons. (This image is in the public domain)
Photographer unknown.

This dark green algae, native to coastal Japan, was first introduced in New York in 1957 and continues to disturb coastal ecosystems along the East coast of North America. This algae will attach to a variety of substrates, and can impact many benthic environments by prohibiting the movement of native animals and smothering shellfish populations.

Siphoned Feather Weed
, Dasysiphonia japonica

Siphones Feather Weed
Image of siphoned feather weed collected in
the Great South Bay in Long Island.
Photo by: Dr. Rebecca Grella

The siphoned feather weed is a fast-growing red algae that is native to Asia and was first seen in Southold, New York in 2009. This aggressive invasive species clones itself asexually and thrives in nitrogen-rich waters, ultimately outcompeting native species for nutrients. As it dies, siphoned feather weed depletes oxygen levels in the environment and, when it dries out, smells like rotting eggs. Read more about this fouling species in the 2018 Newsday Article about Dasysiphonia japonica.

, Undaria pinnatifida
Wakame is a large, brown kelp that is considered one of the world's worst invasive species. Though it has not yet invaded New York waters, it has invaded the Pacific coast of North America and, because it is such a widespread invader and has a large temperature tolerance, it is expected to invade the East coast of the United States. Learn more about this species on the Wakame NEMESIS webpage.

Asian Horseshoe Crab
Since 2011, Asian horseshoe crab species (Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda) have been imported into the United States from Southeast Asia to be used as bait in the eel and whelk commercial fishing industries. Even though the animals imported are dead, the parasites or disease causing pathogens on or within them may not be. The introduction of these non-native, unwanted organisms can impact local fisheries, including the Atlantic horseshoe crab (Limulus polyphemus) and the animals that depend on their populations, and endanger human health. Learn more about the Asian horseshoe crabs in the USA and the IUCN's effort to end their importation.