What is an Invasive Species?

Scientific name: Codium fragile  Common name: Deadman’s Fingers, Green Fleece
Contributed by Abby Deitz
Bigelow Laboratory for Ocean Sciences
West Boothbay Harbor, ME 04575
One of the most harmful disturbances to any ecological system is invasive species, a growing, generic and expensive problem that is not limited to the marine environment (1). Due to increased global commerce and transportation, the marine environment is experiencing extensive ecological changes in response to invasive species (2). Known by a variety of names such as bioinvaders, nonindigenous, exotic, or simply nuisance species, these organisms can become abundant when introduced to a new ecosystem. Such introductions may be mediated by humans, either by accident or deliberately, or they may occur through natural processes such as ocean currents. A non-indigenous species may be present but remain in low abundance until something changes that increases its ability to compete with native species. This could be a change in temperature that increases reproduction, or a change in abundance of a previous competitor or predator that enables the invasive to become established. Once established, the invasive might change the pattern of a community for a long time. Another, often neglected issue, is the role of introduced species in the spread of infectious diseases (3). In contrast, some invasive species cause few changes in the native community (2).

A principal concern, because widespread and at least theoretically avoidable or reducible, is the human role in the increasing occurrence of marine invasives. Biological invasions can dramatically alter community structure and affect ecosystem function, causing considerable economic damage, and threaten biodiversity. Besides the development of land for human use, the intentional or accidental introduction of new species by humans is a leading cause global diversity losses (4). Biodiversity loss promotes invasion and, unfortunately, successful invasions may further decrease biodiversity (5).

The sum of these effects has prompted interest in understanding why and how invasions occur. Many marine organisms experience population cycles that fluctuate. This ebb and flow within populations may contribute to the inability of communities to resist invasion, especially when a population is at a low point. In some cases, invasive species suffer less from their indigenous parasites–organisms that would normally restrict growth rate, control population density and limit individual sizes (6). Research has shown that communities that have abundance and diversity among native species, appear to be better protected from invasion, particularly in the case of available space (5).

How is the Gulf of Maine affected by invasive species?

A growing number of marine invertebrates have relocated to the waters of the Gulf of Maine. Research has pointed to two regional trends that may be responsible for expansion of marine invasives to the Gulf of Maine: (a) warming of coastal waters, and (b) lower biodiversity, a characteristic of “stressed habitats.” (6). Urbanization and development of shore land leads to “stressed habitats” that experience an increase in human activity and pollution (8). Global warming may relax the temperature limit that previously protected the Gulf of Maine from invaders native to warmer or more temperate waters, allowing them to establish and persist (9).

Invasive species have broad impact by occupying space and competing for food, altering gene pools of native organisms through crossbreeding, shifting predator/prey relationships, and potentially spreading disease/parasites. Examples of high-impact invasives include:

  • Codium fragile, a green alga believed to have originated in Japan, has proven to be opportunistic seizing space after the collapse of kelp beds, once established its thick, bushy structure prevents foraging of the ocean bottom by organisms like ground fish and lobsters;
  • Membranipora membranacea a surface growing bryozoan, originating in Europe, is often found growing on kelp. Bryozoans are colonies of very small animals that form a tough, stiff crust over the flexible surface of the kelp blade. Its greatest impact on the subtidal ecosystem has been its role in the loss of kelp beds. Kelp blades that are heavily encrusted with Membranipora, become brittle and susceptible to breakage during storms;
  • pathogens, most common in aquaculture, are transferred with infectious stock (8);
  • nuisance problems of fouling on gear, nets, and other structures.

A list of prominent, established species, that are not native to the Gulf of Maine, are listed below.

Scientific name Common Name
Algae (Chlorophyta) green algae
Codium fragile Deadman’s Fingers, Green Fleece
Algae (Rhodophyta) red algae
Bonnemaisonia hamifera Red Alga
Grateloupia turuturu Red Alga
Lomentaria clavellosa Red Alga
Lomentaria orcadensis Red Alga
Neosiphonia harveyi Red Alga
Porifera (Sponges)
Halichondria bowerbankia Bread-Crumb Sponge
Cnidaria (Hydroids, Anemones, and Jellyfish)
Cordylophora caspia Colonial Hydroid
Diadumene lineata Striped Anemone
Sagartia elegans Purple Anemone
Polychaeta (Segmented Worms)
Janua pagenstecheri (formerly known as Spirorbis pagenstecheri) Bristleworm
Gastropoda (Snails)
Littorina littorea Common Periwinkle Snail
Bivalvia (Clams, Oysters, and Mussels)
Ostrea edulis European Oyster
Arthropoda (Crabs, Shrimp)
Praunus flexuosus Mysid Shrimp
Ianiropsis sp. Isopod
Caprella mutica Skeleton Shrimp
Microdeutopus gryllotalpa Amphipod
Carcinus maenas European Green Crab
Hemigrapsus sanguineus Asian Shore Crab
Anisolabis maritima Maritime Earwig
Bryozoa (Ectoprocta)
Barentsia benedeni Entoproct
Bugula neritina Bryozoan
Membranipora membranacea Lacy Crust Bryozoan
Ascidiacea (Tunicates)
Ascidiella aspersa Tunicate
Botrylloides violaceus Tunicate
Botryllus schlosseri Golden Star Tunicate
Didemnum lahillei Tunicate
Diplosoma listerianum Tunicate
Molgula manhattensis Sea Grapes
Styela canopus (formerly Styela partita) Tunicate
Styela clava Club tunicate
Haplosporidium nelsoni Eastern Oyster (Crassostrea virginica) parasite
Bonamia ostreae European Oyster (Ostrea edulis) parasite
Perkinsus marinus Eastern Oyster (Crassostrea virginica) parasite

How do invasive species arrive in the Gulf of Maine?

There are a variety of ways that marine invasive species find themselves in the Gulf of Maine. The process by which many of these species are transported appears to be related to ballast water (1; also a means for the transport of pathogens, 11). Empty cargo ships carry millions of gallons of ballast water to stabilize the vessel for ocean crossings. When a ship reaches its destination it releases water in preparation for bringing cargo aboard. The water that is released may carry in it living organisms. Other methods of transportation include:

  • hull attachment on vessels of all sizes (including trailered recreational boats and the trailers themselves) and other marine platforms such as barges or dry docks that are moved between ports;
  • algae used as packing material for fishery products;
  • fouling, or accumulation, of organisms in fishing nets.

Introductions of species also occur when aquarium hobbyists release (on purpose or accidentally) exotic species, and when aquaculture operations are mismanaged.

What is the role of the GoM Area Program in understanding invasive species?

“The GoM Area Program exists to explore and describe the region’s biodiversity and explain the patterns of distribution and abundance and the processes that control them” To be comprehensive, we need to understand the extent to which invasive species are a part of the Gulf of Maine ecosystem. Identifying invasive species and their immediate and long-term effects on native habitats is essential to characterizing the present-day Gulf of Maine and how it meets future challenges.

For more information…


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