What are invasive species? Any so-called natural community is usually under the influence of human activities at many levels. Humans add nutrients, exploit some species, often nearly to extinction, and alter habitats by chopping down vegetation and filling in of wetlands. One of the more subtle effects is the purposeful and accidental facilitation of transport of organisms from distant places to our local habitats. Many of these introductions happen on purpose. We import pets, edible plants and animals, but we also accidentally transport other species in ship ballast water, on hulls, and along with other purposeful introductions such as exotic shellfish. Many of these alien (or invasive) species arrive and simply die away because the local habitat is inappropriate or there are too few individuals to start a viable population. But all too often, invasive species arrive and become very abundant here, even to the point of exerting strong and unwelcome local ecological effects.

Unfortunately, many residents of West Meadow habitats are invasive species and are conspicuous and even causing detrimental effects on our local flora and fauna. Here are some examples.

Green crab (also known as the Shore crab). This crab, Carcinus maenas, is very abundant especially on sandy bottoms below the low tide level. Some local fishers will know this crab, because it is often used for bait, especially to catch tautog. Sounds good so far. The crab invaded our shores from Europe, arrived to the south at least 100 years ago and has since spread ever northward, now occurring in New England throughout the Gulf of Maine shoreline. However, it is a predator and efficiently locates and eats mollusks, especially shallow-burrowing juvenile hard clams (Mercenaria mercenaria) and soft-shell clams (Mya arenaria). It is probably a major predator on juvenile bivalves and this greatly reduces the chances for hard clams, which live near the surface, to grow to maturity. In the Gulf of Maine, this crab has been expanding recently and its digging activities have been destroying eel grass beds and juvenile soft shell clams. The soft shell clam Mya arenaria is being grown in the Gulf of Maine under protective plastic screens, to reduce predation.

This green crab was caught in West Meadow Creek in August, 2022.

Asian Shore Crab. Hemigrapsus sanguineus appeared in New Jersey in 1988, arrived in Long Island Sound about 20 years ago, and is now the most abundant crab on intertidal cobbly beaches. It, too, feeds voraciously on a variety of foods, especially molluscs and is capable of killing the bulk of our smaller marine snails. It has a square-shaped shell with three spines on each side. Females can produce four clutches of eggs per season, each with ca. 50,000 eggs. As a result, the species has increased rapidly in our area and is now the most abundant intertidal crab. Since its arrival, smaller snails such as the common periwinkle have nearly disappeared on our rocky shores.

Periwinkle. The common periwinkle Littorina littorea is probably the most abundant snail to be found on hard rocks and cobbles, but it, too, is an invader. It arrived in Nova Scotia or Labrador in the 19th century and has spread southward to Virginia coastal waters. Some have argued that the periwinkle has reduced the ecological range of our common mudsnail Ilyanassa obsoletus and prevents it from laying eggs in optimal rocky habitats. Periwinkles have been in decline probably because of the invasion of the Asian shore crab.

Sea Squirts. Okay, these forms are not part of our everyday experience, but in the last decade or so a number of species of sea squirts have invaded the Long Island Sound region and are rapidly overgrowing and displacing the native rocky subtidal biota. You would notice this if you snorkel and examine the lovely organisms that grow on the undersides of floating docks and on pilings below the low water line. These habitats are dominated by sea anemones, bryozoa, sea squirts, hydrozoans, many of which are colonial and are often brightly colored.

There are a number of species of invasive sea squirts but the most abundant up to recently was the colonial Botrylloides violaceus, which comes from the Pacific. It is often a bright orange color.

Dead Man’s Fingers, Codium fragile. This green seaweed has finger-like projections and is often found stranded on the shoreline. It originates in Asian waters and is a major pest throughout the world, often smothering desirable native organisms like oysters. At one time it was spectacularly abundant in Long Island Sound, but it has declined in recent years.

Codium fragile http://www.unb.ca/cemar/Codium_fragile.JPG