Some have big, pointy teeth. Some let loose a celebration of purple, flowery spikes. Some live in large seashells that would fit nicely in any collector's stash.
But they all have one thing in common: They don't belong in Chesapeake Bay.
"Invasive species" take many forms. Here is a look at 10 that threaten the bay's fragile ecosystem in one way or another.
1. BLUE AND FLATHEAD CATFISH
Description: Blue catfish are silvery in color and have a deeply forked tail. Flatheads look different than most catfish species; they have a squarish tail and brownish, speckled sides. Both can weigh 100 pounds or more.
Native region: The Mississippi, Missouri and Ohio river basins
How they got here: Blue catfish were released in the James, Rappahannock and York rivers in the 1970s and '80s as a game fish. Flathead were brought to the James in the 1960s, also for recreational fishermen.
Prevalence: Blue catfish can be found in the main stem and in every tributary. Electrofishing in the tidal parts of the James and Rappahannock show they can make up as much as 75 percent of the total fish biomass. Flathead tend to stay in the upper portions of tributaries, where water has a lower concentration of salinity.
Effect on the bay: Blue catfish present the bigger concern because of their wider range and rapid growth in numbers. They consume native fish and compete with native predators for food.
2. MUTE SWANS
Description: A large swan with all-white plumage and an orange beak, with black around the eyes. They are less vocal than other swan species, given to grunt and hiss instead.
Native region: Europe, Asia and northern Africa.
How they got here: Brought to America from Europe in the 1800s as decoration for parks, zoos and private estates. Turned wild in Maryland when five escaped from a collection along the Miles River in Talbot County in 1962.
Prevalence: In Maryland, mute swans numbered 3,955 in 1999. Today, all but 100 have been eradicated.
Effect on the bay: Their preferred food is underwater plants, the bedrock of the bay's ecosystem. Unlike native tundra swans, mute swans don't migrate, so they continue to feed during the critical summer growing season.
3. NORTHERN SNAKEHEAD
Description: Described by the Washington Post as "one ugly fish" and "looking like a weird cross" of a Burmese python, a barracuda and an electric eel.
Native region: China
How they got here: Found in a pond near Crofton, Md., in 2002, they were quickly eradicated. But snakeheads soon reappeared in tributaries in Maryland and Virginia, likely castoffs from unwitting aquarium owners.
Prevalence: Snakeheads have found the Potomac River to their liking. Scientists say they will probably never be fully eradicated. All that can be done is to manage the prolific reproducers.
Effect on the bay: Consumes native fish and competes with larger fish for food. It may also transmit disease and parasites, including some that affect humans.
Description: Large rodents that grow up to 2 feet long and 12-15 pounds. They have dense, grayish-brown fur, rounded tails and comically large, orange front teeth.
Native region: South America
How they got here: Released in the 1940s after fur farms failed.
Prevalence: At one time, 35,000-50,000 inhabited Blackwater National Wildlife Refuge. Managers reported in 2004 they had eradicated nutria from the park. From 2000-2011, the federal Chesapeake Bay Nutria Eradication Project devoted $13.8 million to efforts to rid Delmarva of the animal. Pockets of nutria are still being found on Delmarva.
Effect on the bay: Eats the roots, rhizomes and tubers of marsh plants, exacerbating wetland loss in the bay. The large front teeth and strong feet are used to dig in the mud for the more-delicate parts of the marsh. These "eatouts" can turn marshes into silty lakes. A nutria can eat up to 25 percent of its body weight each day.
Description: In Greek, "phragma" translates to "fence," and that gives a good idea of its appearance. The reed-like plant can grow up to 20 feet in height. Feathery plumes wave in the breeze atop the stalks. Foliage is gray-green in the growing season and brown in the fall.
Native region: Europe and Asia.
How they got here: Whisked to North America in the ballast of ships in the 19th century.
Prevalence: Grows all around the Chesapeake in fresh and brackish wetlands and along river banks.
Effect on the bay: Forms dense stands that muscle out native plants, blocking light to their seeds and occupying growing space. Some scientists contend they may help the bay in some ways, however. Unlike other native plants, they keep pace with sea level rise, providing a breakwater that could preserve marshland, they argue. Blackwater Marsh near Cambridge, the largest of the bay's marshes, can credit phragmites for its survival.
Invasive species in Chesapeake Bay that didn't quite make this list:
• Emerald ash borer
• Chinese mitten crab
• European gypsy moth
• Japanese stilt grass
• Rusty crayfish
• Didymo, aka "rock snot"
• Water lettuce
HOW YOU CAN HELP
• Familiarize yourself with what invasive species look like and report colonies to authorities.
• Do your homework before planting an unfamiliar specimen in your yard. Ask the seller whether it is invasive.
• Keep from disturbing too many areas on your property, because invasive plants are honed to take root and multiply in such areas.
• Consider planting native flowers, shrubs and trees.
• Clean boat hulls thoroughly before transporting them to new waterways to ward off aquatic hitchhikers.
• Don't toss aquarium species or unused bait into drains or waterways.
• Wear rubber-soled wading boots instead of felt, which can more easily transport small organisms, and disinfect gear with detergent or a salt solution when moving among waterways.
6. PURPLE LOOSESTRIFE
Description: The most eye-pleasing species on this list, purple loosestrife unfurls purple or magenta flowery spikes in mid- to late-summer. It's a tall, branching herb that can reach 8 feet in height. Leaves are long and thin, pointed at the end and smooth on the edges. Stems are square and a little fuzzy.
Native region: Europe and Asia
How they got here: Seeds also clung to the wool of sheep brought over by early colonists. Early New Englanders planted loosestrife for its perceived medicinal qualities. Imported intentionally in the 1800s as an ornamental plant, unintentionally in ship ballast.
Prevalence: Throughout the Chesapeake watershed in dense colonies.
Effect on the bay: Loosestrife has been known to displace up to 50 percent of native plants in wetland areas. In the Chesapeake, it's a threat to cattails and bulrushes. In some places, it has replaced valuable habitat for animals such as muskrats and the endangered bog turtle.
7. ZEBRA MUSSELS
Description: A small bivalve, usually no more than 2 inches in length, with distinctive light and dark stripes on a triangular or D-shaped shell.
Native region: The Caspian and Black seas of Eastern Europe
How they got here: First discovered in North America in 1988 in the Great Lakes region, zebra mussels probably arrived via ship ballast. By 2002, they had spread to the upper Susquehanna River.
Prevalence: Found often in thick, barnacle-like clumps on solid surfaces in the upper bay, the Susquehanna and its tributaries.
Effect on the bay: Attaches to freshwater mussels, clams and crayfish, and smothers them. They can also damage municipal pipes and boats by clinging to them in large colonies. These filter feeders can consume food otherwise eaten by native fish.
8. WATER CHESTNUT
Description: An annual herb with floating, triangular-shaped leaves linked to a central stem rooted in the sediment below.
Native region: Europe
How they got here: First North American sighting in Concord, Mass., in 1859. First Maryland sighting in 1923 in a 2-acre swath in the Potomac near Washington, D.C. It started as an ornamental in ponds but soon escaped to the wild.
Prevalence: Found in Maryland in the Bird and Sassafras rivers.
Effect on the bay: The thick mats can darken the bay bottom, shading out native aquatic plants and setting off repercussions up the food chain. Not to mention the spines on its seeds can pierce shoe leather. Although volunteer-led efforts have reduced water chestnut "harvests" to just a handful of bushels a year, the seeds can remain viable for up to 12 years.
9. GREEN CRAB
Description: Despite the name, green crabs may be brown, gray, red and, yes, green. Look for black spots on the "palms" of its claws. They are smaller than native blue crabs, their bodies reaching only 3 inches in diameter.
Native region: Northeastern Atlantic
How they got here: Stowaways in the dry ballast of 19th century ships or clinging to the hulls in mossy crevices.
Prevalence: Relatively scarce but, if found, it's in the coastal bays or the mouth of the Chesapeake. Green crabs were first reported in the Chesapeake region in 2007 by a Maryland Department of Natural Resources employee, who caught one in the upper Manokin River.
Effect on the bay: No known impacts on the bay itself, but it disrupted the clam industry in New England for decades. Environmental officials fear it could threaten efforts to restore shellfish populations in coastal bays. But the region marks the crabs' southernmost range, so warming seas caused by global climate change could nudge it out of Delmarva's waters entirely in the future. So, kudos for that, greenhouse gases.
10. VEINED RAPA WHELK
Description: A hungry snail that lives in a large, rounded shell that is as wide as it is long — about 6 or 7 inches. Color may be gray to reddish-brown, with dark brown dashes on the spiral ribs. The inside of the shell is a deep orange. If you found it on the beach, you'd want to keep it.
Native region: The western Pacific, the Sea of Japan, Yellow Sea, East China Sea and Bohai Sea
How they got here: The going theory is via ship ballast.
Prevalence: First discovered in the lower bay by Virginia Institute of Marine Science researchers in 1998, it can now be found along the bottom of the western shore of Virginia from New Point Comfort to Cape Henry and up the James and York rivers. Their exact numbers are unknown.
Effect on the bay: They have a voracious appetite for shellfish and may pose a threat to the clam fishery.
Sources: Maryland Sea Grant, Maryland Department of Environment, Maryland Department of Natural Resources, Chesapeake Bay Program, U.S. Fish and Wildlife Service, Smithsonian Environmental Research Center, U.S. Geological Survey.