Freshwater Invertebrates


Dragonfly larva are fierce predators of fish and Dragonfly LarvaFreshwater invertebrates make up an overwhelming amount of Alabama's biodiversity. The most commonly encountered freshwater invertebrates of Alabama are represented by seven phyla: Annelida, Arthropoda, Mollusca, Aschelminthes, Cnidaria, Platyhelminthes, and Porifera. These invertebrates all spend at least a portion, if not all, of their lifecycle in aquatic ecosystems. Many are valued for their roles either as food sources in their biological communities, as biological indicators of a healthy habitat, or as endemic species that exist only in Alabama and nowhere else in the world.

Alabama is home to 180 different species of mussels from the phyla Mollusca, which accounts for about two thirds of all the mussels found in North America, making Alabama the state with the most types of freshwater mussel. Alabama also has 83 species of crayfish, which is more than any other state. Six of these crayfish species live only in caves or underground streams in Alabama. There are also 102 known species of aquatic snails (many of which are extinct), including six endemic aquatic snails such as the endangered cylindrical lioplax (Lioplax cyclostomaformis), the endangered flat pebblesnail (Lepyriam showalteri), the endangered plicate rocksnail (Leptoxis ampla), the threatened painted rocksnail (Leptoxis taeniata), the threatened round rocksnail (Leptoxis ampla), The tiny Alabama cave shrimp (Palaemonias alabamae) is Alabama Cave Shrimpand the threatened lacy elimia (Elimia crenatella). All six of these are found only in the Mobile River Basin. These animals often go unnoticed by the public, and thus conservation and protection of Alabama's freshwater invertebrates and education about them are of great importance.

Phylum Annelida 

The phylum Annelida is made up of four classes, Hirudinea, Oligochaeta, Polychaeta, and Archiannelida. The class Hirudinea contains leeches, which are aquatic worms with bodies made up of 34 segments. They inhabit lakes, marshes, ponds, and streams with little to no currents. They prefer warm and shallow waters where they can hide from the light. Most leeches are tolerant of polluted waters because some species are able to survive several days without oxygen. Leeches are either blood-sucking parasites or scavengers; there are very few species that are exclusively blood-suckers. The class Oligochaeta is made up of slightly smaller aquatic segmented worms, such as the cave worm (specifically from the order Branchiobdellida) that can live in any source of water, including polluted water, due to their low requirement for oxygen. They are structurally similar to the common terrestrial earthworm. All oligochaetes are hermaphroditic, which means that individuals have both male and female reproductive organs. Although they are hermaphrodites, they can reproduce both asexually and sexually. Oligochaetes gather nutrients by ingesting ooze from the bottom of their freshwater habitats. The class Polychaeta consists of eyeless tubifex worms. These worms live in the sediments at the bottom of sewage-polluted water sources that are at least one meter deep. Tubifex worms consume mud and extract nutrients, and during this process they construct mud tubes. The class Archiannelida is made up of small primitive worms that look very similar to larval stage polychaetes. They are externally unsegmented.

Phylum Arthropoda 

The phylum Arthropoda is represented in Alabama by three classes: Arachnida, Crustacea, and Insecta. The class Arachnida includes water spiders and water mites. Water spiders typically spend their time moving across the surface of the water, but can stay submerged under water for up to 45 minutes. The crayfish, also called crawfish or crawdad, is CrayfishWater mites live entirely underwater. Typically, they will use their legs to swim awkwardly; those that are unable to do this spend their time crawling on plants or substrates at the bottom of the water sources. They are usually found along the shorelines of lakes, ponds, streams, rivers, and marshes.

The class Crustacea is made up of five orders, Amphipoda, Cladocera, Copepoda, Decapoda, and Isopoda. The order Amphipoda are commonly called scuds, which are very small, omnivorous, shrimp-like crustaceans that live in pools, ponds, brooks, streams, and lakes. Cladocera are small zooplankton, commonly called water fleas, or daphnia, which live in most freshwater habitats. Species in the order Copepoda, like those in Cladocera, are very small and have many visible segments. Copepods vary greatly in their feeding behaviors, ranging from parasites to detritus-feeders that eat dead organic matter in the water. The most common freshwater members of Decapoda are commonly known as crayfish. They are omnivorous scavengers that burrow in the sediments of all freshwater ecosystems, ranging from permanent to ephemeral freshwater habitats. The order Isopoda contains sowbugs, which are also omnivorous scavengers that tend to live under rocks or debris in shallow freshwater habitats, but they are also very common in groundwater systems.

The class Insecta is very large, consisting of nine orders: Coleoptera, Collembola, Diptera, Ephemeroptera, Hemiptera, Megaloptera, Odonata, Plecoptera, and Trichoptera. The order Coleoptera is made up of diving beetles, riffle beetles, water penny beetles, and whirligig beetles. Collembola is an order of animals commonly known as springtails. The order Water striders are predatory insects of the order Water StriderDiptera consists of biting midges (no-see-ums), black flies, crane flies, deer flies, horse flies, midges, mosquitoes, rattailed-maggots, and watersnipe flies. Mayflies make up the order known as Ephemeroptera. The order Hemiptera contains backswimmers, water boatman, water scorpions, and water striders. Megaloptera is made up of alderflies, dobsonflies (otherwise known as hellgrammites at the larval stage), and fishflies. The order Odonata consists of dragonflies and damselflies. Stoneflies make up the order Plecoptera, and caddis flies make up the order Trichoptera.

Phylum Mollusca 

The phylum Mollusca is the most important phylum of freshwater invertebrates in Alabama, containing 61 percent of the world's mussels. This phylum consists of as many as eight classes, with two, the Gastropoda and the Bivalvia, being the only classes represented in the state. The Gastropoda includes gilled snails, limpets, pouch snails, and orb snails. All gastropods remain permanently attached to their shell through the columellar muscle, which also allows them to retract their head and foot into the shell. Gilled snails, such as the rocksnail and hornsnail, have leaf-like gills on their mantle, which is the part that protrudes outside the shell, and a shell that is spiraled clockwise. They feed primarily on algae or other decaying material on the bottom substrates of free-flowing streams and rivers. Limpets have a broad cone-shaped shell and feed mostly on algae or other decaying matter. They are hermaphroditic, which allows them to reproduce either by mating or by self-fertilization. Pouch snails have a shell that is spiraled counterclockwise and breathe with lungs rather than gills. They are hermaphrodites like the limpets. Ramshorn snails have flat coiled shells and feed primarily on algae and other decaying organic material.

The fanshell (Cyprogenia stegaria), of the phylum Mollusca, FanshellThe class Bivalvia consists of the bivalve mollusks such as fingernail clams, freshwater mussels, Asian clams, and zebra mussels. Zebra mussels attach themselves to submerged objects in clusters, but the other species move freely through the substrate. The fingernail clam is a very small filter feeder that is native to the southeastern United States but occurs worldwide. Freshwater mussels are also natives, and they come in a variety of shapes and sizes. They can be round, oval, rectangular, trapezoidal, or triangular and most typically appear as separate male and female organisms. Some species are hermaphroditic, however. Larvae of freshwater mussels are called glochidia, and they parasitize fish for up to several months before they fall off and finish their development in the sediments of a stream. The Asian clam is an invasive species with a triangular shell that first appeared in North America in 1938 in the Columbia River in the Pacific Northwest. The clam produces large amounts of offspring, allowing it to overtake the native mollusks. Zebra mussels are another invasive species that look like small clams with zebra-like patterns on their shells. They are native to the Caspian Sea in Asia and were probably brought to the United States in the 1980s in the ballast water of ships. They are a major problem in some areas that they have invaded (although not Alabama) because of their high reproductive rates and their ability to attach to virtually anything.

Phylum Aschelminthes 

Rotifers (Keratella cochlearis), of the phylum Aschelminthes, are RotiferThe phylum Aschelminthes is made up of one class, Rotifera. The rotifers are microscopic, transparent organisms that have an elongated, cylindrical body made up of a head, midsection, and foot. These organisms can be found either attached to a submerged object or swimming freely. They are omnivorous and will sometimes even consume each other. They can be found living in puddles, ponds, damp soil, lakes, sand, or even rock crevices.

Phylum Cnidaria 

The only class of the phylum Cnidaria that lives in freshwater is Hydrozoa, referred to as hydras. They have radially symmetrical, columnar bodies with between five and eight tentacles on only one end of the body. Hydras are small (between only 1 and 25 mm long) and carnivorous, eating anything from tiny aquatic invertebrates to small fish. They can reproduce asexually or sexually if hermaphroditic.

Phylum Platyhelminthes 

The phylum Platyhelminthes contains one class, Turbellaria, known as free-living flatworms. Turbellaria are very small, elongated worms that can be flat, spindle-shaped, leaflike, or cylindrical. Most of their bodies are covered with cilia, which are hair-like structures that the flatworm uses to move itself. Turbellaria reproduce both sexually and asexually, by splitting in two. Flatworms can be found in unpolluted freshwater sources in the substrates at the bottom, or on the underside of the surface of the water.

Phylum Porifera 

One class, Demospongea, commonly known as freshwater sponges, represents the phylum Porifera. These sponges are green, brown, or yellow and grow in mat-like sheets partly attached to submerged objects that are flowing with the water. Sponges do not have any organs. They reproduce sexually either as hermaphrodites or as separate male and female organisms. Freshwater sponges are typically found in shallow slow-moving water. They feed by straining small particles from the water that flows through them.

Additional Resources  

Common Aquatic Flora and Fauna of the Tennessee Valley. Booklet 4, Water Quality Series, Chattanooga, Tenn.: Tennessee Valley Authority, 1994.

Mirachi, Ralph E., ed. Alabama Wildlife Volume One, A Checklist of Vertebrates and Selected Invertebrates: Aquatic Mollusks, Fishes, Amphibians,  Reptiles, Birds, and Mammals. Tuscaloosa: University of Alabama Press, 2004.

Smith, Douglas Grant. Pennak's Freshwater Invertebrates of the United States: Porifera to Crustacea. New York: John Wiley & Sons, 2001.

Thorp, James H., and Alan P. Covich, eds. Ecology and Classification of North American Freshwater Invertebrates. San Diego: Academic Press, 1991.

Stefanie Haba
Auburn University


Published August 13, 2007
Last updated June 13, 2013