Aquatic Invertebrate Facts

Meek's Crayfish
Aquatic Invertebrate Facts

With their diversity of body forms and behaviors, aquatic invertebrates are simply fascinating. They are part of our rich natural heritage and deserve to be known better.

  • Invertebrates are animals without a backbone, like crayfish, clams, snails, leeches, and insects.
  • The bodies of many invertebrates, including crayfish and insects, are supported by external “skeletons” much like body armor. In order to grow, these jointed-legged animals (arthropods) shed their exoskeletons as they develop larger new ones.
  • Snails and clams increase the size of their calcium-enriched shells as they grow.
  • Leeches, planarians, hydras, and other soft-bodied invertebrates can bend, expand, and contract for different activities such as hunting, digesting, moving, and hiding.
  • Aquatic invertebrates may live entirely beneath the water, or they may live on the surface or on the plants surrounding it. Some breathe air, others breathe water. To move, they walk, swim, float, skate, fly, or glide on their bellies — or they may not move much at all.
  • Many insects lead double lives. Dragonflies, mayflies, stoneflies, caddis flies, dobsonflies, and more are aquatic as juveniles. Then, when they undergo their final molt, they leave the water and become winged adults that fly in the air.
  • Other aquatic insects and spiders remain aquatic their whole lives. Water boatmen, predaceous diving beetles, whirligig beetles, water striders, fishing spiders, and others spend all their days hunting in and around water. Some in this group can fly, however, and are attracted to lights at night.

Aquatic invertebrates are vital to water habitats

  • They are vital links in the aquatic food chain, conveying nutrients from plants and algae to larger organisms such as fish, frogs, salamanders, reptiles, birds, and mammals.
  • Many fish depend on aquatic invertebrates for food. Some fish specialize in eating aquatic snails. Some eat mainly crayfish. Others prefer stoneflies, or mosquito larvae — and so on.
  • Hellgrammites, crayfish, and others are popular bait for anglers. Many artificial lures resemble real-life invertebrates.
  • Invertebrates can tell us about water quality. They are sensitive to changes in sediment load, pollutants, pH, and more. If their numbers decline, it could mean something’s going wrong that we need to pay attention to.
  • Some species — notably mosquitoes — are vectors for disease, with serious consequences for people. Many other aquatic invertebrates eat those species.
  • Harvesting freshwater mussels for button manufacturing used to be a major industry in Missouri.
  • Many people eat crayfish, and Missouri has recreational fisheries for them.
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View aquatic invertebrates in the field guide
Water springtails congregate in water above soggy dead leaves
Adult water springtails are bluish gray, often with reddish appendages. Their maximum length is about 1/8 inch.
Julianna Schroeder
1/12
Photo of a spotted fishing spider perched on the water's surface amid floating duckweed plants
Both the spotted fishing spider and minute duckweed plants are able to float on the surface film of water.
Susan Ferber
2/12
Photo of backswimmer, side view
Backswimmers rest at the water surface tilted head-downward, with the abdomen tip protruding from the water. The oarlike hind legs are usually extended downward at angles to the body.
Jim Rathert
3/12
Photo of a fishfly larva crawling among rocks in an aquarium.
Fishfly larvae look a lot like their cousins the hellgrammites, but they lack cottony or hairy gill tufts along the abdomen, and they have 2 short, fleshy tails at the hind tip.
anonymous
4/12
Photo of an aquatic isopod in an aquarium, crawling on a rock.
Some freshwater isopods look a lot like the familiar land isopods that live under rocks in gardens.
Cliff White
5/12
Photo of amphipod on a rock
Cliff White
6/12
Photo of a gravid Mississippi grass shrimp in an aquarium.
Of Missouri’s two species of freshwater shrimp, the Mississippi grass shrimp is by far the most common and widespread.
Jim Rathert
7/12
Photo of a grassland crayfish, also called prairie crayfish.
The grassland crayfish inhabits prairies and grasslands in northern and western Missouri.
Chris Lukhaup
8/12
Photo of a bristly cave crayfish, viewed from the side.
The bristly cave crayfish lives in caves in the Springfield Plateau region of the Ozarks.
Jim Rathert
9/12
ramshorn snail
This ramshorn snail is probably Helisoma subcrenatum, the rough ramshorn. Note the prominent growth lines on the shell that mirror the ongoing development of the outer lip. This individual, along with many others of the same species, was found at Maramec Spring Park, south of St. James. This species has a wide distribution.
MDC staff
10/12
Leech on rocks underwater
Leech. Many leeches have attractive speckled or striped patterns. A leech's head is usually at the more tapered (narrower) end.
Jim Rathert
11/12
Bryozoan colony cut in half
Bryozoan colony, cut in half. Bryozoan colonies may be found free-floating or attached to submerged rocks, dock supports, logs, plastic debris, or vegetation.
MDC staff
12/12
Water springtails congregate in water above soggy dead leaves
Photo of a spotted fishing spider perched on the water's surface amid floating duckweed plants
Photo of backswimmer, side view
Photo of a fishfly larva crawling among rocks in an aquarium.
Photo of an aquatic isopod in an aquarium, crawling on a rock.
Photo of amphipod on a rock
Photo of a gravid Mississippi grass shrimp in an aquarium.
Photo of a grassland crayfish, also called prairie crayfish.
Photo of a bristly cave crayfish, viewed from the side.
ramshorn snail
Leech on rocks underwater
Bryozoan colony cut in half