Aquatic Life: Invertebrate

An invertebrate is an animal that doesn't have a backbone or any form of internal skeleton. Instead, they have some form of exoskeleton, which gives their bodies structure and stability. We shall divide our aquatic invertebrates up in groups that we can more easily analyze. The first group of invertebrates that we will look at is the mollusks. Mollusks are typically split into three groups: Pelecypods (bi-valve or double-shelled), Gastropods (uni-valve or single-shell), and Cephalopods (no external shell). Cephalopods are things such as squid, cuttlefish and octopus, all of which are marine species. Our focus will be on Pelecypods and Gastropods, which can be found in freshwater ecosystems as well as marine ecosystems.

 

Pelycepods are typically filter feeders. As water moves through their shells and siphoned into their bodies, oxygen and nutrients are filtered out of the water and into the tissues of the organism. The water is then siphoned back out once the oxygen and food has been depleted. There are two different types of pelycepods in freshwater ecosystems. There are freshwater mussels and there are freshwater clams. Both types have gills through which they obtain oxygen from the water. Because these organisms are gill breathers, water quality is very important for their survival, however, some are more tolerant than others. Freshwater clams are greatly affected by changes in water quality and are very often one of the first groups of aquatic organisms to disappear when water quality starts to decline. Some of the freshwater mussels on the other hand, manage to tolerate the change but their numbers tend to drop significantly if the water degradation persists for a long period of time. When present in large numbers, they play an important role by filtering a lot of harmful nutrients out of the water that might harm other organisms. Low numbers of mussels might suggest that impairment of the stream is occurring or has already happened. There are several species of freshwater clams and mussels found in North Carolina. Many of these are declining and some are endangered. Freshwater clams and mussels are usually found in sandy bottom streams but occasionally occur in rocky streams as well. They are a favorite food source for many species of fish, particularly bream species (bluegill, red-breasted sunfish, red-eared sunfish, etc.) that are also known as shellcrackers.   

 

The other group of mollusks in freshwater ecosystems is the gastropods or snails that are all single-shelled. Snails can be divided into two groups, those that breathe with gills (prosobranchs, pronounced proso-branck) and those that breathe with lungs (pulmonate). These snails typically feed on detritus (decaying matter) and aquatic plants such as algae. Snails with lungs tolerate poor water quality conditions very well. They are typically bottom-dwellers, in fairly shallow water, that will occasionally crawl up a rock to the surface to take a breath and then go back to the bottom again.

 

There are three types of pulmonate snails: spiral shell, coiled shell, and conical shell. Gilled snails or prosobranchs on the other hand, require good water quality conditions. They prefer fast moving, highly oxygenated water. These snails are usually found in very shallow water attached to rocks where they dine on the microscopic algae and detritus that accumulate. Prosobranchs are all spiral shelled and have a leathery "shoe-like" structure on the bottom of their foot called an operculum. These snails are also right-handed whereas spiral shelled pulmonate snails are left-handed and lack an operculum. Freshwater snails are an important food source for fish. Prosobranchs, which are most of the time fairly small, are also a favorite food source for crayfish, which are another important water quality indicator.