Classification of Organisms

The International Code of Zoological Nomenclature divides organisms into groupings going from very general to very specific. This manual uses these scientific names along with common names. To help clarify which grouping is used, the taxonomic classification system is listed below, in order from most general (kingdom) to most specific (species).

Kingdom - Benthic organisms are in the Animal Kingdom.

Phylum - Platyhelminthes, Annelida, Mollusca and Arthropoda are examples of benthic phyla.

Class - Insecta and Oligochaeta are examples of classes of benthic organisms.

Order - This is the level of identification for most organisms in this manual. Examples include Plecoptera (stoneflies) and Ephemeroptera (mayflies).

Family - Although many organisms are difficult to identify to this level without a microscope, tolerance groupings are more accurate when done with families. Some information on families (for Orders Diptera and Ephemeroptera) is included in the manual.

Genus, Species - These two parts of the classification system make up the scientific name unique to each organism. This should be written Genus species, as in Homo sapiens, the scientific name for humans.

Macroinvertebrate Descriptions
Information for this section came primarily from Thorp and Covich, 1991. The phylum and class names are in bold print, and the common names follow in parentheses.

Phylum Platyhelminthes (Flatworms)
Class Turbellaria (Planaria):
Unlike most other flatworms, almost all turbellarians are free-living. Order Tricladida has many fresh water representatives. The most common Texas genus is Dugesia, common name planaria. They have only one body opening, the pharynx; they lack an anus. The body is elongated and relatively soft. Their two eyespots near the anterior end give the organism a crosseyed look. They usually live in slower riffles. Gas exchange in planarians occurs through the body wall and is therefore not very efficient. It is suggested that they may have a limited ability to cope with reduced oxygen levels. Most planarians are negatively phototropic; they move away from light. They are usually found on the underside of rocks or other river debris. Their role in the ecosystem is one of predation or scavenging. They are a food source for other macroinvertebrates and some vertebrates.

Phylum Mollusca
Class Pelecypoda (Clams and Mussels):
These organisms, also known as bivalves, are mollusks with two shells held together with a hinge. They have a long life span. Their growth and reproductive rates are sensitive to environmental variations. Because most bivalves move very little during their adult life, they are sensitive to conditions in the surrounding area. Bivalves are filter feeders; they filter out phytoplankton and zooplankton that float in the water. They serve as a food source for a variety of organisms. Various forms of pollution can greatly affect their populations. Of particular importance are heavy metals, chemical wastes and organic sewage effluent. Low levels of dissolved oxygen can also be detrimental to this group. Of particular importance in Texas rivers is the Asiatic Clam (Corbicula), an exotic clam introduced from China. It is easily distinguished from native clams and mussels by its thick, ridged shell with toothed hinge and triangular shape. Native clams and mussels have a thinner, smoother shell with an integrated tough membranous hinge and are more oval or oblong in shape.

Class Gastropoda (Snails and Limpets):
This is a class of single shelled mollusks. They are herbivores or detritivores and are able to tolerate temperature variation from 0 - 40 degrees Celsius. They can give a more varied picture of water quality than bivalves because of their mobility. Limpets have cone-shaped shells and are generally considered to be grazers, choosing diatoms as their main food source. Freshwater mollusks prefer alkaline water (pH of 8-9) with sufficient amounts of calcium for shell production. The waters of Central Texas provide both of these. Pouch snails (left-handed shell) breathe air and can tolerate water of poor quality. Gilled snails (right-handed shell) breathe oxygen dissolved in the water and prefer water of good quality. The difference makes these snails useful bioindicators.

To determine if the shell is left or right-handed, use the following procedure: Hold the snail with the opening towards the observer and with the tip upward. Observe which side the opening is on. See the Key and Key Card for sample diagrams.

Phylum Aschelminthes or Nematoda (Roundworms):
Roundworms are usually distinguished from many other worms by their smooth body, lacking segments. They exhibit a characteristic constant whiplike movement. These organisms are considered reliable indicators of water pollution. Heavy metals and other pollutants settle out and attach to the organic matter in the sediments. Nematodes living in the sediments ingest the materials. Some species of nematodes can withstand high levels of pollutants, and they can be a dominant species in heavily polluted areas. Many nematodes feed on microorganisms and waste products of benthic animals. Some species of nematodes are found in anaerobic sediments. Extreme habitats for freshwater nematodes include low-temperature snow pools and high-temperature hot springs. One species was found in a pool at 63.1 degrees Celsius, the hottest habitat for any multicelled life form.

Phylum Annelida
Class Hirudinea (Leeches):
Leeches are dorsoventrally flattened with an oral and caudal sucker. Leeches form an important component of the benthos in a wide range of aquatic habitats. They feed on chironomids, amphipods, mollusks and oligochaetes and provide a food source for some fish. The literature reports that leeches have lived for extended periods in anoxic as well as hyperoxic conditions. Some research shows that leeches tend to adjust their oxygen intake based on what is available in the environment. Other research indicates that leeches are regulators, displaying a constant oxygen uptake over a range of oxygen concentrations.

Phylum Annelida
Class Oligochaete (Aquatic Segmented Worms):
These organisms look like terrestrial earthworms, but are much reduced in size. Traditionally, segmented worms have been used as indicators of water quality. They can inhabit sediments with high levels of pollutants and low-oxygen levels. These organisms live in sediments that are sandy, silty or muddy. They are also found in pockets of such sediments in rocky habitats. Other available habitat includes aquatic vegetation of quiet rivers and ponds. Their behavior is influenced by temperature, light and physical disturbances. Most of their habitat differentiation appears to be based on food availability. Water with increased amounts of organic pollutants and decreased amounts of oxygen have greater populations of tubifex worms, a type of oligochaete.

Phylum Arthropoda
Class Arachnida
Hydracarina (Water Mites):
The term hydracarina is not a taxonomic term such as class, order or family. It refers to many families of mites that are restricted to fresh water. Water mites are the only arachnid group that is generally adapted to fresh water. They have the appearance of a tiny spider or tick, with most species being either red or green. Larval mites regularly parasitize 20-50% of the adult aquatic insects, and are specialized to exploit narrow ranges of physical and chemical regimes as well as the biological attributes of their host as parasites. Preliminary studies have demonstrated that water mites are excellent indicators of water quality because they react strongly to chemical and physical disturbances. However, due to their small size and enigmatic habits, they are often overlooked in biological studies. Water mites can inhabit a variety of aquatic environments and have been specifically grouped by habitat. Riffle mites are soft bodied, but adults of most other groups are strongly flattened and well sclerotized (hardened). These are the mites that will be commonly encountered in CRWN biomonitoring. Other groups include interstitial mites, living deeper in sand and gravel deposits, lake mites that are excellent swimmers and temporary pool mites adapted to withstand the dry phase of ephemeral pools.

Phylum Arthropoda
Class Crustacea
All crustaceans are distinguished from other arthropods by the presence of two pairs of antennae. They are similar to insects in that they have a hard exoskeleton and three major body regions: head, thorax and abdomen. In many groups, the head and thorax is fused into a cephalothorax. Differences in orders (Isopoda, Amphipoda, Decapoda) are described below. Order Isopoda and Order Amphipoda (Sow Bugs and Scuds): These two orders share the characteristic of having seven pairs of legs attached to the thorax, with the first one or two pairs specialized for grasping food. Aquatic isopods and amphipods generally share the following five ecological similarities:

A requirement for high concentrations of dissolved oxygen, thus often limiting them to cool, clean waters.
A distinct behavioral pattern of vertical migration;
A limited ability to move upstream;
Obtain much of their energy feeding on bottom substrate; and
Serve as important prey to fish.

They can be herbivores, omnivores or detritivores and are well known as consumers of organic particulate matter. Few can tolerate fluctuating DO levels or temperatures. Isopods are flattened dorsoventrally and have seven pairs of walking legs, with the posterior ones longer than the anterior ones. Only the first pair of legs is modified for grasping food. Terrestrial isopods are often known as "pillbugs". Amphipods are compressed laterally, and often bear a superficial resemblance to tiny shrimp. Their first two pairs of legs are modified for feeding. The first two pairs of regular walking legs are flexed forward and the last three backward, giving amphipods a bent or almost coiled appearance.

Order Decapoda (Crayfish, Fresh Water Shrimp):
The major distinguishing characteristic for crayfish is the presence of pincers on the end of each of the first three pairs of legs. Shrimp only have these on the first two pairs of legs. On crayfish, the first pair of pincers can be well developed and even deliver a somewhat painful pinch. Also, crayfish are dorsoventrally flattened, while shrimp are compressed laterally much like Isopods. These organisms are widely distributed in the freshwater ecosystem, but they seem to prefer shallow, littoral areas. The crayfish genera most likely to be found in Texas are Orconectes, Procambarus and Cambarus (in East Texas). There is a wide range of tolerance levels between these three groups, and they are difficult to distinguish in the field. Crayfish are found in a wide range of habitats and are important to the ecosystem for their ability to process organic material. Most are omnivorous opportunists. Furtado (coauthor of this manual) says if they can find it, they will eat it. Crayfish are also the prey of fish and birds found in these habitats. Decapods are sensitive to lowered pH (increased acid or hydrogen ions). According to "Save Our Streams" (Izaak Walton League 1992), crayfish are considered somewhat tolerant of pollutants, but need a water supply that has good levels of DO. However, it is noted that some crayfish species can extract oxygen from the air if DO levels in the water are insufficient.

Phylum Arthropoda
Class Insecta (Insects):
Many insects go through a complete metamorphosis: adults lay eggs; these hatch into larvae (growth stage), which develop into pupae (maturation stage, sometimes encased in a cocoon) which then emerges into an adult. These larvae bear little resemblance to the adult form. Examples of this include Dipteran larvae, worm like organisms, which look nothing, like the adult flies. Other insects (Ephemeroptera, Plecoptera, Odonata, Hemiptera) go through incomplete metamorphosis: adults lays eggs, these hatch into larvae that go through both growth and maturation before emerging into adults. These larvae often resemble tiny adults. Those species with wings as adults have wing pads as larvae. Although the more scientific term is larva, this stage of incomplete metamorphosis is often referred to as a nymph.

Aquatic environments provide habitat for an incredible variety of aquatic insects, both larval forms and adults. Listed below are some of the more common aquatic insect larvae found in the lower Colorado River and its tributaries.

Orders of Common Insects-Aquatic Stages

Order Ephemeroptera: Mayflies
1. Widespread; found in a wide variety of habitats.
2. Abdomen with ten segments and lateral or dorsal gills.
3. Majority has three long, thin "tails"; a few only have two.
4. Most have one claw at the end of each segmented leg.
5. Herbivores (feed on algae and other aquatic plants); detritivores (feed on detritus-leaf litter, etc.); predators (feed on other animals).
6. Different families prefer different habitats:
  1. Leptophlebiidae are highly intolerant of pollution. They have exposed abdominal lateral gills and have somewhat cylindrical or flattened bodies. Gills are double or forked on segments 2 to 6, giving them the common name of pronggills.
  2. Heptageniidae have much more flattened bodies, with horizontally oriented heads, thus the common name flatheaded mayflies. These are fairly intolerant of pollution.
  3. Caenidae (larger) and Tricorythidae (very small) are tolerant to sedimentation and lower DO levels. Their anterior gills are covered by an operculum.
  4. Ephemerellidae have soft bodies. They have feathery, lateral gills except on abdominal segment 2.
  5. Oligoneuriidae are very large mayflies with fringed hairs on forelegs. They are fairly pollution intolerant.
  6. Ephemeridae are large sized, with filamentous gills that extend upward over their backs. They tolerate low DO levels.
  7. Baetidae are small larvae, some with very long antennae up to two times the width of the head. They are fairly pollution intolerant.
Order Plecoptera: Stoneflies
1. Adapted to cooler temperatures than many other benthic groups.
2. Usually lack abdominal gills, or present on first two segments. Some have gills ventrally and laterally on thorax.
3. Very sensitive to low DO levels, not found in streams with high fluctuations of DO.
4. Two long, thin tails. Larvae have wing pads.
5. Two claws on end of each leg.
6. Most nymphs emerge from stream in spring or summer.
7. Mostly predators, some detritivores and herbivores.
Order Trichoptera: Caddisfly
1. Fleshy wormlike bodies with three pair of legs near head.
2. Hardened sclerites on first one to three segments behind head.
3. Two anal prolegs, each with one hook.
4. Many build cases or spin webs.
5. Predators/ case builders (herbivores)/ net builders (omnivores).
Order Megaloptera: Dobsonflies/Alderflies
1. Very large and aggressive.
2. Two anal prolegs, each with two hooks (dobsonfly) or long single tail filament (alderfly).
3. Body sometimes with long lateral filaments, some with gill tufts at base.
4. Intolerant of pollution and low DO.
Order Odonata: Dragonflies, Damselflies
1. Retractable lower lip (labium), some are spoon-shaped.
2. Somewhat tolerant to pollution, usually found in slower moving water or along stream edges.
3. Suborder Anisoptera (dragonfly)- almond-shaped robust abdomen with three short platelike "tails". Adult holds wings horizontally, perpendicular to body.
4. Suborder Zygoptera (damselfly)- long thin abdomen with three posterior bladelike gills. Adult holds wings parallel to body.
5. Mostly predators.
Order Hemiptera: Water Striders, Giant Water Bug, Water Boatman
1. Mouth in the form of a beak, usually extending ventrally.
2. Abdomen lacks gills, filaments and tails.
3. Air breathers, often skimming /walking on surface or scuttling along the bottom.
4. Good swimmers; since they can change location easily, they're not particularly significant as indicator organisms, but need to be counted for overall diversity.
5. Predators, detritivores, piercing herbivores.
Order Coleoptera: Beetles - Water Pennies, Riffle Beetles
1. Wide variety of appearance, habitat and feeding habits.
2. Riffle Beetle-small brown/black aquatic adults, larvae similar to caddisfly, with distinct crescent shape; intolerant to pollution.
3. Water Penny-larvae have distinct flattened round body, with legs hidden underneath.
4. Many other larvae look like small Megalopteran larvae.
5. Most without hooked anal prolegs.
6. Adults have a hardened look, not soft tissue.
Order Diptera: Flies
1. No true legs (some with prolegs-unsegmented legs), many with no apparent head.
2. Often abundant and tolerant of poor water quality .
3. Variety of Families:
  1. Simuliidae: Black fly - often abundant, attached to surface of rocks. Unique 'bowling pin' shape, swollen abdomen has sucker for attachment.
  2. Chironimidae: Midges - narrow wormlike bodies, with distinct head; one pair of prolegs on first thoracic and last abdominal segment. Characteristic vertical "wiggler" movement often observed. Some types have deep red coloration produced by dissolved hemoglobins in blood, giving them the name "bloodworm".
  3. Tabanidae: Deer fly - no obvious head or prolegs, cylindrical maggotlike body with fleshy rings. Whitish, yellowish, greenish or brownish.
  4. Tipulidae: Crane fly - body ends in a disk with eye-like spiracles used for breathing (when needed). Disk is fringed with hairs and often with fleshy lobes.
  5. Athericidae: Snipe fly - elongated, with head mostly retracted into thorax. Two long, fringed filaments are at the end of the abdomen.
  6. Stratiomyidae: Soldier fly - small distinct head on hard body, often appearing stiff and lifeless. A circle of bristles at posterior end acts as a float.
  7. Ceratopogonidae: Biting Midges - very long hairlike body, often wiggling horizontally in water.
Order Lepidoptera: Aquatic Moth Larvae
1. Typical caterpillar appearance, distinct head, three pairs of thoracic legs.
2. Aquatic species often have numerous filamentous gills on sides of body.
3. Abdominal segments 3-7 have paired ring shaped prolegs.
4. Herbivores.

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