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Unit 10 Notes: Adaptations and Organisms in Freshwater Habitats
2013
I. Freshwater Macroinvertebrates
A. General Characteristics
Freshwater macroinvertebrates are animals without backbones that live in or near the bottom of freshwater
ponds, lakes, streams, and rivers for all or part of their life cycle. They are very important indicators of the
health of these bodies of water, reflecting the quality of the water and the conditions of the habitat. Freshwater
insects are a large group within the macroinvertebrates found in bodies of freshwater. They make up 90% of the
organisms living at the bottom of a stream. Organisms that live on the bottom are called benthic. Of the 2
million known insect species, there are over 80,000 species that fall into the macroinvertebrate category.
Examples include mayflies, dragonflies, and beetles.
Other benthic macroinvertebrates that do not fall into the insect group include crayfish, clams, snails, worms,
and leeches.
B. Aquatic Adaptations
Freshwater macroinvertebrates are usually grouped by three characteristics: how they move, how they feed, and
their tolerance to pollution (intolerant, moderately tolerant, or tolerant)
How They Move:
In order to adapt to the aquatic environment, macroinvertebrates have to be able to move or maintain their
position in the water.
Type
Description
Planktonic
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Divers
Swimmers
Clingers
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Skaters
Sprawlers
Climbers
Burrowers
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Adapted for movement on the water’s surface
Scavenge on organisms caught in the surface film
May float and swim in open water or float at the surface to get oxygen or food; can dive when
alarmed
Inhabit open water (limnetic zone) of lakes, ponds, and bogs (lentic systems)
Can swim by rowing with the hind legs in lentic habitats and lotic pools
Adapted for fish-like swimming in lotic and lentic habitats
Construct shelters, have long claws and flattened bodies for attaching to rock or other surfaces in
lotic riffles and wave-swept rocky littoral zones
Live on the surface of floating aquatic plants or fine sediments
Adaptations include long legs
Live on overhanging branches, logs, roots, or aquatic macrophytes
Adapted for climbing plants or debris
Inhabit fine sediments of streams (pools) and lakes
Adapted for digging
How They Feed:
Feeding Group
Food Preference
Feeding Habits
Shredders
Collector-gatherers
Collector-filterers
Scrapers
Piercers
Live or dead aquatic plant material
Small bits of decomposed organic matter
Small bits of decomposed organic matter
Algae, diatoms, bacteria, and fungi found attached to
plants or rocks
Live aquatic plants
Predators
Live animals
Chew, bore, or gouge
Gather deposits from the bottom
Filter matter out of the water
Scrape food from the surface it is
found on
Pierce into the plant and suck fluids
out
Engulf whole animals, or pierce into
the animal and suck fluids out
Pollution Tolerance
Aquatic macroinvertebrates are also put into categories based on their tolerance to pollution. Tolerance values range from
0 to 10 with 0 being the least tolerant, and 10 the most tolerant. Tolerance values are assigned to those organisms used as
pollution indicators. Freshwater macroinvertebrates are used as indicators of pollution for many reasons:
 Important part of all aquatic ecosystems
 Most live or stay over a small area
 Found in all types of aquatic habitats
 Life cycles of most groups are well
documented
 Fairly easy to collect
 Have different levels of tolerance to an
environmental disturbance
II. Freshwater Fish
A. General Characteristics
Fish are cold-blooded animals, meaning their body temperature changes with the temperature of their
surroundings. They have backbones and live their entire lives in water, breathing with gills and moving
with fins. There are 247 species of freshwater fish in Texas alone. Some of these species are able to live
in brackish (partly salty) water, such as in estuaries. All fish play an important role in the aquatic
ecosystem.
B. Fish Habitat
Each species of fish has its own distribution and range of tolerance in which it can survive or thrive. The
basic requirements for a fish’s survival include prey, cover, suitable water temperature, and dissolved
oxygen levels.
Habitat Types with Representative Fish Species
Habitat Types
Fish Species
Streams
Riffles (high dissolved oxygen)
Pools (calm)
Backwater areas (low dissolved oxygen)
Darters, shiners
Sunfish, bass, catfish, suckers
Gar, bowfin, pirate perch
Reservoirs
Open water
Shorelines with aquatic vegetation
Quiet coves
Shad, white bass, striped bass
Sunfish, bass, shiners
Gar, suckers
C. Feeding Groups
Fish feed at all levels of the food chain, from bottom scavengers to top-level predators. Fish can be
categorized into groups based on food preferences. Fish in each of these groups have form and structure
adaptations to allow more efficient feeding.
Trophic Group
Food Preferences of Fish
Food Preferences
Adaptations
Piscivores
(Predators)
Fish that eat other fish
Invertivores
Omnivores
Fish that eat invertebrates (insects,
worms, crayfish, mollusks, etc.)
Fish that eat any available food
Herbivores
Fish that eat plant material
Large eyes for better sight,
well-developed lateral line to
detect vibrations of prey in
the water
Feed on bottom – fleshy lips
(suckers) or taste buds on
their barbels (catfish) to
detect prey
Use pharyngeal teeth (a
modified gill arch in the
throat) for crushing plant
material
Examples / Adaptations
Large and small mouth
bass,Flathead catfish,
Gar, crappie
Blue sucker, red shiner,
bluegill, longear sunfish
Shad, carp, channel
catfish
Grass carp, stoneroller
D. Pollution Tolerance
Fish can be used to evaluate the water quality and health of an aquatic ecosystem. Fish are categorized as either
intolerant, tolerant, or intermediate.
Freshwater fish are used for many reasons:
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Live in water for their entire life
Have long life spans
Easy to identify in the field because of
established key characteristics of fish species
Fairly easy to collect
Wide range of tolerance levels
Fish can indicate the following about water pollution:
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Long-term and short-term water quality trends
Impacts of various pollutants
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Distribution, life histories, and pollution
tolerances for most fish species are well
documented
Fish communities are persistent and can
recover from natural disturbances
Impact of changes to the physical habitat
Improvements or declines in water quality
III. Common Freshwater Reptiles
A. Characteristics
Reptiles are air-breathing vertebrates that are cold-blooded, meaning their body temperature is determined by
their surroundings. Reptiles control their body temperature by moving to warmer or cooler areas as necessary.
There are many groups of reptiles found in freshwater ecosystems:
a. Crocodiles and Alligators
There are 21 know species of alligators, crocodiles, and caimans in the world, but only two are native to
the United States—the American alligator, and the American crocodile. The American alligator (Alligator
mississippiensis) is native to Texas, but the crocodile is not. The American crocodile is native to the
southern-most tip of Florida.
The American alligator is the largest reptile in North America. It can grow to more than 19 feet long. They
inhabit the eastern part of Texas, and most of the Gulf of Mexico coast. They are found in rivers, swamps,
lakes, and bayous, and can tolerate the brackish water of coastal marshes. During the coldest months and
in dry seasons, they dig deep holes in muddy banks, where they hibernate. They are carnivorous predators
that feed mostly on fish, snakes, turtles, small mammals, and waterfowl. The alligator can chew
underwater, but must surface to swallow. When breeding, the female builds a large mound of mud and
leaves, and drops up to 60 hard-shelled eggs into the center. The female stays to protect the nest for the 9
weeks it takes for them to hatch.
b. Turtles
Turtles are the oldest living reptiles. Freshwater turtles have a toothless, horny beak and a shell of bony
dermal plates, usually covered with horny shields, enclosing a soft body. The head, limbs, and tail may be
drawn into the shell when necessary. Turtles have scaly skin and are often seed basking on rocks and logs.
They lay eggs in cavities dug out by the female. Texas is home to 35 species, with about 240 found
worldwide. There are 4 families of Turtles found in Texas: Snapping turtles, soft-shelled turtles, box and
water turtles, and musk and mud turtles.
c. Snakes
Snakes are a limbless, scaled reptile with a long tapering body. Over 2300 species of snakes are known
worldwide. They live all over the world, but are absent from many islands, such as Ireland, New Zealand,
and Hawaii. There are two families of snakes in Texas: vipers (poisonous) and Colubrids (nonpoisonous)
IV. Freshwater Amphibians
A. General Characteristics
Amphibians have many characteristics that are in between fish and reptiles. The work “amphibian” means
“living a double life”. Larvae of amphibians have gills and live in water, while adults breathe air through either
their skin or lungs. Amphibians, like reptiles, are vertebrates that are cold-blooded. They have moist, glandular
skin and their toes lack claws. They lay gelatinous-covered eggs in moist areas. Groups include frogs, toads,
salamanders, and newts.
Frogs have smooth skin, webbed feet, no tail, and have long legs for leaping, while toads have warty
skin and short legs for hopping. They both feed on insects, crayfish, minnows, and on almost anything
smaller than themselves.
V. Freshwater Aquatic Plants
A. General Characteristics
Aquatic plants have adapted to living in a variety of water conditions ranging from swift-flowing to stagnant. They
are an important part of an aquatic ecosystem. Plants affect water quality by adding oxygen to the water column
and by stabilizing bottom sediments and shorelines to reduce turbidity. They also provide protection, feeding
areas, and spawning areas for fish, invertebrates, and waterfowl.
B. Groups – Aquatic plants are grouped in three categories, according to their growth habits
a. Floating plants – plants that float freely on the surface and plants that are rooted on the bottom with
their leaves floating on the surface
b. Emergent plants – plants that are rooted to the bottom and grow above the water along shorelines and
in shallow water areas.
c. Submerged plants – plants that are generally rooted at the bottom and are completely underwater,
except for seed heads or flowers.
Nearly all aquatic plants live in the littoral zone (near-shore area.) The extent to which they move away from shore
depends on the depth that light penetrates into the water column. In large lakes, plants may inhabit only shallow nearshore areas. In shallow lakes, ponds, and slower-moving streams and rivers, aquatic plants may inhabit the entire water
body. Free-floating plants cause problems by covering a water body and reducing the amount of light available to other
aquatic organisms.
VI. Nonnative and Invasive Species
Most Texas streams or reservoirs are home to at least one nonnative species (those species that enter an ecosystem
beyond their historic ranges). In Texas, at least 35 nonnative species have become established in aquatic habitats.
How Did They Get Here?
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Increasing local food supplies
Authorized enhancement of sport and
commercial fisheries
Manipulationg aquatic ecosystems to control
pests or nuisance plants
Improving the appearance of an area
Unauthorized stocking of ponds or waterways
by fishermen, etc.
Negative Impacts of Non-native species
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Prey on native fish
Altering established aquatic food webs
Introducing new parasites or diseases
Altering habitats
Damaging the genetic integrity of native
species by hybridization
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Hitchhiking on boats (snails, plants)
Releases from personal aquariums
(plectosimus)
Escape from aquaculture facilities
Canal systems
Angler bait buckets
Out-competing native species for resources
Clogging irrigation canals
Endangering commercial and recreational
fishing due to lower numbers of native species