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River-Lab 5 Guide Manual – Plankton Pages
PLANKTON PAGES
Phytoplankton
Phytoplankton include the microscopic green algae, the yellow-green algae (e.g. diatoms),
dinoflagellates and blue-green bacteria. They all make their own food through photosynthesis.
They are found in all types of water—fresh and salt water; polluted and clean; hot and cold. They
occur mostly as single-celled organisms, but many form filaments (strands) or colonies (clumps
of cells). They are the base of food chains that start in the water. Phytoplankton are also very
important because these microscopic algae that live in the oceans produce the majority of oxygen
for our planet.
Green Algae
General Information
There are over 6,000 species of green algae, most of which live in fresh water. Freshwater
microscopic green algae are single-celled organisms that can occur individually, as a long string
(strand, filament), or as a slippery film or mat on submerged rocks and vegetation (sticks, logs,
leaves). The green color comes from the chloroplasts in the algal cell. The chloroplasts contain
chlorophyll—the substance that performs photosynthesis in plants and plant-like organisms.
Through photosynthesis, green algae can make the food they need to grow. Green algae are
called producers because they use sunlight to make the food they need to grow (they do not
“eat”). Since they are producers, green algae are the base of aquatic food chains. They are a
major source of food for freshwater animals. Green algae produce oxygen as a by-product of
photosynthesis.
Spirogyra
Description and Habitat
Spirogyra is a common green alga found in freshwater in quiet areas of streams, rivers, ponds,
and marshes. This phytoplankton grows in long thin un-branched strands (filaments, strings) the
thickness of a human hair. There are over 400 species of Spirogyra. The filaments can form
clumps or mats that can be seen just under the surface of the water. Rectangular-shaped cells are
connected end-to-end to form the filaments. Inside each cell are long spiral-shaped chloroplasts
containing the chlorophyll. Spirogyra is named for these spiral chloroplasts; the chlorophyll is
used in photosynthesis to enable Spirogyra to make its own food. Oxygen is produced as a byproduct of this process.
Spirogyra
○ Green
○ Thread-like
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Reproduction
Spirogyra reproduces in two ways. One way is through simple cell division: the cell divides into
two separate new cells, making the strand longer. Spirogyra can also reproduce when two
strands touch. The strands exchange material through tubes that form. This contact produces a
special cell that can live through very hot, cold, or dry conditions. When conditions improve, a
new strand of Spirogyra can grow from this cell.
Eaten by
Like all green algae, spirogyra is a producer (see above for General Information on green algae)
because they use sunlight to make the food they need to grow (they do not “eat”). Spirogyra is
an organism at the base of aquatic food chains. They are eaten by seed shrimp, aquatic worms of
all types, freshwater snails, and fish.
Desmids
Description and Habitat
Desmids are single-celled green algae that have many shapes—circular, crescent-shaped,
elongated, star-shaped, and spherical. They have two equally-shaped sides (symmetrical).
Desmids do not have flagella, but they do have knobs, spines, granules that can be seen sticking
out of the cell wall. There are over 500 species of desmids around the world.
Desmids range in size from 0.01mm to 0.4 mm.
Desmids are found in freshwater in areas that are still and non-polluted. They either float or are
attached to plants or rocks in the water. They grow best in acid conditions. Some can move by
secreting mucus through pores in the cell wall. The desmid glides over the mucus. They move
toward light in order to photosynthesize.
Reproduction
Desmids reproduce in two ways. One is through cell division, which creates two exact copies of
a desmid from one. The other way is also similar to Spirogyra: two cells touch, and a special cell
forms which can grow into a new desmid.
Eaten by
Like all green algae, desmids are producers (see above General Information on Green Algae)
and are one of the organisms at the base of aquatic food chains. Desmids are eaten by water
fleas, copepods, rotifers, insect larvae, seed shrimp, and nematodes.
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Volvox
Description and Habitat
Volvox is a hollow, ball-shaped colony made up of a single outer layer of green algae cells.
There are 500 or more cells per colony, linked together with jelly-like strands. The ball is semitransparent. A colony can grow up to 2 mm. Each cell has two flagella that beat together to
move the colony through the water with a smooth rolling motion. Each cell also has a red
“eyespot” that detects light and a cup-shaped chloroplast containing its chlorophyll.
Volvox are found in sunny, clean freshwater areas with lots of nutrients, such as the still water
areas of rivers and streams, ponds, lakes, ditches, and even puddles.
Reproduction
Volvox cells make “daughter cells” inside the center of the ball through repeated cell division.
When there are enough cells inside, the parent colony disintegrates and releases the cells.
Eaten by
Like all green algae, Volvox are producers (see above General Information on Green Algae)
and are an organism at the base of aquatic food chains. Volvox are eaten by rotifers, seed shrimp,
aquatic worms of all types, copepods, and water fleas.
Chlorella
Description and Habitat
Chlorella is a spherical, single-celled green phytoplankton which is found individually or
clumped together into a ball. They are very small—0.002-0.10 mm. They do not have flagella.
They are found in freshwater and saltwater and in soils. They also live inside hydras (see page X)
and freshwater sponges, making those organisms appear green.
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Reproduction
Chlorella multiplies by simple cell division and when conditions are right they multiply rapidly.
Eaten by
Like all green algae, Chlorella are producers (see above General Information on Green Algae)
and are an organism at the base of the aquatic food chain. Chlorella are eaten by water fleas,
copepods, seed shrimp, rotifers, and aquatic worms.
Yellow-Green Algae
General Information
Yellow-green algae are photosynthetic phytoplankton that are found mostly in freshwater but
some species are found in saltwater, and in soils. There are over 600 species, many of which
have flagella (whip-like tail). They are single-celled organisms and similar to green algae, they
can occur individually, as colonies, as strands, or as slimy mats. Yellow-green algae contain
chlorophyll—the substance that performs photosynthesis in plants and plant-like organisms.
They are yellow-green in coloring because they contain yellow pigments in addition to the green
chlorophyll. The yellow pigment helps the chlorophyll capture light for photosynthesis.
Through photosynthesis, yellow-green algae can make the food it needs to grow. Yellow-green
algae are called producers because they use sunlight to make the food they need to grow (they do
not “eat”). Since they are producers, yellow-green algae are organisms at the base of aquatic
food chains. They are a major source of food for freshwater animals. Yellow-green algae are
important in oxygen production in the ocean.
Diatoms
Description and Habitat
Diatoms are yellow-green algae (phytoplankton.) They are one of the most abundant of all
organisms that live in fresh and salt water. They contain chlorophyll for photosynthesis, and salt
water species are important in oxygen production in the ocean. .Diatoms reproduce by simple cell division. The cell divides into two separate new cells.
Diatoms
○ Geometric
○ Glass-like color of cell wall
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Eaten by
Like all yellow-green algae, Diatoms are producers and one of the organisms at the base of
aquatic food chains. (See above General Information on Yellow-Green Algae.) Diatoms are
eaten by dinoflagellates, copepods, water fleas, seed shrimp, rotifers, insect larvae, aquatic
worms of all types, other small aquatic animals, and fish.
Blue-Green Bacteria--Cyanobacteria
Description and Habitat
Blue-green bacteria used to be called blue-green algae. Since they have features different from
algae and features more like bacteria, they have been renamed. These differences include the
material its cell wall is made of as well as the way material is arranged inside the cell.
These bacteria are named for the blue-green color (cyan) of the pigments inside the cell that help
the green chlorophyll capture light for photosynthesis. Chlorophyll is the substance that
performs photosynthesis in plants and plant-like organisms. Through photosynthesis, blue-green
bacteria can make the food it needs to grow. Blue-green bacteria are called producers because
they use sunlight to grow (they do not “eat”). Since they are producers, blue-green bacteria are
at the base of aquatic food chains. They are a major source of food for freshwater animals.
Blue-green bacteria produce oxygen as a by-product of photosynthesis. They are important
oxygen producers.
Most blue-green bacteria are single-celled but some form chains. Some have a gel-like substance
surrounding them. These blue-green bacteria have no flagella but can glide on the gel.
Blue-green bacteria live mostly in freshwater habitats. Some do live in the ocean. There are over
1500 species. They grow attached to rocks, plants, and are in bottom debris among the mud in
quiet water areas that are rich in nutrients.
Blue-green bacteria can convert nitrogen gas into a usable form of nitrogen (needed for growth)
for itself and for the other phytoplankton and plants in their habitat.
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Reproduction
Blue-green bacteria reproduce by dividing into new two cells.
Eaten by
Blue-green bacteria are producers and are one of the organisms at the base of aquatic food chains.
Blue-green bacteria are eaten by fish, copepods, water fleas, seed shrimp, rotifers, and aquatic
worms of all types.
The blue-green bacterium, Nostoc, is the most common in freshwater. It grows in long strands
that are enclosed in a mass of a gel-like substance.
Some blue-green bacteria, such as Anabaena, produce toxins that may cause animals that drink
the water to become sick and die.
Zooplankton
General Information
Zooplankton is made up of microscopic animals and animal-like living organisms. They cannot
make their own food. They are consumers. They live by eating phytoplankton or other
zooplankton. Zooplankton are complex organisms composed of more than one cell, yet most
zooplankton are usually less than one millimeter in length. Zooplankton are transparent—nearly
invisible. Zooplankton are very weak swimmers and are mostly carried about by the current.
The most common and numerous zooplankton are the microscopic crustaceans, related to shrimp.
There are two types of zooplankton—permanent and temporary. Permanent zooplankton
consists of organisms that remain microscopic for their entire life cycle. They have many
strange shapes and appendages. Temporary zooplankton consists of the microscopic immature
and egg stages of fish, insects, crustaceans, shellfish, etc. Some of these immature states do not
look at all like the adult. As they grow larger, these animals depend on smaller plankton as their
food.
The descriptions that follow are of the permanent freshwater zooplankton.
Water Fleas
Description and Habitat
Water fleas are permanent members of the freshwater plankton community. There are hundreds
of species. They are one of the most numerous forms of zooplankton in fresh water. They are
found everywhere in nutrient-rich waters except for water that is fast- flowing or polluted. They
range in size from 0.1mm to 3mm. The antennae beat to move them in fast, jerky, circles
through the water. The jerky movements resemble the hopping of a flea—hence their name.
The water flea’s body is enclosed by a carapace made of two shell-like flaps that are hinged at
the back, similar to a clam shell. This carapace covering is thin and allows
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light to pass through. The internal structures can be seen when viewed with a microscope. The
heart can even be seen beating. Inside the carapace, the water flea body is made up of a heart,
tiny brain, compound eye, mouth, digestive system, and an egg chamber. The two pair of
antennae and five to six pairs of hairy legs are usually not covered by the carapace. The eye is a
very prominent feature and can detect light.
Water Fleas
○ Eggs inside body
○ Transparent
○ When legs kick, water flea "spins."
Food
Water fleas eat microscopic phytoplankton, rotifers, bacteria, and tiny pieces of organic debris.
They beat their legs rapidly, creating a water current that brings food towards them to collect on
the “hairy” legs. The food is then passed up along the legs into the mouth.
Eaten By
Water fleas are a very important food source for young and adult fish such as largemouth bass.
Up to ninety-five percent of the contents of some fish stomachs are water fleas. Water fleas are
also eaten by hydra, copepods, planarians, nematodes, mites, and immature and adult aquatic
insects.
Reproduction
During the spring and through the summer, water fleas produce female babies from unfertilized
eggs. They carry between two and fifty eggs in the egg chamber (in their body). Female water
fleas can have a new brood of eggs every 10 days. When the eggs hatch, young water fleas look
just like the adults, only much smaller. The young molt several times in the two weeks it takes
them to mature. Once mature, they can start to have eggs and hatch babies. If a female had 50
babies and those 50 babies had 50 babies, after four weeks there could be 125,000 water fleas.
[50 x 50 x 50=125,000] After ten weeks (five generations), there could be as many as 15 billion
descendents. But not all
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water fleas survive to reproduce. So the actual number of water fleas after ten weeks would be
smaller than 15 billion but could still be an enormous amount.
In stressful conditions, or at the end of the growing season, the females will produce both
females and males that can mate. Mating produces a special egg (resting egg) that is capable of
surviving the stressful conditions of winter. In spring, or when conditions improve, these eggs
produce more females.
Copepods
Description and Habitat
Copepods are members of the freshwater plankton community. There are over 6,000 kinds of
copepods. They live in both fresh and salt water but are more numerous in salt water than
fresh—the opposite of water fleas. Freshwater copepods live among the vegetation in quiet
backwaters of streams and rivers or open areas of ponds and lakes. They are one of the first
kinds of zooplankton to emerge in spring from resting eggs that have rested in the bottom mud
over the winter. Some adult copepods survive in the bottom mud over the winter and become
active in spring.
Copepods are crustaceans; they are related to larger crustaceans such as shrimp and lobsters.
Crustaceans have a hard exoskeleton—a covering over the body. Copepods are grayish-brown.
They molt this covering as they grow. The body has nine segments connected by flexible tissue.
Female copepods carry their egg sac(s) on the outside of their bodies near the tails. Copepods
have a few pair of legs near the mouth that are used in eating. They have five pair of swimming
legs outside of the exoskeleton and two antennae on the head. The antennae beat like tiny oars.
They use the antennae and the swimming legs to swim through the water—hence their name:
cope=oar + pod=foot. They swim in fast jerks by kicking the legs backward and pressing their
antennae straight back. This kick sends them through the water. After kicking itself forward or
upward, the copepod spreads out its antennae, which act as a parachute to slow down the
copepod body as it sinks.
Food
Copepods eat single-celled organisms such as diatoms and bacteria; smaller zooplankton than
themselves such as water fleas and copepods; and decaying plants and animals. They grab their
food with the legs near the mouth and pass the food into the mouth. Under a microscope, the
food can be seen going into the digestive tract.
Eaten by
Copepods are eaten by fish larvae, young and small fish, other copepods, planarians, nematodes,
mites, and aquatic insects. They are a very important source of food for fish.
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Cyclops Copepod
The Cyclops copepod is named because it appears to have only one eye. The eye is a
combination of two eyespots that can see light and detect moving objects. Cyclops copepod is
shaped like an upside-down bowling pin, with the head area wider than the tail. Cyclops
copepods range in size from ½ mm to 3mm in length. Cyclops antennae are long—about onethird as long as its body. Male antennae are more irregularly-shaped than the female antennae,
and have pincers (claws) on them to hold the female when mating.
Female cyclops copepods have an egg sac on each side of the body. Each sac holds an average
of thirty eggs. The eggs are fertilized inside the female. The female carries the eggs in the two
egg sacs for one to five days. Under the right conditions, a female could produce eggs three
times in one month, totaling 180 eggs per month. If all her eggs hatched, one female copepod
could produce 1000 eggs by fall. However, not all will become adults, since they can be eaten
by other animals in all stages of their lives.
The eggs hatch into a larval form that looks very different from the adult. It takes six months to
a year for the young to mature. They molt eleven times as they grow into the adult form.
Cyclops
o Upside-down bowling pin shape
o 2 egg sacs on outside of body
Canthocamptus Copepod
The body of the Canthocamptus copepod is shaped differently than the Cyclops. Its head is only
slightly larger than the body segments which barely taper toward the tail. It has short antennae
that do not go past the first body segment. The males’ antennae are wider than the females’.
Females usually have only one egg sac. Canthocamptus can be found crawling around on rocks
in the quiet areas of a river.
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Canthocamptus
○ Slightly tapered body shape
○ Short antenna
○ 1 egg sac
Seed Shrimp
Description and Habitat
Seed shrimp are members of the permanent freshwater plankton community. There are over
2,000 species of seed shrimp. They are named “seed shrimp” because they look like a seed with
a shrimp inside. A hard shell-like carapace protects the seed shrimp’s soft body and head. The
carapace has two parts connected by a hinge, similar to a clam shell. The carapace ranges in
color from yellow to black and is covered by many little hairs. The seed shrimp’s coloring
prevents its insides from being seen when viewed through a microscope. Seed shrimp live on
and among the submerged plants and debris in quiet areas of a river or stream. Without a
microscope, the seed shrimp looks like a tiny black dot moving smoothly through the water.
When viewed under a microscope, a seed shrimp does not look alive until its legs and antennae
stick out from the carapace and it swims away. A seed shrimp is usually about 1mm wide.
Inside the carapace, the body has five to eight pair of legs, two pair of antennae and a digestive
system with a mouth. Seed shrimp have one, two, or three “eyespots” that detect light and dark.
The antennae and all the hairs on the carapace and legs help the organism sense objects around it.
Seed shrimp
○ Opaque – can't see inside of shell to body
○ Oval shape
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Food
Seed shrimp eat almost anything, including bacteria, molds, algae (single-celled or colonial),
other microscopic life, and decaying plants and animals.
Eaten by
Seed shrimp are eaten by water mites, nematodes, mallards and other ducks, and a wading bird
called a Great Egret. They are only a small part of the food eaten by fish.
Reproduction
Seed shrimp reproduce in two ways. Unfertilized eggs develop inside a female and eggs develop
after a male has fertilized them. All eggs are raised in a chamber inside the female. Just-hatched
seed shrimp do not look like the adults—they look more like immature water fleas. Seed shrimp
molt many times before becoming mature and looking like the adults.
Rotifers
Description and Habitat
Rotifers are members of the permanent freshwater plankton community. They are found
attached to anything under water in quiet areas of the freshwater system. There are over 2,000
types of rotifers, most of which (95%) live in freshwater. They are the smallest multi-celled
animals in the world—0.01mm to 0.5mm.
A rotifer body is a soft, flexible bag-like structure that can change its shape by contracting or
stretching out. This body has toe-like projections on one end and a pair of disks with cilia (tiny
hair-like projections) that rotate around a mouth at the other (“head”) end. The rotating disks
make the rotifer to swim through the water in a spiraling motion. The bag-like body surrounds a
fluid-filled cavity that contains the rotifer’s digestive system, consisting of its mouth, jaws,
stomach, and intestines. This cavity also contains the rotifer’s brain, eyespots, and reproductive
system. The rotifer also moves in an inch-worm style. It brings it “toes” forward under its
“head,” and then extends its “head” and body forward.
Rotifers
○ Move very quickly
○ Transparent
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Food
The rotating disks also create a current that brings food to the rotifer’s mouth. Rotifers eat dead
and decaying organic matter, single-celled algae, blue-green bacteria and other single-celled
microscopic organisms. Before the food goes into the stomach, it is crushed and ground up by a
jaw-like structure.
Eaten By
Rotifers are eaten by worms, copepods, water fleas, and fish.
Reproduction
Female rotifers can produce eggs without fertilization. For some species, males have never been
found. Some species lay eggs on vegetation in the water where the eggs remain to develop and
hatch. In other species the eggs develop within the body and hatch as small rotifers. At certain
times, males are produced that can mate with a female. The egg from mating develops inside the
female and hatches into a form called a zygote that can survive extreme temperatures and heat.
The adult rotifer also can protect itself from drought or excessive heat by producing a thick body
covering that protects it while it waits for better conditions. Rotifers live from two to twenty
days, depending on the species.
Annelids (Segmented Worms)
Description and Habitat
Annelids are a large family of worms that have segmented bodies. There are over 9,000 types of
annelids, including earthworms and leeches. One type of annelid, the bristle worm, has several
species. On land some bristle worms grow from 1 to 180 meters long. However, freshwater
planktonic bristle worms grow to only 40 mm.
Each segment of a bristle worm’s body, except the first, has bundles of bristles. Each segment
has muscles that go around the body and others that go down the body. The worm crawls by
pushing the bristles against surfaces such as rocks, debris, and mud at the bottom of quiet waters
of the freshwater system. Bristle worms can swim by whipping their bodies back and forth very
fast in the water. An annelid’s body is a tube-within-a-tube. The inner tube is a digestive tract
with a mouth at one end and wastes coming out at the opposite end. Some worms are transparent.
You can see the digesting food in the inner tube through a microscope. Some annelids have
“eyespots” that detect light and dark.
Freshwater microscopic bristle worms live in soil and among the debris in the quiet water areas
of the freshwater system.
Annelids
○ Bristles on sides of worm
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Food
Bristle worms eat the decaying plants and animal matter in the mud as they borrow through it. In
the mud are algae, diatoms, and other decaying plants and animals. These worms play an
important role in cleaning up dead plants and algae, which helps to maintain the oxygen supply
in the water.
Eaten By
Bristle worms are eaten by fish of all sizes, water mites, and hydra.
Reproduction
Planktonic bristle worms can reproduce in two ways. Planktonic bristle worms have both male
and female parts on the same worm. Mating between two individuals is necessary to produce
young worms. Planktonic bristle worms can also reproduce by “budding.” In this process, a
small lump forms on the end of a worm. After developing four or more segments, this section
will break off, forming a new worm.
Nematodes (Round Worms)
Description and Habitat
Nematodes are worms that do not have segments. There are over 20,000 species, 1000 of which
live in freshwater habitats. Some nematodes are harmful (if ingested) to larger animals,
including humans and animals such as dogs and cats. Nematodes live in the top of the mud on
the bottom of quiet waters of streams, rivers, ponds, lakes, and wetlands.
Planktonic nematodes look like tiny thread-like hairs whipping around in the water. Their bodies
are small—0.3mm to 1 cm long—and are pointed at both ends. The body is basically a tubewithin-a-tube. The inner tube is the digestive system. One end of this tube is the worm’s mouth.
The other end excretes wastes. The outer tube is protected by a tough and flexible shell-like
covering called a cuticle. To grow, nematodes molt this protective covering.
Nematodes
○ No bristles
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Food
Nematodes eat a wide variety of food—bacteria, fungi, algae, small animals, and dead and
decaying matter. They eat much dead and decaying matter, thus helping to maintain the oxygen
supply in the water. The food is pulled into the mouth and is crushed inside by a jaw-like
“throat” just below the mouth.
Eaten By
Nematodes are eaten by fish, water mites and hydra.
Reproduction
Both a male and a female are required for nematode reproduction. After fertilization, a female
deposits her eggs in the mud. Nematodes can lay as many as 200,000 eggs in a day! The eggs
hatch into larvae which then develop into tiny worms that resemble the adult. Young worms
molt four times before becoming a mature adult.
Planarians
Description and Habitat
Planarians are gray, brown, or black, flat, soft, worm-like organisms that live in salt and fresh
water. In freshwater they live in both quiet waters and flowing waters in a river basin. They
hide under rocks and among submerged leaves and other debris on the bottom of the water.
Planarians range in size from 3-12 mm. The flat body does not have a body cavity. The head is
triangular-shaped and contains two “eyespots” that are not centered. This makes the planarian
look cross-eyed. The “eyespots” are not real eyes; they only detect light, which planarians avoid.
The mouth is on the underside of the body in the center of the animal. The mouth both ingests
food and excretes wastes. The mouth releases digestive enzymes to the outside of the body to
begin the digestion process and then sucks the food inside. Planarians have cilia on the
underside of the body that beat to enable the animal to glide on a moist surface. Planarians can
regrow lost or damaged parts of the body.
Planarians
○ Triangular-shaped head
○ Flat
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Food
Planarians eat diatoms, water fleas, copepods, aquatic insects, and dead and decaying plants and
animals. Many are scavengers and eat a lot of dead and decaying matter, thus helping to
maintain the oxygen supply in the water.
Eaten By
Planarians are eaten by fish, water mites and hydra.
Reproduction
Planarians reproduce in two ways. They have both male parts and female parts on the same
animal, but mating between two separate individuals is necessary. After fertilization, an egg
develops inside the female. The egg is released with a protective capsule around it. A very
small planarian hatches one week later. Planarians can also reproduce by budding. A new
worm grows from a part of the adult worm and then breaks off.
Water Mites
Description and Habitat
Water mites look like small, fat spiders and belong to the spider family. Water mites live in quiet,
shallow areas of the river system, in and among the bottom debris and rooted water plants. Most
water mites live in freshwater; only a few are found in saltwater. They are active during the day
all year long—even under the ice in winter.
Mites vary in size—1—5 mm. Their rounded bodies are often brightly colored—red, yellow,
green, or blue. They have two double eyes and two “palps” (parts used for capturing food and
eating), all at the front end of the body. Their four pairs of hairy-looking legs are also attached
near their front end. Mites can run on the water’s surface, on the bottom debris, or on plants.
They can also swim by moving their legs very fast, but, if they stop moving, they will sink. The
hair-like projections on their legs slow down their sinking.
Water mites
○ Often red
○ Looks like a spider
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Food
Some mites eat tiny worms, other mites, water fleas, and copepods. They grab the animal with
their palps, pierce it with special razor-sharp jaws, and suck out the body juices, thus killing the
prey. Some mites feed on larger aquatic animals by sucking out the juices, which may kill the
animal, depending on its size. Some mites are scavengers and eat dead and decaying plants and
animals.
Eaten By
Mites are eaten by hydra, aquatic insects, water fleas, copepods, and small fish.
Reproduction
After mating, female water mites lay 20-400 bright red eggs on the undersides of leaves of
aquatic plants. After one to six weeks the eggs hatch into tiny larvae that do not yet look like the
adult. These larvae attach to water insects such as dragonfly larvae. They suck juices from these
animals. Due to the tiny size of the mite larvae, the sucking doesn’t kill the much larger insect
larvae. When the mite larvae are big enough, they change into a form called a nymph that
resembles the adult but is not yet mature. The nymph eats the same food as an adult water mite
as it grows. The nymph then it will attach to a plant and develop (through metamorphosis) into
the mature adult.
Hydra
Description and Habitat
Hydra are freshwater cousins of jellyfish, coral, and sea anemones but are very small (up to
3mm). They are plentiful in unpolluted water. They attach to aquatic plants and submerged
rocks, leaves, twigs, etc. in ponds, lakes, rivers, and streams.
Hydra have a soft, sac-like body with an opening—the mouth— surrounded by five to eight
tentacles at the top of the body. The mouth leads to the digestive system, which is a hollow tube.
The hydra’s wastes are also eliminated through the mouth. The tentacles and the whole outside
of the body are covered with stinging cells that the hydra use to inject poison into their prey to
paralyze them. Hydra are gray to brown but some appear green if green phytoplankton, Chlorella,
live inside. The hydra and the algae help each other: the hydra uses the oxygen produced by the
algae for the hydra’s respiration, and the algae use the carbon dioxide produced by the hydra for
the algae’s photosynthesis.
A hydra attaches itself to surfaces under the water with a sticky glue-like substance. It can
change its size by contracting and extending its body while it is attached. To move, a hydra can
detach and float in the water current or glide on its foot like a snail. A hydra “somersaults” by
attaching its foot onto some underwater surface and attaching its mouth and tentacles alongside.
The hydra then releases its foot, re-attaching it in a place within its reach and then releases its
mouth and tentacles and brings them up to re-attach alongside the foot. Repeating this process
moves the hydra in whatever direction it chooses to go.
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© 2009 Mill River Wetland Committee, Inc.
River-Lab 5 Guide Manual – Plankton Pages
Hydra
○ Tentacles
Food
Hydra are carnivores: they eat other animals such as worms, copepods, water fleas, and just
hatched fish. While attached, hydra sweep their tentacles, looking for prey.
Eaten By
Hydra are eaten by fish and aquatic insects.
Reproduction
When food is plentiful, hydra reproduce by budding. A small swelling appears on the adult and
grows until a new hydra is formed. The new hydra then breaks off. Under the right conditions, a
bud can be formed every few days. Hydra also reproduce by mating. Under certain conditions,
one hydra will grow female parts, and another will grow male parts. After the eggs are fertilized,
they are covered by a tough coating and are laid among the vegetation. The eggs will “rest” until
conditions are better for the hatching of the young hydra.
© 2009 Mill River Wetland Committee, Inc.
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