Freshwater
Animals
Note Ctenophores and Echinodermata
are marine phyla; all other phyla have
representatives in freshwater.
Phylum Porifera: The sponges
• Sponges are mainly marine
but there are ~25 freshwater
species described.
• Primitive multicellular animals;
do not have organs, but do
have specialized cells for
feeding, digestion.
• Filter feeders; sieve particles
from the water as it flows into
pores.
• Some species have symbiotic
algae (similar to corals), mainly
green algae Chlorella.
Can be important consumers of
microbes as small as bacteria
Cnidaria: Celenterates
(Hydra & Jellies)
• Most species marine; all display radial symmetry and possess
nematocysts (defense cells).
• Hydra is probably most common freshwater cnidarian:
– May have symbiotic algae, Chlorella.
– Most often sessile; epiphytic
Platyhelminthes and Nemertea:
• Turbellaria (free-living flatworms)
are most common in freshwater
benthic habitats or epiphytic.
• Some planaria acquire
nematocysts for defense by
ingesting hydra (marine forms do so
as well).
• Trematoda (flukes), a major group
of animal parasites; some with
aquatic phase in life cycle.
• Schistosomiasis is a disease of
the human intestinal tract caused by
a Schistosoma fluke. A similar fluke
causes swimmers itch in humans
we get infected as an alternate host
(normally water fowl).
• Nemertea have an anus and
closed circulatory system.
Schistosoma life history:
Nematoda: The roundworms
•
Non-segmented roundworms;
common in all kinds of aquatic
habitats (damp soil, freshwater,
marine).
•
Generally benthic infauna (within
sediments).
•
Feeding strategy varied: detrivores,
herbivores, carnivores (including
predation on other nematodes).
•
Some species are common and
important parasites of animals
(including humans), often using
insects as host vectors. (e.g. “river
blindness”, onchocerciasis
transmitted by blackfly (Simuliidae).
(and rotifers)
Rotatoria (Rotifers)
• 2000 freshwater species, more diverse than in marine
habitats!
• Possess an advanced digestive system including
mastax (to grind food down), stomach, intestine, anus.
Possess a nervous system and sensory organs (eyes).
• Sexual reproduction produces a dormant cyst and
several species can survive years of drought.
• “Wheel organ” is ciliated and creates a whirlpool-like
flow to bring prey to mouth. Some modified for ambush
capture.
• Predators of bacteria and small protists like
nanoflagellates.
• May be planktonic or attached by “foot” (with “toes”).
Mollusca
Soft-bodied, unsegmented; possess a head, muscular
foot, stomach/viscera, and often grow a calcareous shell.
• Class Gastropoda: snails and
limpets
– 500 species of freshwater snails
– One-piece shell and a radula
Conical that
shellis used to
(file-like structure)
scrape food from surfaces
– Snails are important grazers of
periphyton (epilithic algae,
bacteria, etc.) but also feed upon
detritus, macrophytes, and dead
tissue
Spiral shell
• Class Bivalvia: clams and
mussels
– Bivalves have a shell with two
halves and gills specialized for filter
feeding.
– Found in the benthos, either
burrowed in sediments or attached
to substrate.
– Adults can withstand periods of
drought by closing shell.
– Some have specialized larval
stages with interesting dispersal
characteristics.
– Some bivalve species are important
invaders of North American aquatic
systems (alien species: Asiatic
clam, zebra mussles) with serious
ecological implications.
Annelida: Segmented worms
• Tubular, segmented body with
specialized digestive system,
terminal mouth and anus.
Familiar representatives include
oligocheates, leeches.
• Oligocheates (like familiar earth
worms): benthic, burrow through
sediment.
– Most ingest organic particles in
sediments (important connection
in food chain); some algal grazers
or predators.
– Resistant to low oxygen and
polluted conditions (good indicator
species; Tubifex).
– Vectors of some important
parasites, e.g. Whirling Disease
Hirudinea
(Leeches)
Whirling Disease of Trout
Tubifex sp.
Myxobolus cerebralis
Arthropoda
• Ubiquitous in all continental surface
waters.
• Important in linkages of ecosystems.
(aquatic and terrestrial)
• All arthropods characterized by:
– Chitinous exoskeleton
– Stiff jointed appendages (mouth, legs, etc.)
Arachnida: Mites and Spiders
• No true aquatic spiders; some able to utilize the
environment on occasion.
• Water mites are diverse and inhabit most surface waters.
• Mostly benthic, but some pelagic species in lakes.
• Almost all predatory (mainly on insects) but some
parasitic (often in larval stage).
Fused
cephalothorax
and abdomen
Mouth
Six pairs of
appendages,
4 pairs of legs most
conspicuous
Subphylum Insecta
• Ten orders contain aquatic species; majority are aquatic
as larvae, emerge as adults.
• Characterized by:
– body divided into head, thorax (three segments), and abdomen
– Single pair antennae, compound eyes, specialized mouthparts
• Order Collembola (springtails): Not true insects.
– Wingless, often eyeless.
– Mostly terrestrial or semiaquatic, often in lakes
– Poorly studied
Order Plecoptera
• Stoneflies most common in streams; some predators
and others detritivores.
• Sensitive to pollution and low oxygen levels, therefore
used as an indicator species.
• Similar in appearance to mayfly larvae except only have
two cerci (filaments) on posterior end, and tend to be
mostly flattened.
• Incomplete metamorphosis
Incomplete Metamorphosis
Order Ephemeroptera: Mayflies
• Aquatic only as larvae; common in streams and in lake
benthos. Feed by scraping, collecting, some predatory
• Many species, typically divided by habitat interaction or
behavior: swimmers, clingers, crawlers, burrowers.
Body shape reflects lifestyle:
–
–
–
–
Clingers flattened dorso-ventrally
Swimmers round & streamlined
Burrowers often have tusk-like mandibles
Crawlers have more streamlined gills than burrowers
Gills
Three
filaments
Mayfly life cycle:
1) Eggs hatch in water, larvae grows.
2) Larvae swims to surface and emerges; metamorphoses into
sub-adult (subimago) form.
3) Subimago matures into adult. Adult mates, female lays eggs
in water, dies.
Order Odonata:
dragonflies, damselflies
• Aquatic as larvae in both streams and lakes (more in
lakes and slow waters). Important predators of other
insect larvae.
• Move by crawling, some by swimming. Prefer thick
aquatic vegetation for cover, debris/litter, rocky cobble, or
burrow in sediments.
• Three-stage lifecycle similar to mayflies. This lifecycle is
termed incomplete metamorphosis.
Order Trichoptera
• Caddisflies mostly lotic, a few lentic species; aquatic as
larvae and pupae
• Variety of lifestyles habits:
– Some build protective cases from materials in the environment,
crawl and graze on periphyton or leaf litter
– Some construct nets for filter-feeding
– Others free-living, predatory
Complete Metamorphosis
Order Megaloptera
• Dobsonflies, alderflies; aquatic as larvae, pupae
• Often large larvae with large mandibles; all predators.
• Aquatic period of life cycle can last several years before
emerging as adults.
• Occur in both lotic and lentic systems.
Order Heteroptera
• The true bugs are mostly terrestrial; aquatic species live
either on surface or submersed.
• Inhabit both lentic and lotic environments; many possess
specialized appendages adapted for swimming or to
facilitate air breathing.
Order Lepidoptera
• Very few aquatic species of moths or butterflies; usually
associated with ponds with dense macrophyte
populations. Some lotic species grazers of periphyton.
• Complete metamorphosis.
Paired, filamentous gills
Order Coleoptera
• Though only about 3% of beetles aquatic, there are so
many species of beetles that this still represents a
significant order of aquatic insects.
• Some submerge water bubbles for air breathing.
• Includes predators, periphyton and macrophyte grazers.
Order Diptera: True Flies
• Largest group of aquatic insects, dominated by family
Chironomidae (midges).
• Also includes nuisances like mosquitoes, black flies…
• Some midge larvae possess hemoglobin as an
adaptation for survival in low oxygen environments.
Subphylum Crustacea
• 4000 species of crustaceans found in freshwaters;
although most are marine.
• Includes many important food chain links:
– Zooplankton species are key as primary consumers (plankton
grazers)
– Benthic omnivores which feed upon detritus, carrion, etc.
• Characterized by:
– Respiration across gills or body surface
– Chitinous exoskeleton, two pairs antennae, paired and jointed
appendages
Ostracoda
•
“Seed shrimp” are benthic species covered by carapace made of mix of chitin
and calcium carbonate.
•
Mostly graze algae or eat detritus.
•
When ostracod dies, carapace resists dissolution; can be found in sediment
cores, fossils.
•
Isotopic composition of recovered carapace can reveal past climate patterns
(temperature).
Copepoda
• Important pelagic zooplankton, but other
species also benthic in streams, lakes and
groundwater, or parasitic.
• Sexual reproduction, characterized by a
many-staged development process divided
between 6 naupliar (analogous to larval) and
6 copepodite (juvenile) stages.
• Interesting patterns of diverse morphology,
adaptations and speciation.
Branchiopoda
• Diverse group including
Cladocera, tadpole shrimp, brine
shrimp, etc.
• Usually found in lentic
environments. Some benthic,
some planktonic.
• Many species have an egg stage
that is resistant to drying and can
withstand long periods out of
water.
Decapoda
• Includes many large (macroinvertebrate) species such as crayfish,
shrimp, crabs.
• Inhabit lentic and lotic environments, including caves, groundwaters,
wetlands.
• Some species have high value as food for humans; these are often
cultured and harvested.
• Crayfish are omnivorous and important benthic consumers; shrimp
are primarily grazers or detritivores.
• Important as both consumers and as prey for larger vertebrates.
Isopoda
• Pillbugs, sowbugs;
terrestrial, marine and
freshwater species.
• Often found in clean,
oxygenated water
(springs, streams,
groundwaters).
• Detritivores and
scavengers.
Amphipoda
• Scuds and side-swimmers; resemble isopods
but are flattened laterally, not dorso-ventrally.
• Omnivorous scavengers; some important
shredders of CPOM; mainly benthic.
The Vertebrates
• In freshwaters, fish are considered the most important
vertebrate species.
• Most diverse aquatic vertebrates, over 24,000 species
described; ~ half found in freshwater.
• Dominant classes:
– Superclass Pertomyzontiformes, jawless fishes
(lampreys)
– Class Chondrichthyes, cartilaginous fishes
– Class Osteichthyes, bony fishes
• Fish can be found as predators,
grazers, scrapers…
• Body form is adapted and
specialized toward certain lifestyle /
behavior:
– Streamlined fishes are specialized
for near constant swimming for
locating and pursuing prey
– Elongate, torpedo-shaped fishes
specialized as sit-and-wait ambush
predators; built for burst speed.
– Benthic species often flattened
dorso-ventrally, with under-slung
mouths
– Deep-bodied, ventrally flattened fish
are specialized for maneuvering in
tight quarters
– Eels are adapted for moving through
narrow spaces
Tetrapods
Other aquatic vertebrates include amphibians, reptiles, birds and mammals.
• Amphibians include salamanders, frogs; mostly
predators but some are algal grazers and detritivores
(especially as larvae).
– Amphibians have recently gained much attention as indicator
species of aquatic pollution and environmental changes.
– Recent evidence suggests a worldwide decline in amphibian
diversity; some correlations to climate change, global warming
processes.
Reptiles, Birds and Mammals
• Many familiar representatives including:
– Turtles, Water snakes, Crocodilians
– Pelicans, ducks, some raptors
– Dolphins, bats, beavers, hippopotamus
These animals are often of interest as high-order
predators, or for ways in which they alter their
environment.
Mostly associated with shallow habitats and aquaticterrestrial interface.