Major Aquatic Invertebrate Taxa
Phylum
Porifera (sponges)
Cnidaria (hydra & jellyfish)
Platyhelminthes (flatworms)
Gastrotricha (gastrotrichs)
Rotifera (rotifers)
Nematoda
(nematodes)
Mollusca (snails/bivalves)
Annelida (oligochaetes/leeches)
Bryozoa (moss animals)
Tardigrada
(water bears)
Arthropoda (arthropods)
Review of major invert classifications
--Radially symmetrical animals
Phylum: Cnidaria
--Bilaterally symmetrical animals
Acoelomates – animals that lack a body cavity:
Phylum: Platyhelminthes
Pseudocoelomates – animals that have a body cavity
but no peritoneum.
Phlya: Gastrotricha, Nematoda, Rotifer
Coelomates – animals with internal body cavity lined
with peritoneum.
Protostomes:
Phyla: Mollusca, Annelida, Tardigrada, Arthropoda
Deuterostomes:
Phyla: Echinodermata, ‘Chordata’
Phylum: Porifera
About 5,000 species worldwide
About 25 species are freshwater
General physiology
Pinacocytes: ‘skin cells’, thin, leathery and tightly packed.
Choanocytes: striking resemblance to choanoflagellates (a
single-celled protist). Their function is to create active
pumping of water and major site of nutrient uptake.
Archaeocytes: These cells are “totipotent”. They can
change into all of the other types of cells. Ingest and
digest food caught by choanocyte collars.
Schlerocytes: Create and excrete spicules.
Reproduction
--All sponges can reproduce sexually
--Generally monoecious and produce eggs and sperm at
different times.
--Produce flagellated parenchymella larva that exit via
exhalent current.
--Larval motility is the principal dispersal mechanism
--Sponges have great powers of regeneration
Feeding
--Sponges feed on fine particulate material in the
inflowing water.
--Food particles generally range from 5- to 50 µm and are
phagocytized by archeocytes.
--After digestion is complete, the archeocytes and
associated wastes are expelled into the water.
Phylum: Cnidaria
Class Hydrozoa (only freshwater rep)
over 3,000 species
only 14 freshwater species
Class Cubozoa (sea wasps)
Class Scyphozoa (jellyfishes)
Class Anthozoa (sea anemones and corals)
Dimorphism in Cnidaria
polyp
medusa
Feeding
Reproduction
--Cnidarians reproduce both asexually and sexually.
Medusa are produced by budding of polyps but not vice
versa.
--Generally dioecious and reproduce sexually during
warmer periods.
--Embryos develop into ciliated free-swimming larva
called planula.
Crespedacusta bowersi –
the only freshwater “jellyfish”
--Cnidarians are carnivorous but have limited powers of
movement.
--Essential to the feeding process are thin, flexible
‘tentacles’.
--Nematocysts are specialized cells located on tentacles
that aid in capture of prey.
--Amino acids released by prey can trigger the tentacles
to ‘bend’ toward the mouth by ciliary action.
--Common foods of Hydra include invertebrates and
sometimes small fish.
Nematocycts – Food, protection, anchoring
Platyhelminthes (flatworms):
Free livingClass Turbellaria (planarians)
mostly predaceous
free living
epidermal rhabdites
Parasitic –
Class Trematoda (internal flukes)
exclusively parasitic
complicated life-cycles
Class Cestoda (tapeworms)
exclusively parasitic
Trematodes --- ‘black spot disease’; ‘yellow grub’
Phylum: Platyhelminthes
Planarian
Dugesia
About 200 species in N. America
Turbellarians: General morphology
gut
epidermis
mesenchyme
neoblasts
rhabdites
General physiology
Triploblastic (3 layers). 0.5 to 5 cm long.
-- Respire/excrete by diffusion.
-- No body cavity other than gut. No anus.
--Rhabdoids: Cells that produce mucus and poison for
prey immobilization/predator deterrence.
--Neoblasts: Small, ameboid like cells that initiate
regeneration of lost parts.
Identification of planarians
Nutritive cells
Simple intestine
3-lobed intestine
Many-lobed intestine
Reproduction
Most turbellarians are monoecious.
--Penal stylets (Penis) that can be used for both
reproduction and defense.
--Embryos develop into a free-swimming, ciliated stage
called Muller’s larva. Short-lived, nonfeeding stage.
--Can produce asexually by fission, fragmentation or
both.
Life history and ecology
--Widely distributed in N. America
--Occur in both lakes and streams
Stream-dwelling species more differentiated than
lake species
--Species diversity increases in temperate areas 20 to
60 species per lake
Some species are univoltine:
produce one generation a year
Most turbellarians are multivoltine:
produce several generations a year
Feeding
--Effective predators on other invertebrates including
rotifers, nematodes, cnidarians, bryozoans, small
crustaceans, annelids and other turbellarians.
--Turbellarians use ‘slime’ to entangle prey. They use a
muscular protrusible pharynx to help ingest prey.
Phylum: Gastrotricha
About 450 species
Fewer than 100 freshwater species
--Nearly ubiquitous in the benthos of freshwater
habitats.
10,000 to 100,000 per m2.
--Colorless animals 50 to 800 um long.
General physiology
--Sensory organs include ventrally located cilia
--Most are photosensitive
--Most species exhibit tactile chemical sense.
--Excretory system consists of a mid-body pair of
protonephridia that empty through pores on the body
surface.
--No circulatory or respiratory system per se.
Reproduction
--Believed to produce 3 types of eggs:
2 types are parthenogenic
1) tachyblastic eggs – develop immediately and
hatch quickly (within 1-2 days)
2) opsiblastic eggs – thick shelled ‘resting’ eggs that
are very resistent to freezing and drying
1 type of sexual reproduction?
--Newly hatched Gastrotrichs already have
parthenogenetic eggs and reach maturity in several days.
--No larval stage in gastrotrichs.
Feeding ecology
--Gastrotrichs typically feed on bacteria, algae,
protozoans, detritus and inorganic particles.
--Bacteria are probably most important.
--Predators include amoeba, cnidarians and midge larvae.
--We know very little about what controls gastrotrich
populations in the natural environment.
Phylum:Rotifera
The ‘wheel animals’
--About 2000 species (mostly freshwater)
--Possess 2 distinctive features:
1) ciliated ‘corona’ near head region
2) a muscular pharynx, the ‘mastax’
--Small, (100-1000 um) herbivores and predators.
--Can be very abundant (up to 1000/L) and found in nearly
all habitats from open water to soils and attached to
plants.
--Exhibit cyclomorphosis
--Two major classes:
1) Bdelloidea
2) Monogononta
Rotifer Diversity
General Rotifer morphology
Rotifer reproduction
--Rotifers are dioecious and males are always smaller
than females.
--Parthenogenesis predominates, but males do appear
sporadically.
-- Mostly oviparous; producing 3-50 eggs.
--Monogononta have only 1 gonad; Bdelloidea has paired
gonads but no males are known.
Rotifer life-cycle
--2 types of females: Amictic and dimictic.
Morphologically indistinguishable, but functionally
distinct.
Functional role
--Because of their sheer abundance and quick turnover
time, they can exert significant grazing pressure on
phytoplankton.
--Can represent up to 50% of zooplankton production in
lakes/ponds.
--Generally have lower ‘filtering rates’ as compared to
crustacean zooplankton (e.g. cladoceran and copepods).
Phylum Nematoda
mouth
pharynx
gut
Reproductive
tract
anus
General biology
--Unsegmented, round worms that range in size from
microscopic to 250 um.
--Live almost anywhere and are a major taxa of sediment
fauna. Can even live in hot springs at 62˚ C!
--Estimates of 10,000 to 30,000 species but may be up to
20 times higher.
--Many species are parasitic on plants and animals; feed
on a variety of food resources.
--Most are dioeceous and no asexual reproduction occurs.
--Exhibit sexual dimorphism; males are smaller and have
curved posterior ends
Implications for Humans
--Humans are hosts to about 50 species of parasitic
nematodes:
--Life cycles of parasitic nematodes not as complex as
trematodes because usually involves only 1 host.
--Common human diseases
pinworm
whipworm
hookworm
intestinal roundworm
Trichinella
Onchocerca “river blindness”
Filaria “elephantiasis”