Major Aquatic Invertebrate Taxa

Major Aquatic Invertebrate Taxa
Porifera (sponges)
Cnidaria (hydra & jellyfish)
Platyhelminthes (flatworms)
Gastrotricha (gastrotrichs)
Rotifera (rotifers)
Mollusca (snails/bivalves)
Annelida (oligochaetes/leeches)
Bryozoa (moss animals)
(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.
Phyla: Mollusca, Annelida, Tardigrada, Arthropoda
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.
--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
--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
--Cnidarians reproduce both asexually and sexually.
Medusa are produced by budding of polyps but not vice
--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
--Essential to the feeding process are thin, flexible
--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
About 200 species in N. America
Turbellarians: General morphology
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
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
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
--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
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
--No circulatory or respiratory system per se.
--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.
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
--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
-- 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
Functional role
--Because of their sheer abundance and quick turnover
time, they can exert significant grazing pressure on
--Can represent up to 50% of zooplankton production in
--Generally have lower ‘filtering rates’ as compared to
crustacean zooplankton (e.g. cladoceran and copepods).
Phylum Nematoda
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
--Life cycles of parasitic nematodes not as complex as
trematodes because usually involves only 1 host.
--Common human diseases
intestinal roundworm
Onchocerca “river blindness”
Filaria “elephantiasis”