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Zooplankton
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Zooplankton
Zooplankton in lakes are composed mainly ofrotifers, cladocerans, and copepods
The zooplankton may also include protozoans, afew coelenterates, larval flatworms, mites, insect
larvae, and fish larval stages
Zooplankton generally range in size from 0.1mm to 4 mm
Fish larvae range from 15 - 20 mm A few "giant" taxa of zooplankton may be as large
as 40 mm
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Rotifers
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Rotifers (Phylum Aschelminthes)
Rotifers evolved in freshwater Only phylum to evolve in freahwater
Rotifers possess a ciliated corona and athickened cuticle (lorica)
Most rotifers are omnivorous
A few taxa are predatory (e.g. Asplanchna)
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Rotifers
The majority of rotifers are notplanktonic (epipelic, epiphytic, orepipsammic)
Rotifer populations appear quickly and
decline quickly (partly a result of theirlack of defenses)
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Reproduction in Rotifers
Males are often absent for manysuccessive generations andreproduction occurs by parthenogenesis
of diploid females
These females are amictic females andthey produce amictic eggs
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Reproduction in Rotifers
Under conditions of environmentalstress, mictic females are produced
The mictic female produces mictic eggsformed by meiosis and are haploid
If mictic eggs are not fertilized, theyhatch and form males that are haploid
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Reproduction in Rotifers
When a mictic egg is fertilized by a male, theyform thick-walled encysted embryos knownas resting eggs
Resting eggs are resistant to adverseenvironmental conditions
Resting eggs may not hatch for several weeks ormonths
Hatching is related to changes in temperature,osmotic pressure, water chemistry, and oxygenconcentration
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Cladocera
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Crustacean zooplankton
The major crustacean zooplankton arecladocerans and copepods
Other crustaceans found in thezooplankton include aquatic insect
larvae, ostracods, and branchiopods(fairy shrimp, tadpole shrimp, brineshrimp)
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Cladocera
Cladocerans range from 0.2 to 3.0 mm All cladocerans have a distinct head
and a bivalve carapace
Cladocerans have a compound eye(light-sensitive organ)
The second antennae provide the majormeans of locomotion
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Cladocera
Cladocerans consume particles byfiltration and are omnivorous
Waving motions of the setose legs pass
stream of water and particles anteriorly
Particles are filtered from the water by
setae and are passed in a food grooveto the mouth
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Reproduction of Cladocera
Males are often absent for manysuccessive generations andreproduction occurs by parthenogenesis
of diploid females (as was observed inthe rotifers)
Eggs are deposited in a brood pouch ina cavity dorsal to the body
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Reproduction of Cladocera
Under unfavorable environmentalconditions or poor food supply, some ofthe eggs develop into males
Females then produce a few haploid
sexual eggs
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Reproduction of Cladocera
After fertilization, the carapace aroundthe brood chamber thickens andencloses the eggs
This encased fertilized egg is called anephippium
Ephippia can withstand severeenvironmental conditions
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Copepods
Copepods are streamlined and rangefrom 0.5 to 4 mm
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Copepods
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Copepods
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Three Major Groups of Copepods
Calanoid
Cyclopoid
Harpactacoid
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Calanoid Copepods
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Calanoid Copepods
Almost exclusively planktonic
Omnivorous, herbivorous, carnivorous
Swims more smoothly than
harpacticoids and cyclopoids
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Cyclopoid Copepods
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Cyclopoid Copepods
Primarily benthic but some forms areplanktonic
Omnivorous, herbivorous, carnivorous
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Harpactacoid Copepoids
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Harpactacoid Copepoids
Almost exclusively benthic
Detritivores
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Reproduction in Copepods
Reproduction occurs by sexual
fertilization and males are alwayspresent
Resting eggs may be formed but bothresting eggs and subitaneous eggs are
formed by sexual reproduction
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Reproduction in Copepods
Eggs hatch into free-swimming larvaecalled nauplii
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Comparison of Reproductive Strategies
Rotifers - multivoltine (ie. many generations
per season) but abundant for short timespans(days-weeks)
Cladocera - multivoltine, can be present forseveral months
Copepoda - present year round Cyclopoids: bivoltine, multivoltine
Calanoids - longer life cycles than cylopoids,
multivoltine
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Vertical Migration
Cladocera and copepods
Most species migrate up in the watercolumn during darkness and return tolower depths during daylight
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Vertical Migration
Nocturnal migration - one period of maximumbiomass in surface waters at night
Twilight migration - two periods of maximumbiomass in surface waters at dawn and dusk
Reverse migration - one period of maximumbiomass in surface waters during daylight
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Vertical Migration
Migration may range from a fewcentimeters to more than 25 m
Adaptive significance of migration Reduces predation by fish and other
predators that require light (sight feeders)
Maximizes growth efficiency (greater atlower temperatures)
Reduces interspecific and intraspecific
competition in grazing
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Rotifers
No clear pattern of migration of rotifershas been observed
Range of migration is less because oflimited powers of locomotion
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Cyclomorphosis
Seasonal polymorphism
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Patterns of Cyclomorphosis
in Rotifers
Elongation in relation to body width Enlargement
Reduction in size Production of lateral spines
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Causes of Cyclomorphosis
in Rotifers
Causes of cyclomorphosis in rotifers Changes in temperature and viscosity
Predation
Competition
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Patterns of Cyclomorphosis
in Cladocera
Extension of helmet
Extension of tail spine
Increase in size
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Causes of Cyclomorphosis
in Cladocera
Changes in temperature and viscosity
Food supply
Water turbulence
Predation
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Cyclomorphism in Copepods
Cyclomorphism is relatively uncommonin copepods
A few taxa exhibit slightly smaller bodysize as temperature increases
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Adaptive Significance
Cyclomorphism is confined toepilimnetic species
Adaptive significance of cyclomorphosisis related to predation and sometimescompetition
Predation by fish
Predation by large zooplankters
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No Cyclomorphosis in Copepods
Copepods do not exhibit extensivepolymorphism and rely on locomotion to
evade predation
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Reactions to Changing Conditions
Change in egg production
Cyclomorphosis
Resting stages Aestivation (asexual)
Diapause (sexual)
Vertical migration
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Environmental Factors
Temperature Egg production increases with increases in
temperature
Feeding rate increases with increase intemperature
Cyclomorphosis
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Environmental Factors
Food availability Clutch size decreases as food availability
decreases
Cyclomorphosis Resting stages
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Environmental Factors
Crowding Cyclomorphosis
Resting stages
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Environmental Factors
Change in light Aestivation
Daily changes in light Vertical migration
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Predator-Prey Interactions
Size-selective Predation by Fish Small body size is selectively
advantageous for zooplankton that are
exposed to predation by fish All planktivorous fish have closely spaced
gill rakers
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Predator-Prey Interactions
All planktivorous fish actively search for andvisually select their zooplankton prey
Searching ability and prey-handling efficiency
increase with increase in fish size
As prey abundance is increased and search
time decreases, smaller-sized classes areeaten less frequently or ignored
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Size-Selective Predation
Planktonic herbivores compete for fineparticulate matter in open waters
Larger zooplankton feed on fineparticulate matter more efficiently andtake larger particles
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Size-Selective Predation
When there is low predation intensity, largerzooplankton competitively eliminate smallerzooplankton
When predation intensity is high, largerzooplankton will be eliminated, allowingsmaller zooplankton to become dominant
When predation is moderate, enough largezooplankton are removed to allow smallerzooplankton to remain
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Vulnerability to Predation
Visibility is important Apparent visibility
Eyespot size
Genetic variability within Cladocera canlead to morphological escape
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Invertebrate Predators
Prey morphology critical for vulnerabilityto grasping predators
Behavioral escape - vertical migration
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Herbivory and Omnivory
Feeding by filtration
Algal morphology important
Selectivity varies between taxa
Some zooplankton thrive on bacteria
Ceriodaphniareproduce well
Daphniaeat but don't reproduce