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Zooplankton
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Zooplankton
Zooplankton in lakes are composed mainly of rotifers, cladocerans, and copepods– The zooplankton may also include protozoans, a
few coelenterates, larval flatworms, mites, insect larvae, and fish larval stages
Zooplankton generally range in size from 0.1 mm 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 a thickened cuticle (lorica)Most rotifers are omnivorous– A few taxa are predatory (e.g. Asplanchna)
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Rotifers
The majority of rotifers are not planktonic (epipelic, epiphytic, or epipsammic)
Rotifer populations appear quickly and decline quickly (partly a result of their lack of defenses)
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Reproduction in Rotifers
Males are often absent for many successive generations and reproduction occurs by parthenogenesis of diploid females
These females are amictic females and they produce amictic eggs
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Reproduction in Rotifers
Under conditions of environmental stress, mictic females are produced
The mictic female produces mictic eggs formed by meiosis and are haploid
If mictic eggs are not fertilized, they hatch and form males that are haploid
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Reproduction in Rotifers
When a mictic egg is fertilized by a male, they form thick-walled encysted embryos known as resting eggsResting eggs are resistant to adverse environmental conditions– Resting eggs may not hatch for several weeks or
months– Hatching is related to changes in temperature,
osmotic pressure, water chemistry, and oxygen concentration
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Cladocera
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Crustacean zooplankton
The major crustacean zooplankton are cladocerans and copepods
Other crustaceans found in the zooplankton include aquatic insect larvae, ostracods, and branchiopods (fairy shrimp, tadpole shrimp, brine shrimp)
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Cladocera
Cladocerans range from 0.2 to 3.0 mmAll cladocerans have a distinct head and a bivalve carapaceCladocerans have a compound eye (light-sensitive organ)The second antennae provide the major means of locomotion
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Cladocera
Cladocerans consume particles by filtration and are omnivorousWaving motions of the setose legs pass stream of water and particles anteriorlyParticles are filtered from the water by setae and are passed in a food groove to the mouth
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Reproduction of Cladocera
Males are often absent for many successive generations and reproduction occurs by parthenogenesis of diploid females (as was observed in the rotifers)
Eggs are deposited in a brood pouch in a cavity dorsal to the body
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Reproduction of Cladocera
Under unfavorable environmental conditions or poor food supply, some of the eggs develop into males
Females then produce a few haploid sexual eggs
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Reproduction of Cladocera
After fertilization, the carapace around the brood chamber thickens and encloses the eggsThis encased fertilized egg is called an ephippiumEphippia can withstand severe environmental conditions
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Copepods
Copepods are streamlined and range from 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 are planktonic
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 CopepodsReproduction occurs by sexual fertilization and males are always present
Resting eggs may be formed but both resting eggs and subitaneous eggs are formed by sexual reproduction
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Reproduction in Copepods
Eggs hatch into free-swimming larvae called nauplii
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Comparison of Reproductive StrategiesRotifers - multivoltine (ie. many generations per season) but abundant for short timespans(days-weeks)
Cladocera - multivoltine, can be present for several 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 water column during darkness and return to lower depths during daylight
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Vertical Migration
Nocturnal migration - one period of maximum biomass in surface waters at night
Twilight migration - two periods of maximum biomass in surface waters at dawn and dusk
Reverse migration - one period of maximum biomass in surface waters during daylight
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Vertical Migration
Migration may range from a few centimeters 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 at
lower temperatures)– Reduces interspecific and intraspecific
competition in grazing
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Rotifers
No clear pattern of migration of rotifers has been observed
Range of migration is less because of limited powers of locomotion
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Cyclomorphosis
Seasonal polymorphism
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Patterns of Cyclomorphosisin Rotifers
Elongation in relation to body widthEnlargementReduction in sizeProduction of lateral spines
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Causes of Cyclomorphosisin Rotifers
Causes of cyclomorphosis in rotifers– Changes in temperature and viscosity– Predation– Competition
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Patterns of Cyclomorphosisin Cladocera
Extension of helmet
Extension of tail spine
Increase in size
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Causes of Cyclomorphosisin Cladocera
Changes in temperature and viscosity
Food supply
Water turbulence
Predation
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Cyclomorphism in Copepods
Cyclomorphism is relatively uncommon in copepods
A few taxa exhibit slightly smaller body size as temperature increases
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Adaptive Significance
Cyclomorphism is confined to epilimnetic species
Adaptive significance of cyclomorphosisis related to predation and sometimes competition– Predation by fish– Predation by large zooplankters
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No Cyclomorphosis in Copepods
Copepods do not exhibit extensive polymorphism 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 in
temperature– 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 and visually 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 are eaten less frequently or ignored
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Size-Selective Predation
Planktonic herbivores compete for fine particulate matter in open waters
Larger zooplankton feed on fine particulate matter more efficiently and take larger particles
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Size-Selective Predation
When there is low predation intensity, larger zooplankton competitively eliminate smaller zooplanktonWhen predation intensity is high, larger zooplankton will be eliminated, allowing smaller zooplankton to become dominantWhen predation is moderate, enough large zooplankton are removed to allow smaller zooplankton to remain
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Vulnerability to Predation
Visibility is important– Apparent visibility– Eyespot size
Genetic variability within Cladocera can lead to morphological escape
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Invertebrate Predators
Prey morphology critical for vulnerability to 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– Ceriodaphnia reproduce well– Daphnia eat but don't reproduce