Zoo Plankton

8/3/2019 Zoo Plankton

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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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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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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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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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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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Food availability Clutch size decreases as food availability

decreases

Cyclomorphosis Resting stages


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Crowding Cyclomorphosis

Resting stages


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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

Zoo Plankton

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