Zooplankton

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

Rotifers

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)

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)

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

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

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

Cladocera

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)

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

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

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

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

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

Copepods

Copepods are streamlined and range from 0.5 to 4 mm

Copepods

Copepods

Three Major Groups of Copepods

Calanoid

Cyclopoid

Harpactacoid

Calanoid Copepods

Calanoid Copepods

Almost exclusively planktonic

Omnivorous, herbivorous, carnivorous

Swims more smoothly than harpacticoids and cyclopoids

Cyclopoid Copepods

Cyclopoid Copepods

Primarily benthic but some forms are planktonic

Omnivorous, herbivorous, carnivorous

Harpactacoid Copepoids

Harpactacoid Copepoids

Almost exclusively benthic

Detritivores

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

Reproduction in Copepods

Eggs hatch into free-swimming larvae called nauplii

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

Vertical Migration

Cladocera and copepods

Most species migrate up in the water column during darkness and return to lower depths during daylight

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

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

Rotifers

No clear pattern of migration of rotifers has been observed

Range of migration is less because of limited powers of locomotion

Cyclomorphosis

Seasonal polymorphism

Patterns of Cyclomorphosisin Rotifers

Elongation in relation to body widthEnlargementReduction in sizeProduction of lateral spines

Causes of Cyclomorphosisin Rotifers

Causes of cyclomorphosis in rotifers– Changes in temperature and viscosity– Predation– Competition

Patterns of Cyclomorphosisin Cladocera

Extension of helmet

Extension of tail spine

Increase in size

Causes of Cyclomorphosisin Cladocera

Changes in temperature and viscosity

Food supply

Water turbulence

Predation

Cyclomorphism in Copepods

Cyclomorphism is relatively uncommon in copepods

A few taxa exhibit slightly smaller body size as temperature increases

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

No Cyclomorphosis in Copepods

Copepods do not exhibit extensive polymorphism and rely on locomotion to evade predation

Reactions to Changing Conditions

Change in egg production

Cyclomorphosis

Resting stages– Aestivation (asexual)– Diapause (sexual)

Vertical migration

Environmental Factors

Temperature– Egg production increases with increases in

temperature– Feeding rate increases with increase in

temperature– Cyclomorphosis

Environmental Factors

Food availability– Clutch size decreases as food availability

decreases– Cyclomorphosis– Resting stages

Environmental Factors

Crowding– Cyclomorphosis– Resting stages

Environmental Factors

Change in light– Aestivation

Daily changes in light– Vertical migration

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

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

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

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

Vulnerability to Predation

Visibility is important– Apparent visibility– Eyespot size

Genetic variability within Cladocera can lead to morphological escape

Invertebrate Predators

Prey morphology critical for vulnerability to grasping predators

Behavioral escape - vertical migration

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