1. Zooplankton

2. Zooplankton Planktonic animals can be found in almost all animal phyla
Most zooplankton belong to 3 major groups: rotifers, Cladocera, and Copepoda

3. Zooplankton One other group may, at times, be important: Protozoa
Spend only portion of lives in plankton (mostly sediment-dwelling)
Feed on bacteria, detritus (little used by other zooplankton)

4. Rotifers Mostly littoral, sessile, but some are completely planktonic
May be dominant zooplankton in some lakes
Omnivorous, small (<12 µm)
Filter-feeding with corona

5. Rotifers Some are predatory Asplanchna
Feed on protozoa, other rotifers, small crustaceans

6. Rotifer Reproduction
Reproduction during most of growing season by diploid female parthenogenesis
Diploid eggs produced via mitosis
Develop into amictic females
Continues for generations during good conditions

7. Rotifer Reproduction Environmental stress causes changes
Drop in temperature
Crowding (food)
Accumulation of pheromones from females
Reduced availability of food components (e.g., vitamin E)

8. Rotifer Reproduction
Mictic females develop, produce haploid eggs via meiosis
Unfertilized eggs develop into males
Mate with mictic females to produce thick-walled resting eggs

9. Rotifer Reproduction
Resting eggs resistant to adverse environmental conditions
Eggs remain in diapause until return of favorable conditions
Hatch into amictic females

10. Rotifer Population Dynamics
Different species exhibit different population peaks
Some in early summer, others in winter/early spring, others multiple times in summer

11. Rotifer Cyclomorphosis
Seasonal polymorphism
Elongation, enlargement or reduction, production of spines

12. Rotifer Cyclomorphosis
Reduce sinking rate in warmer water
Cope with larger prey
Better resist predation
Brachionus

13. Rotifer Cyclomorphosis
Spines prevent ingestion of Brachionus by Asplanchna
Formation of spines induced by organic substance (kairomone) produced by predator

14. Rotifer Use by Fish Too small to be important as food for most fish
May be important in diets of some larval fish
Rotifers are potential prey for predatory copepods
Vertical migration upward at midday to avoid copepods

15. Cladocera
Small crustaceans ( mm) with head, and body covered by bivalve carapace
Swim by using large 2nd antennae
Filter phytoplankton, detritus for food (some are predators)

16. Cladocera Size of phytoplankton ingested proportional to body size
Rate of filter feeding increases with size and temperature
Selective filtering by cladocerans can remove big “chunks” of the phytoplankton, and alter phytoplankton succession

17. Cladocera Reproduction
Reproduction similar to that of rotifers
Parthenogenesis by diploid females throughout most of the growing season
Continues until interrupted by unfavorable conditions

18. Cladocera Reproduction
Temperature reductions, drying, reduced photoperiod, crowding (competition for food), decrease in food size/quality

19. Cladocera Reproduction
Some eggs develop into diploid males
Females produce haploid eggs
Mate with males
Fertilized eggs overwinter in thickened brood pouch - ephippium

20. Cladocera Reproduction
Ephippia can withstand severe conditions
Can be transported by birds to other waters
Hatch under favorable conditions into parthenogenetic females

21. Population Dynamics Similar to those of rotifers
Some overwinter as adults, others as resting eggs (ephippia)
Increased food and temperature enhance production

22. Diurnal Vertical Migrations
Most migrate to surface at dusk, downward at dawn (light intensity the stimulus)
Movements may be >50 m and rapid (20 m/hr)

23. Diurnal Vertical Migrations
Reasons for migration:
1) avoid visual feeding fish in epilimnion by coming up to feed on phytoplankton after dark
2) improve food utilization - filter faster in warmer water, assimilate better in cooler water

24. Cladoceran Cyclomorphosis
Extension of head to form helmet
Increase of caudal spine length
Caused by increased temperature, turbulence, photoperiod, food

25. Cladoceran Cyclomorphosis
Advantage of allowing for continued growth of transparent peripheral structures without enlarging central portion of body visible to fish
Reduces predation

26. Food for Fish!
Large species favored by many fish (visual and filter-feeders)
More energy return from bigger species
Eliminates large forms, small ones flourish (big forms often predatory)

27. Copepoda Microcrustaceans in same size range as cladocerans
Several different groups based on differences in body structure
2 major groups: cyclopoids and calanoids

28. Copepoda
Cyclopoids - short 1st antennae

29. Copepoda
Calanoids - long 1st antennae

30. Copepoda
Cyclopoids - most are littoral, but few are open-water planktonic forms
All seize food particles and bring them to mouth - raptorial

31. Copepoda
Most are predators (eat zooplankton), but some are herbivores (phytoplankton)
Move by swimming with legs

32. Copepoda
Calanoids almost strictly open-water planktonic, seldom in littoral areas
Primarily filter feeders on algae, detritus (filtering appendages near mouth - maxillae)

33. Copepod Reproduction No parthenogensis Both males, females present
Sexual reproduction always present
Fertilized eggs carried attached to female’s abdomen

34. Copepod Reproduction Eggs hatch into nauplius larvae - 3 prs. of legs
Grow and molt several times to become copepodite
Grow and molt more before becoming adult

35. Copepod Reproduction
Longer period of time from egg to adult than in rotifers, cladocerans
May have resting eggs (overwinter), or diapause in egg or copepodite stage

36. Community Dynamics
Predation by cyclopoid copepods may kill up to 30% of nauplii or copepodites (of own or other species)

37. Community Dynamics
This predation may result in vertical, seasonal separation of similar species

38. Community Dynamics Same diurnal vertical migrations as cladocerans
No cyclomorphosis
Great as fish food!

39. Zooplankton Generalities
Assimilation efficiency ~50%
Increased with higher temps.
Decreased with increased food availability

40. Zooplankton Generalities
Highest assimilation when feeding on prime food - right type, size
Lower when feeding on bacteria
Lowest when feeding on detritus

41. Zooplankton Generalities
Seldom evenly distributed
Avoidance of shore
At mercy of epilimnetic water movements, especially Langmuir spirals

42. Zooplankton Generalities
Productivity correlated with phytoplankton production
Filter-feeders have higher productivity than predators

43. Zooplankton Generalities
Many zooplankton abundant in littoral areas
Associated with macrophytes, sediments
Abundance related to plant surface area, algal/detrital abundance

44. Zooplankton Generalities
Abundance generally highest in spring and fall, lowest in mid-summer
Lows correspond with heavy predation by insect larvae, small fish