Zooplankton

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 (0.2-3.0 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