1. Diversity of Aquatic Organisms Zooplankton Part 41

2. Zooplankton Why study zooplankton?
Important link between primary producers (algae) and fish production
Indicators of lake history and health
Good models for basic ecological and evolutionary principals

3. Size categories of zooplankton
Macrozooplankton
Larger than 200 um
Mostly crustacean species and some meroplankton (fish and insect larvae, zebra mussel larvae)
Microzooplankton
Smaller than 200 um
Mostly rotifers, also protozoans
Crustacean
Rotifer
homepage.ntlworld.com/f.longrigg/rotifers.html homepage.ntlworld.com/f.longrigg/rotifers.html

4. Taxonomic Groups Protozoa (animal-like protists)
Cilates (e.g. Paramecium)
Zooflagellates (e.g. Giardia)
Rhizopoda (Amoebas)
Phylum Rotifera
Rotifers
Phylum Arthropoda
Crustacea
Branchiopoda (mainly fresh-water water fleas)
(Daphnia, Bosmina, Leptodora, Bythotrephes, etc)
Copepoda (both FW and marine)
Calanoid, cyclopoid, and harpactocoid groups
Malacostraca
Mysid shrimp, amphipods
Ostracoda
Insecta
Dipteran larvae

5. Rotifers ~1500 species, size between 0.04 and 2.5 mm
Generation time is a few days (temperature dependent)
Can tolerate lower Water Residence Time than larger zooplankton
Populations respond quickly to an increase in food resources
Most are filter-feeders
Use corona to create feeding current
A few are predaceous (Asplanchna)
Prey species may exhibit cyclomorphorosis
Corona
Asplanchna
cyclot.hp.infoseek.co.jp/wamusi/mituude1.jpg

6. Rotifer Reproduction
Rotifers usually reproduce asexually through Parthenogenesis
Entire population is diploid female. Eggs are produced which are clones of the mother
When would this be an advantage over sexual reproduction?
Haploid males produced under stressful conditions
science.kennesaw.edu/

7. Branchiopods Most are filter feeders
feeding appendages act as electrostatic filters
Feed on phytoplankton, bacteria, protozoa
Often the dominant herbivores in lake systems (before zebra mussels)
Generation time is a few weeks
Some are predators on other cladocerans and rotifers (Leptodora, Bythotrephes)
webs.lander.edu/rsfox/rsfoximages3/clad99L_x550_x_643x.gif
Bosmina
Leptodora /
Bythotrephes
www2.biologie.uni-halle.de/zool/dev_biol/lect/pk_lim/limno/Daphnia.jpg

8. Ephippium with 2 resting eggs
Male Daphnia
First antennae
Branchiopods are usually parthenogenetic
Produce eggs (clones) after each molt (~3-30 eggs)
Sexual reproduction can be triggered by
Low food condition
Decreasing photoperiod
Overcrowding
Resting eggs are contained in an Ephippium
Can be dried, transported on legs of waterfowl
Eggs from Ephippia buried in sediments can survive decades
Ephippium with 2 resting eggs

9. Copepods
Three major groups: Cyclopoida, Calanoida, (pelagic) and Harpactocoida (benthic)
Always sexual reproduction, near equal numbers of males and females
Cyclopoids
Life Cycle
Shorter generation time (1-2 months)
Eggs hatch as nauplius larvae
Several juvenile molts (copepodids) before becoming adults
Juvenile stage may enter diapause in fall and re-activate in the spring
Adult male
Adult female
Female with egg sacs
centexnaturalist.com

10. Copepods Calanoids Life Cycle
Longer generation time (6 months or more)
Eggs hatch as nauplius larvae
Several juvenile molts (copepodids) before becoming adults
No diapause
Adult female
Adult male
www5.pbrc.hawaii.edu

11. Other zooplankton Mysis relicta (opossum shrimp) Amphipods (scuds)
Found in the hypolimnion of the deeper great lakes (Lake Superior, L. Michigan, Huron..)
Important food source for fish
Predators on other zooplankton
Amphipods (scuds)
Mainly benthic, eat settled algae
Important (and disappearing!) food resource for Great Lakes fish
Ostracods (seed shrimp)
Mainly in littoral zones
limnology.wisc.edu/
web2.uwindsor.ca

12. Zooplankton Feeding
Rotifers –filter feeding or predaceous (Asplanchna)
Cladocerans
Most are omnivorous filter feeders (Daphnia, Bosmina)
Relatively indiscriminant feeders (efficient with high quality food)
Use legs to create a feeding current and concentrate food particles
High filtering rates, high birth rates 
may overgraze phytoplankton
Some are raptorial predators (Leptodora, Bythotrephes)
Legs modified for grabbing prey, mouthparts for shredding
Cyclopoid copepods
All raptorial predators on phytoplankton and small zooplankton
Not very efficient when most particles are high quality food, but can be efficient when many particles are low quality.
Calanoid copepods
Many have dual feeding mode
Feeding current with constant filtering
Raptorial feeding – particles sensed at a distance
Very efficient, good when food is scarce (oligotrophic lakes)

14. Avoidance of Predation
Cyclomorphosis
Daphnia and others may grow long helmets and tail spines (inducible defenses) to help avoid invertebrate predators

15. Cyclomorphosis Why not have long spines and helmets all the time?
Energetically expensive, lowers reproductive rate
Interferes with feeding in some cases (Bosmina)

16. Avoidance of Predation
Escape Tactics
Calanoid copepod “jump” reaction
Flex 1st antennae to achieve burst of speed, acceleration up to 30G and 100 body lengths/second
“Dead Man” response
Some small cladocerans (Bosmina, Diaphanosoma) may avoid detection by invertebrate predators by stopping all activity and allowing themselves to sink slowly
Bosmina
Diaphanosoma
Epischura

17. Avoidance of Fish Predation
Diel (Daily) Vertical Migration (DVM)
Most zooplanktivorous fish are visual predators that feed during the day.
Larger zooplankton species typically migrate downward during the during the day to avoid being seen by fish
Migrate upward at night to feed on algae near the surface, more dispersed at night
Hypoxia in the hypolimnion can be a barrier to migration, but many zooplankton can tolerate lower DO than fish, therefore hypoxia can also serve as a refuge.
During day, may find high density of zooplankton in the metalimnion

18. Avoidance of Fish Predation
Try to be as transparent as possible
Especially true for large invertebrate predators (Chaoborus, Leptodora) and slow-moving species (Daphnia, Bosmina, etc)
Leptodora
Chaoborus
“Phantom midge”

19. Predation on Zooplankton
Brooks and Dodson, Science 1965, (Hrbacek 1962)
Found lakes with herring (Alosa), an efficient zooplankivorous fish were dominated by small zooplankton species (Bosmina, Tropocyclops)
Lakes without herring had large zooplankton species (Daphnia, large calanoid copepods, Leptodora)
Natural experiment – herring were added to a lake in 1950s, zooplankton samples from before (1940s) and after (1960s) showed the same shift to smaller species.

21. Avoidance of Predation
Brooks and Dodson, Science 1965 (text pp )
Formulated the “Size Efficiency Hypothesis”
Fish preferentially eat larger zooplankton species
In the absence of fish predation large zooplankton species will dominate because they are more efficient feeders
They can eat a larger size range of particles
Metabolic costs are proportionally lower
The SEH turned out to be a partial answer. It applies to some species some of the time.
Additional studies by S. Dodson and others showed that for zooplankton, avoiding invertebrate predators is more important than grazing efficiency
When fish are absent, invertebrate predators become more abundant
Invert predators preferentially consume small zooplankton species, giving larger species a size refuge