Diversity of Aquatic Organisms Zooplankton Part 4 1

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

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 http://www.uwosh.edu/faculty_staff/sandrin/proteomics/images/prot%20pics/daphnia.jpg

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

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 www.tolweb.org/tree/ToLimages/cava042p.jpg Asplanchna www.microscopy-uk.org.uk/mag/artfeb02/fresh/freshimg/rotifers.jpg cyclot.hp.infoseek.co.jp/wamusi/mituude1.jpg

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/

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 www.cof.orst.edu/project/plankton/bos.jpg /www.internal.eawag.ch/~steiner/Zoo/Bilder Bythotrephes www2.biologie.uni-halle.de/zool/dev_biol/lect/pk_lim/limno/Daphnia.jpg

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

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 http://www.emporia.edu/biosci/invert/lab6/copnaup1.jpg centexnaturalist.com

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 www.glerl.noaa.gov www.sahfos.ac.u www5.pbrc.hawaii.edu

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 www.gpmatthews.nildram.co.uk/

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)

http://www.youtube.com/watch?v=P16YdZEbLZk&feature=related http://www.youtube.com/watch?v=wWus4q-A8pU http://www.youtube.com/watch?v=Ne5Gw-qBCoA http://www.youtube.com/watch?v=7DU9tYJGmFs&feature=related http://www.youtube.com/watch?v=Mxps7wNFxgk http://www.youtube.com/watch?v=93ao5aKp3E0&feature=related

Avoidance of Predation Cyclomorphosis Daphnia and others may grow long helmets and tail spines (inducible defenses) to help avoid invertebrate predators www.unb.ca www.esf.edu/efb/schulz/Art.html www.nature.com

Cyclomorphosis Why not have long spines and helmets all the time? Energetically expensive, lowers reproductive rate Interferes with feeding in some cases (Bosmina) www.ecostudies.org/images www.fbl.ku.dk

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 www.microscopy-uk.org.uk Diaphanosoma www.glerl.noaa.gov Epischura www.lbm.go.jp/emuseum

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 www.wellesley.edu

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” www.lbm.go.jp/emuseum www.internal.eawag.ch www.microscopy-uk.org.uk

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.

Avoidance of Predation Brooks and Dodson, Science 1965 (text pp 168-175) 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