Zooplankton

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

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)

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

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

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

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)

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

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

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

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

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

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

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

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)

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

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

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

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

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

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

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)

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

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

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

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)

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

Copepoda Cyclopoids - short 1st antennae

Copepoda Calanoids - long 1st antennae

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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