Transcript Zooplankton Generalities

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 sedimentdwelling)
 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

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

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 midsummer
 Lows correspond
with heavy
predation by insect
larvae, small fish