10/27/00
Aquatic Biology
Zooplankton I: Zooplankton life histories and adaptations
I. Introduction to the Zooplankton
A. Use a videotape of lake sample from your files
II. Roles played by zooplankton mainly relate to providing a link between bacteria and phytoplankton and the larger consumers in lake
ecosystems.
A. Secondary production - ingest and process plant and bacterial primary (or decomposer, microbial loop) and convert it into
animal material
B. Primary consumers - the majority of zooplankton are herbivorous (or microbivorous) with three components making up the
majority of their diet.
1. phytoplankton
2. bacteria
3. organic and inorganic POM
C. Eaten by the secondary consumers 1. Almost all zooplankton are good food items, especially for fish larvae and small fish species (most fish however are
not Obligate "planktivores" due to below(2-3)
2. Size of zooplankton is often inappropriate for "nutritious" feeding and zoop use cyclomorphosis against predator
populations
Daphnia can starve on particles sized < 10 , so size works both ways
3. Populations are very unstable as the phytoplankton popns "cycle" through time
III. Major groupings and life history adaptations of Zooplankton
A. Microzooplankton
1. Protozoans - flagellates and ciliates
a. feed on detritus, bacteria, and v. small algae]
b. especially abundant in shallow ponds like lake valentine
c. their contribution to the biomass of consumers is quite minor, but they do serve to connect bacterial
components with larger zooplankton n the fod chain
d. are filter feeders in the pelagic and littoral zones with little really known about the group as a whole or
specific species in general.
2. Rotifers a. in their own phylum next to the nematodes (superphylum aschelminthes) as the only other pseudocoelomates
b. Mostly on the small size 40 m to 2.5mm but mostly around 100m and planktonic (1/4) to attached (3/4)
c. Thus they must be filter feeders and grazers on the decomposers in the microbial loop especially in high
organic (acidic to neutral lakes)
d. Life history:
Are found (when would you expect?) in early summer to late summer when food is available.
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i. Amictic ™ (2N) via parthenogenesis produces amictic eggs which develop in one day ->
amictic ™ ->AE-> A™ about 20 - 40 times more
ii. If (a) stress in food supply, (b) decreased T, or (c) changes in photosynthetic rate [ as
Algae increase in diet and Protozoans decline, there is an increase in d- -tocopherol (Vitamin E) in
the diet esp. in green algae typical of fall bloom (vitamin E essential for sprmatogenesis)]
AE->Mictic ™ (2N) which produces normal mictic eggs (N)(by normal meiosis)
ME-> male if unfertilized (males lack digestive system and are just sperm clone machines
that are ready to copulate within one hour of hatching
ME-> if fertilized they develop into a thick walled encysted resting stage
Resting stage senses T, osmotic pressure, light, water chemistry, O2??? and hatches as an
amictic ™ .
e. Feeding strategies are based upon the structure of their sclerotized jaws (trophi)
i. malleate trophi are comblike and are crunchers and grinders in the plnaktonic community
ii. virgate are suction cup piston plunger types which suck juices out of plant and animal and
thus are littoral attached and epiphytic
iii. forcipate - predators with a trophi that extends out of oral grooves and grabs food littoral and planktonic
f. are selective filter feeders (12 - 50 m) [ few are raptorial predacious] with starved being less selective than
well fed and selectivity increasing with increasing filter clogging food particle size (yeasts vs algae) (ingested
food takes from 3 to 20 minutes to pass through the digestive tract.
yeas t food
FR
(# eaten
/ rotifer/time)
bacteria
algae
inrease in pseudotrocal
s creening
Food dens ity
g. population growth rate is dependent thus on density and quality of food
16.4 microg/ml
Popn growth
rate
Food dens ity
Brachionus, Keratella, Asplanchna
B. Mesozooplankton
1.
CLADOCERA are early summer filter feeders that use their thoracic legs to set up a current and a "post-abdominal" spine as
a toothpick to reject too large particles
- the rate of food processing is thus determined by
a. beating rate
b. Width of carapace opening
c. rate of PAS rejection
- work by Gerritsen and Porter on feeding rates
flow experiments shows that flow through the sieves is laminar and due to slow flow and water viscosity the sieve plate
acts as a wall to the water --- so how do they feed selectively????
- styrene spheres of different sizes used on sp. Daphnia magna
Ho: Sieve model: all particles less than 1.0m will pass through the filter = "slit size"
- allowed animals to feed for 30 minutes and killed and examined the ingested particls (sizeed and counted)
- Experiment 1 - untreated spheres - results smallest not fed on as much as expected and the larger were selected more
than expected.
- conclusion not a true passive sieving but selective grazing
- Experiment 2 treated amide to neutralize the natural - charge and they filtered in proportion (observed = expected)
- Conclusion surface properties determine capture efficiency for a given food item leading to neutral being captured
more readily than charged particles Thus it is not size selection but surface chemistry selection
- problems acid rain kills filter feeders as effects charges>>>>>
Are parthenogenic most of the year
A few 2N ™'s produce 1N
eggs
2N ™
Amictic 2N eggs
in "brood sac"
Most 2N ™'s
produce eggs which
develop into 2N
s
Adult molts to release
the young from the
brood pouch - no free
living larval stages
Baby 2N ™ small
adults, with many
feeding & "food" sizes;
2 - 7 adult instars with
new eggs in each instar
sperm
Copulation results in fertilized eggs in the brood
sac which develops a thickened surrounding
carapace making a resistant black egg case called
the ephippium. These are especially formed as
days get shorter (light) and then sink (or float)
Eggs hatch with long
days = light
Developing into…
2.
COPEPODS
a. are sexual - Why?
and ™'s
2 lateral egg sacs in cylopoid
&
1 medial in calinoids
Molt into copepodites and
these molt 5- 6 more times
increasing # appendages each
molt
Hatch into free
swimming larvae called
nauplii
Resting eggs which
diapause in the
sediments
The many sizes of young play very important roles in the food web.
b.
feeding
Calenoid copepods make up 30% of the pelagic zooplankton (herbivorous)
Algae are really low in concentration in most waters and so feeding becomes a major issue.
Originally these were thought to be filter feeders but there were problems associated with this hypothesis.
- must scan large amounts of water to get enough food to survive
- must avoid the pull of gravity as they sink
Solutions
- Flow fields created with anterior appendages that optimally "sieves water
-sampling is done via algal chemicals
- gravity is used as an orientation device resulting in a species specific hunting for the best patches in which to
feed - cruising vs feeding
III.
Filter feeding is a very distinct energy extraction technique
A. Two separate processes
1. Filtering rate - the rate of extraction by filter feeding
2. Feeding rate - (includes FR above)
- ingestion - take into the digestive system (what are associated problems)
- digestion - breakdown and absorption - (what are problems with eating phytoplankton?)
B. Study # 1 Rigler subjected crustacea to different densities of phytoplankton with the following results
Filterin g rate
105 cells
Filtering rate slows down as gut packing
occurs. WE need to take apart the
physiology of the organism to understand
this phenomenon.
# phytoplankton
Why?
1.
2.
3.
4.
5.
Thoracic limbs slow down
Carapace narrows
Rejection rate increases
Is a size related phenomenon
- increased size leads to increased energy needs
- leads to increased need for higher feeding activity
Process is internally regulated - why is this necessary?
If full  need to decrease feeding, but if in an optimal environment, without internal regulation the feeder would go on feeding
and "waste" energy.