Zooplankton - Montana State University by maclaren1

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									Reproduction and Life Histories of Zooplankton
Rotifers and Cladocerans reproduce by diploid female parthenogenesis (females produce young without
mating) throughout much of the growing season. During periods of stress (low,food, light, temp.,
crowding) meiosis occurs leading to the formation of haploid males and females.

The haploid males and females mate and the fertilized, diploid eggs are produced which are encased in a
heavy shell or ephippium. These resting eggs are shed when the female molts and overwinter in the
sediments and will develop into amictic diploid females
These resting eggs are very durable and can remain a viable “seed bank” of organisms for decades.

After the resting or parthenogenic eggs hatch, the Cladoceran undergoes a number of rapid instar stages
which differ mainly in size. Temperature is the main factor that determines growth rate. A Daphnia gives
birth to her first brood in about 48 hours at a water temperature of 25C.
However, the fecundity or numbers of young/brood are more influenced by availability and quality of food.

Copepods are bisexual and carry their fertilized eggs in external egg sacs.

As with the Cladocera, Copepod egg production is determined by temperature and clutch size is determined
by food quality.

Copepod eggs hatch into free-swimming larvae or nauplii that are morphologically very different from the
adult forms. The nauplii go through six molting (naupliar) stages.
The nauplii then transform into copepodites and go through five additional molts before assuming adult
form.

Development of a Copepod from egg to adult is much longer than compared to Rotifers and Cladocerans

The Life Cycle of Copepods
The life cycle of copepods can be divided into two periods:
a period of active growth
a period of slowed growth or diapause caused by the onset of low temperatures, reduced light, reduced
food, or anoxia.

The degree to which a copepod population enters diapause, the instar stage when diapause occurs and the
location (water, sediments) of diapause is highly variable among species.

The life cycles of the Calinoid vs. Cyclopoid copepods differ in that the seasonal life cycles of the Calinoid
copepods (time from nauplii to adult stages, active growth period) are longer than those of the Cyclopoids.

Spatial and Seasonal Distributions of the Zooplankton


Distribution of Rotifers
Rotifer populations may be distributed seasonally as well as spatially and depend upon a number of factors:
Many species are perennial and will exhibit maximum densities in the summer, much as do the
Cladocerans.
Other species are better suited to a limited temperature range, some preferring colder (cold stenotherms)
temperatures, and will be most numerous in early spring or winter.
 Some species will have two population maxima/year (probably nutrient dependent like the Diatoms).
Some species are specialized for low oxygen conditions.

Distribution of Cladocerans
Like the Rotifers, Cladoceran populations are distributed seasonally as well as spatially and depend upon a
number of factors:
Different species of Cladocerans do best under different water chemistry conditions (hard vs. soft water,
different lake prod.).

Some Cladocerans will overwinter as adults, while most overwinter in a resting egg stage. Aestivating
populations usually develop population maxima in the spring and again in the fall

Temperature increases in the spring hasten growth and shorten the time between successive broods.

Availability of food determines the number of eggs/brood
Maximum predation of Cladocera occurs in the summer.

Diurnal (Diel) Movements of Zooplankton

Many of the zooplankton, particularly the Cladocerans, undergo daily vertical migrations in the water
column.
The general direction of movement is toward the surface during the nighttime hours and toward the bottom
during the daytime. Light stimulus of the eyespots seems the main stimulus for Diel migration.
Rates of vertical movement are highly variable from <2m/hr for Rotifers to >20m/hr for some Cladocerans
and Copepods.

The degree or length of vertical movement will be greatest in clear lakes and less in highly productive
lakes. The reason for vertical migration is unclear but is probably related to avoidance of predation by
small fish which are mainly visual feeders.

Horizontal Distribution of Zooplankton
Pelagic Copepods and Cladocerans tend to move away from the littoral zone (why?). These organisms
apparently are responding to changes in light intensity and angle that occur as the shoreline is approached.

As the zooplankters are weak swimmers, the organisms undergo a great deal of random re-distribution due
to water movements such as Langmuir circulations (remember those?) and often become trapped on the
surface film or in the metalimnion.

Seasonal Polymorphosis
Seasonal polymorphism or cyclomorphosis is found among many of the zooplankters, but is most marked
among the Cladocerans. These adaptations are thought to be in response to predation pressure
Some ways that Rotifers change their form are by:
 elongation of the body
 reducing body size (less conspicuous)
producing spines

Seasonal polymorphosis among the Cladocera involves lengthening the helmet (in Daphnia) and the anal
spine.

Most of the growth that is due to seasonal polymorphosis occurs in the extremities, presumably to allow the
animal to escape with vital parts intact.

As a rule, Copepods do not undergo seasonal polymorphosis. This is probably because the Copepods are
faster, more erratic swimmers and are thus more likely to escape predation than rotifers or cladocerans.

Factors Inducing Seasonal Polymorphosis
The cues to undergo polymorphosis consist of a combination of environmental factors:
increased Temp.
food availability
turbulence
photoperiod
Other Body Adaptations
Zooplankton in clear, non-productive lakes tend to be smaller and clear-bodied whereas those in productive
lakes tend to be larger-bodied and colored. Zooplankton in high-altitude lakes that receive a lot of UV
radiation are often very dark.

The Cladoceran Holopedium is encased in a gelationous coating which may be an anti-predation
adaptation.

Competition among Cladocerans
Feeding among the Cladocera encompasses a wide range of food sizes and levels of the food web (detritus,
periphyton, phytoplankton, other zoops.)
Adaptations to different types of lakes clear, productive, high altitude, and ephemeral lakes
Adaptations to different temp. water chem., and light regimes.

Competition among Copepods
Many of the Cyclopoid copepods are predaceous and affect Copepod competition at both the inter and intra
(cannibalism of immature stages) species levels through predation.

Competition is reduced among the copepods by:
variations in the timing and duration of diapause
seasonal and vertical separations between species
utilization of different-sized food particles

Bottom-Up Control of Zooplankton
Food selectivity depends upon the morphology of the cilia (rotifers) or setae (Cladocerans, Herbivorous
Copepods) in a filter feeding organism.
Feeding rates are inversely proportional to food concentration
Feeding rates are directly proportional to body size and increasing temperature
Food size is directly proportional to body size.

The effectiveness of zooplankton grazing varies seasonally and among different types of lakes. During
most of the year, filtering by zooplankton is low (<15%/day). However, during the peak of summer,
grazing by zooplankton can remove large amounts of phytoplankton and have a dramatic effect on phyto.
productivity. Conversely, the sizes and amounts of available phytoplankton have a dramatic effect upon
zooplankton populations.

Algal species succession can be severely altered by grazing. Smaller algae tend to pass through the
digestive tracts of zooplankton intact and can even undergo growth within the nutrient-rich digestive tract.

Assimilation efficiency among zooplankton is usually < 50%. Efficiency increases with temperature and
decreases with increasing food concentration. Efficiencies are lowest with detrital particles and highest
with size-appropriate algae.

Top-Down Control of Zooplankton
Planktivorous fish are a major factor in regulating both the size structure and abundance of zooplankton.

Size selection of prey is governed by combination of the energy return/prey item, gape limitation, and prey
item abundance.
Initially, only larger prey are taken, then fish switch to smaller prey as the large prey supply is exhausted.

When size selection by fish is not a factor, the larger forms of zooplankton will dominate as smaller
invertebrates (also gape-limited) such as carnivorous Copepods prey upon small zooplankton.

								
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