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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