IslandWood students often observe zooplankton species under microscopes. The word zooplankton comes from the Greek zoon, meaning "animal," and planktos meaning "wander" or "drifter." Zooplankton are generally weak swimmers that drift at the mercy of water currents in both marine and fresh water habitats. Many, however, have locomotive abilities. For example, zooplankton often move vertically through the water column over a 24-hour period, a phenomenon called diel vertical migration. Typically, they migrate to surface waters at night and move deeper during they day. This is thought to be a predator avoidance adaptation.
Zooplankton play a crucial role in aquatic food chains. They link microscopic plant life to larger organisms like fish and whales. They often graze on phytoplankton (Gk. "plant-drifter") suspended in the surface waters or attached to substrates. In spring, when phytoplankton grow quickly to sunlight and warmth, zooplankton populations increase in tandem. Water clarity is closely correllated to zooplankton populations due to their consumption of phytoplankton and other microscopic plant life suspended in the water column.
The zooplankton of IslandWood
Fascinating questions of inquiry for students observing zooplankton include "How can you describe its movement?", "What appendages does it use for movement?", and "What is its body shape?" Many types of zooplankton have unique appendages, movements, and body shapes.
Remember, zooplankton is merely a term to describe a group of organisms with a drifting lifestyle. It is not a taxonomic distinction. Some of the zooplankton we observe from Mac's Pond are also macroinvertebrates, but most are too small to see without aid of a microscope. Many of the macroinvertebrates we catch are benthic (bottom) dwellers. These include dragonfly larvae, mayfly larvae, and caddisflies. Don't bother confusing the kids with these distinctions, but it would be interesting to discuss adaptations in the context of different aquatic lifestyles.
The most commonly seen zooplankton in Mac's Pond samples is Daphnia. Daphnia are members of the Cladocera order, the water fleas. Members of the Daphnia genus are capable of reproducing parthenogenically (without fertilization from a male). Daphnia produce mostly female offspring, but during times of poor environmental conditions, such as winter or drought, males are produced. Fertilized eggs are capable of "resting" during tough conditions and can endure dessication and freezing. When ideal conditions return, these resting eggs come to life and the reproductive cycle begins again.
Daphnia is a model organism for many scientists. Its transparent body allows for close observation of physiological reactions. For example, scientists have observedDaphnia under the effects of caffeine and alcohol. Students can easily measure their heart rates and see internal organisms.
Daphnia and other Cladocerans have a hopping movement resulting from the thrusting of their massive second antennae. Students can perform the "Daphnia dance" by hopping around with hands above the head like antennae.
Students can observe rotifers filter feeding with their unique crown appendage.
Members of the phylum Rotifera, the rotifers, are known as the "wheel-bearers"
They have a crown of cilia that, when in motion, bears resemblance to a wheel. Although it may appear otherwise, the cilia do not rotate around the crown! The cilia aid in movement and in trapping food.
Not all rotifers are planktonic. Some inch along surfaces and others are sessile.
Copepods - "the world's strongest animal"
Students can observe copepods powerful movements.
Copepod comes from the Greek "oar-foot" and refers to these animals' paddle-like appendages.
'Most of the crustacean copepods have a teardrop shaped body with large antennae. Their locomotion results from two mechanisms of propulsion. One mechanism is their continuously moving feeding legs that create a current. The other mechanism results from 4-5 swimming or jumping legs that allow it to exhibit quick bursts to escape predators. The combination of these two mechanisms led to one study's claim that copepods are the "world's strongest animal" with a surprising strength for its size.
External link to article explaining study: www.sciencedaily.com/releases/2010/05/100512172444.htm
Hanging out "in the plankton"...