Zooplankton
Plankton is composed of the phytoplankton ("the plants of the sea") and zooplankton (zoh-plankton) which are typically the tiny animals found near the surface in aquatic environments. Like phytoplankton, zooplankton are usually weak swimmers and usually just drift along with the currents. Plankton are comprised of two main groups, permanent members of the plankton, called holoplankton (such as diatoms, radiolarians, dinoflagellates, foraminifera, amphipods, krill, copepods, salps, etc.), and temporary members (such as most larval forms of sea urchins, sea stars, crustaceans, marine worms, some marine snails, most fish, etc.), which are called meroplankton. Along with phytoplankton, zooplankton are key components of marine ecosystems forming the base of most marine food webs.
Taxonomy
Zooplankton are classified by size and/or by developmental stage. Size categories include: picoplankton that measure less than 2 micrometers, nanoplankton measure between 2-20 micrometers, microplankton measure between 20-200 micrometers, mesoplankton measure between 0.2-20 millimeters, macroplankton measure between 20-200 millimeters, and the megaplankton, which measure over 200 millimeters (almost 8 inches). There are two categories used to classify zooplankton by their stage of development: meroplankton and holoplankton. Meroplankton are actually larvae that eventually change into worms, mollusks, crustaceans, coral, echinoderms, fishes, or insects. Holoplankton remain plankton for their entire life cycle and include pteropods, chaetognaths, larvaceans, siphonophores, and copepods.
Meroplankton and holoplankton are a component of almost every taxonomic group. However, the most common plankton are protists, nanoplanktonic flagellates, cnidarians, ctenophores, rotifers, chaetognatha, veliger larvae, copepods, cladocera, euphausids, krill and tunicates. Protists produce energy by photosynthesis and form the base of marine food webs as primary producers. Protozoa are also protists and are similar to animals. Protozoa make up a huge part of micro and nanozooplankton, such as amoebas, ciliates, and flagellates. These animals do not photosynthesize energy. Some amoebas such as those classified as Foraminifera and Actinopoda have hard skeletons, usually larger than 2 millimeters in diameter, that help form deep-sea sediment.
Nanoplanktonic Flagellates
Zooplankton also include the nanoplanktonic flagellates that help keep bacteria populations under control. They are characterized by either a long tail used for swimming (flagellates) or by hair-like structures called cilia (ciliates). Some dinoflagellates have a net-like structure called a protoplasmic net—used to capture and eat prey that are typically larger in size than bacteria. Some dinoflagellate species are also responsible for harmful fish kills and the infamous red tides. Ciliates are capable of catching bacteria, other protists and phytoplankton.
Mixotrophs are an amazing organism that are half plant and half animal. Mixotrophs have the ability to ingest other organisms through phagocytosis (phago: "to eat" + cytosis: "cells" = the process of engulfing other cells for ingestion) but also contain functional photosynthetic structures.
Cnidarians
Cnidaria is a phylum that contains the colonial siphonophores and the scyphozoans—also known as the true jellyfish. Both of these animals are predators and have stinging tentacles. They are not found often in fresh water and in the ocean they inhabit the layers closer to the surface. Comb jellies or ctenophores were previously classified under Cnidaria but have recently been distinguished from other jellyfish because they lack the characteristic stinging cells of other jellyfish known as nematocysts. Comb jellies effectively keep copepod zooplankton levels in check through predation.
Phylum Rotifera
There are about 2,375 species of rotifers in freshwater and only 125 in the ocean. Most of the rotifers are non-motile (not able to move) but about 100 species are holoplanktonic. Rotifers eat bacteria, detritus, other rotifers, algae or protozoa. Rotifers are highly efficient reproducers. They are able to reproduce asexually (without a mate) when environmental conditions are good, and sexually when environmental conditions are stressful. This ability allows rotifers to conserve energy in good conditions and adapt to their environment in stressful conditions. Adaptation is possible through sexual reproduction because a variety of offspring are produced, allowing the individuals best suited to the environment to survive.
Chaetognatha
The chaetognatha or Arrow worms are mostly holoplanktonic and are abundant worldwide. These transparent worms are approximately 3 cm long and have fins on the sides of their bodies.
Marine Gastropods
Another type of zooplankton include the larvae of benthic mollusks usually found in coastal waters, such as marine gastropods including heteropods or pteropods.
Polychaeta
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Polychaeta means "many-bristled" (as opposed to the Oligochaeta which are "few-bristled"), and indeed the polychaetes are sometimes referred to as bristle worms. More than 10,000 species are described in this class. Common representatives include the lugworm (Arenicola marina) and the sandworm or clam worm Nereis.
Copepods
Phylum Arthropoda > Subphylum Crustacea > Order Copepoda
Most macrozooplankton are copepods found in marine and freshwater ecosystems. Copepods swim using an antenna and frontal structures on their bodies. They eat phytoplankton and detritus, and occasionally other zooplankton smaller in size.
Cladocerans
Phylum Arthropoda > Subphylum Crustacea > Order Cladocera
Claudocera are planktonic crustaceans found in coastal waters. They swim using an antenna, like copepods, but instead of using their first antenna—they use the second antenna. They appear to have two sections to their body but it's only an illusion caused by a folded outer shell. Cladocerans eat phytoplankton and other zooplankton. Like many species of zooplankton, cladocerans migrate to the surface at night. This is referred to as “diurnal migration”.
Krill
Krill, classified under Euphausids, are found all over the world. They can be 3 cm large and are an important source of food for many types of whales. In cold waters, krill often feed on diatoms, a type of phytoplankton. In warmer water they prefer to eat other animals. » More about krill (Euphausia superba)
Insect Larvae
The larvae of the midge Chaoborus is the only widely known insect larvae classified as plankton. Chaoborus comes up during the night to float with other plankton and eats many types of zooplankton in lakes.
Tunicates
Some tunicates are planktonic, such as the holoplanktonic classes Appendicularia and Thaliasia. Both are filter feeders; Appendicularia consumes small food particles using a mucous filter. Other types of tunicates are benthic and are only planktonic during their larval stages.
Adaptations
All species of plankton have been forced to develop certain structural adaptations to be able to float in the water column. Adaptations include: flat bodies, lateral spines, oil droplets, floats filled with gases, sheaths made of gel-like substances, and ion replacement. The flat body and spines allow some species of plankton to resist sinking by increasing the surface area of their bodies while minimizing the volume. All other adaptations keep plankton from sinking quickly to the bottom. Zooplankton have also adapted mechanisms to deter fish (their heaviest predator) including: transparent bodies, bright colors, bad tastes, red coloring in deeper water, and cyclomorphosis. Cyclomorphosis occurs when predators release chemicals in the water that signal zooplankton, such as rotifers or cladocerans, to increase their spines and protective shields.
Night Swimming
Many types of zooplankton migrate deeper into the water during the day and come up at night. The migration of species appears to be dependent on location rather than particular species types. All plankton migrate differently based on factors like age, sex and the season. The amount of light is probably the major factor in the extent of migratory behavior. It seems that zooplankton move around most in low light and least in higher light situations. It's possible that zooplankton migrate to lower levels during the day so they are less visible to predators relying on vision. At night, zooplankton can sneak up to the surface and snack on phytoplankton relatively safely. The lower metabolism occurring in colder waters during the day may also be a factor in the migration of zooplankton. This way, zooplankton can save energy by feeding in the cooler, night waters. The fact that different species of zooplankton have varying migration times may be the result of a partitioning of resources.
Communities of Zooplankton
Specific species of zooplankton occupy particular marine habitats. Each species is uniquely adapted to factors like light, temperature, turbulence, and salinity in its environment. Zooplankton on one side of the Gulf Stream are different species from those on the other side. These characteristics of different species of zooplankton can sometimes help scientists distinguish one water mass from another.
Zooplankton are also sensitive to their environment and like phytoplankton—a change in zooplankton concentration can indicate a subtle environmental change. Zooplankton are highly responsive to nutrient levels, temperatures, pollution, food that is not nutritious, levels of light, and increases in predation. As well as providing an essential link in the marine food chain (which is an understatement), the diversity of species, amount of biomass and abundance of zooplankton communities can be used to determine the health of an ecosystem.
Other factors that influence the distribution of zooplankton include predation, reproduction, community interactions and the amount of available resources. Zooplankton can also be predators of algae or protozoa causing the sizes of these organisms to change through evolution. When light concentrations in boreal or temperate areas increase in the spring, phytoplankton communities increase in number. Because zooplankton feed on phytoplankton, the numbers of zooplankton increase in response during the spring. However, the reaction of zooplankton is dependent on the species type as well, so sometimes the zooplankton numbers increase only in the summer with another spike in the fall.
Zooplankton are also affected by levels of pH, heavy metals, calcium, and aluminum. Nutrients like nitrogen and phosphorus will affect the prey of zooplankton (like algae, protozoa and bacteria), indirectly affecting zooplankton survival. Scientists are still putting together pieces of the zooplankton puzzle. Some questions include how nutrient levels found in algae can influence the growth and behavior of zooplankton. Another question important to marine and human life is how toxins and pollution will affect this crucial link in the food chain.
References
Share your thoughts
Feedback & Citation
Sea Life News :: ScienceDailyFirst-of-its-kind study reveals surprising ecological effects of earthquake and tsunami
The reappearance of long-forgotten habitats and the resurgence of species unseen for years may not be among the expected effects of a natural disaster. Yet that's exactly what researchers have found on the sandy beaches of south central Chile, after an 8.8-magnitude earthquake and devastating tsunami in 2010. Their study also revealed a preview of the problems wrought by sea level rise -- a major symptom of climate change.
Old fish makes new splash: Coelacanth find rewrites history of the ancient fish
Coelacanths, an ancient group of fishes once thought to be long extinct, made headlines in 1938 when one of their modern relatives was caught off the coast of South Africa. Now coelacanths are making another splash.
Marine food chain becomes clearer with new revelations about prey distribution
A new study has found that each step of the marine food chain is clearly controlled by the trophic level below it -- and the driving factor influencing that relationship is not the abundance of prey, but how that prey is distributed.
Impaired recovery of Atlantic cod: Forage fish or other factors?
Biologists suggest the delay in recovery of Atlantic cod on the eastern Scotian Shelf could be attributed to increased predation by grey seals or other governing factors and not the effect of forage fish as previously thought.
Global warming refuge discovered near at-risk Pacific island nation of Kiribati
Scientists predict ocean temperatures will rise in the equatorial Pacific by the end of the century, wreaking havoc on coral reef ecosystems. But a new study shows that climate change could cause ocean currents to operate in a way that mitigates warming near a handful of islands right on the equator.
Pacific islands on equator may become refuge for corals in a warming climate due to changes in ocean currents
Scientists have predicted that ocean temperatures will rise in the equatorial Pacific by the end of the century, wreaking havoc on coral reef ecosystems. But a new study shows that climate change could cause ocean currents to operate in a surprising way and mitigate the warming near a handful of islands right on the equator. As a result these Pacific islands may become isolated refuges for corals and fish.
Scientists provide first large-scale estimate of reef shark losses in the Pacific Ocean
First study to provide estimates of reef shark losses in the Pacific Ocean are sobering. Researchers noted the enormous detrimental effect that humans have on reef sharks.
Purple sea urchin metamorphosis controlled by histamine
Now that hay fever season has started, sufferers are well aware of the effect of histamines. However it is easy to forget that histamine is also a neurotransmitter involved in controlling memories, regulating sleep, and controlling secretion of gastric acid. New research shows that for the purple sea urchin histamine is also responsible for controlling metamorphosis from a free swimming larval form to the spiny adult living on the sea floor.
Smalleye pigmy sharks' bellies shine: They glow for camouflage
Smalleye pigmy sharks have an eye-catching party trick: Their bellies glow. However, instead of being a giveaway, biologists have shown that the fish's shiny undersides probably provide camouflage. They also discovered that the pigmy shark and another glowing fish, the lantern shark, regulate their glow using the similar mechanisms, although the pigmy shark is probably more closely related to their common ancient ancestor.
Eight species of wild fish have been detected in aquaculture feed
Researchers have for the first time analyzed a DNA fragment from commercial feed for aquarium cichlids, aquaculture of salmon and marine fish in aquariums. The results show that in order to manufacture this feed, eight species of high trophic level fish have been used, some of them coming directly from extractive fisheries.
Did bone ease acid for early land crawlers?
Scientists have proposed that the bony structures in the skin of many early four-legged creatures might have been there to relieve acid buildup in bodily fluids. Analysis of their anatomy suggests that as they ventured out of water, the animals would have had trouble getting rid of enough CO2 to prevent acid buildup.
Geophysicists employ novel method to identify sources of global sea level rise
As the Earth's climate warms, a melting ice sheet produces a distinct pattern of sea level change known as its sea level fingerprint. Now, geophysicists have found a way to identify the sea level fingerprint left by a particular ice sheet, and possibly enable a more precise estimate of its impact on global sea levels.
Fish larvae find the reef by orienting: The earlier the better
For the first time, a numerical study incorporates horizontal larval fish navigation skills into realistic 3D flow fields, creating a powerful tool that spells out how larvae use environmental cues to find their way back to the reef after being out on the open ocean. This model can be used for a wide variety of marine species.
Extent of Illinois' Asian carp problem detailed
Asian carp now make up more than 60 percent of the total fish biomass in one of Illinois' major river systems.
Freeing loggerhead turtles comes at a price
Scientists have studied loggerhead turtles' re-adaptation to the environment. The results show that after a lengthy recovery in rehabilitation centers these animals display changes in behavior and may not adapt well to being free.
Deadly jellyfish weapons unraveled
Researchers have analyzed proteins of stinging cells in the hydra freshwater polyp. The results of their research reveal a complex mixture of toxic and structural proteins that can explain the extraordinary toxicity and biophysical properties of these unique cells. They also show how the energy for discharging the toxin can be stored in the stinging cells and released at extraordinary speed.
Jellyfish on the rise in world's coastal ecosytems
Jellyfish are increasing in the majority of the world's coastal ecosystems, according to the first global study of jellyfish abundance.
Cod has a key role in the whole Baltic Sea
A new investigation put in evidence the key role of cod as regulator of the whole Baltic Sea ecosystem. The study shows that when the cod population in the central Baltic increases, it spreads into larger areas and spills over into adjacent marginal systems where it usually does not occur, as for example the Gulf of Riga.
BP oil spill, two years later: Natural recovery far greater than expected
This Friday, April 20, will mark two years since the explosion aboard the Deepwater Horizon oil rig caused vast quantities of crude oil to flow into the Gulf of Mexico. But despite the size of the spill, "the natural recovery is far greater than what anybody hoped when it happened," said a professor of biology. "The fears of most people -- that there would be a catastrophic collapse of the ecosystem in the Gulf -- never materialized."
Understanding of hearing in baleen whales amplified
For decades, scientists have known that dolphins and other toothed whales have specialized fats associated with their jaws, which efficiently convey sound waves from the ocean to their ears. But until now, the hearing systems of their toothless grazing cousins, baleen whales, remained a mystery, largely because specimens to study are hard to get. Now, a new study has shown that some baleen whales also have fats leading to their ears.
Help us continue to share the wonders of the ocean with the world, raise awareness of marine conservation issues and their solutions, and support marine conservation scientists and students involved in the marine life sciences. Join the MarineBio Conservation Society or make a donation today. We would like to sincerely thank all of our members, donors, and sponsors, we simply could not have achieved what we have without you and we look forward to doing even more.
