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.
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.
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.
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.
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.
Another type of zooplankton include the larvae of benthic mollusks usually found in coastal waters, such as marine gastropods including heteropods or pteropods.
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.
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.
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, 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)
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.
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.
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.
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.
Introduction to the Dinoflagellata
Antarctic krill — Australian Antarctic Division
Karl Havens, Professor, Zooplankton Ecology – Department of Fisheries and Aquatic Sciences, University of Florida / IFAS
Robert W. Sanders, “Zooplankton”, in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.756950