Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player
 

  • Nederlands
  • Deutsch
Search in the Encyclopedia

Dieren en planten

Animals   Crustaceans   Copepods   Marine fauna   Animal plankton   Sea fish   Seabirds   Marine mammals   
Zooplankton, NIOZ, www.nioz.nl

Animal (or zoo)plankton

Plankton is the collective term for all free-floating organisms in water. Zooplankton account for the animal category. Zooplankton are able to move around themselves, but are not strong enough to fight the currents. This category ranges from microscopically small one-celled organisms to larger animals such as jellyfish. The free-floating larvae of fish and other larger marine animals also fall under the zooplankton. Three-quarters of all of the plankton biomass in the North Sea consists of the zooplankton copepod.

  • Species and sizes

    Zooplankton comes in all sorts of shapes and sizes. The smallest species consist of one cell. It cannot be detected with the naked eye; however their fantastic shapes can be admired under a microscope. As far as numbers are concerned, the one-celled ciliates form the largest group within the zooplankton in the coastal waters. Not all zooplankton are so small. Copepods and larvae from fish as well as benthic animals can grow to a few millimeters. Krill are shrimp-like animals approximately three centimeters long. Krill form the most important food source in the oceans for many whales. Jellyfish grow much larger than krill: some species can reach more than one meter in diameter.

  • Tricks for staying afloat

    Just like phytoplankton, a number of zooplankton species possess protrusions which enlarge their ability to float. One-celled zooplankton in particular often possess this adaptation. A number of one-celled species also use drops of oil as a floating organ. Fish larvae have a yolk sack which increases their floating ability immensely. Copepods use their long antennae to stay afloat. They can also move around in a vertical direction: sometimes with a speed of 60 meters per hour. Krill can cover 100 to 400 meters in an hour. Jellyfish move about by contracting their umbrella-like bell. The swimming combs of the sea gooseberries consist of a number of plates. Each plate contains ciliate hairs which make stroking movements. In this way, every species has its own way of remaining afloat and thereby remaining in the area where their food is.

  • Ups and downs

    Zooplankton is transported passively by the sea current in a horizontal direction. Active displacement is necessary to move in a vertical direction . Zooplankton is found in deep water during the day but moves up to just under the surface at night to feed upon the algae. It probably avoids daylight in order to stay out of sight of predators that hunt by sight. This layer of daily rising and sinking zooplankton is readily detected with sonar. The phenomenon was already observed during the Second World War.

  • Plant and meat consumers
    Een zwemmend ribkwalletje met lange tentakels
    Swimming sea gooseberry, Foto Fitis, www.fotofitis.nl

    Of all the zooplankton species, copepods in particular feed upon vegetal plankton. They filter their food out of the water with a fine-meshed net of bristle hairs located on sections of the mouth. In this way, they are able to distinguish between the different species of phytoplankton: they show a definite preference for diatoms. Most other zooplankton feed upon the copepods. That makes copepods the most important link between the phytoplankton and marine animals.

  • Link between plants and animals
    Food web in the marine environment, Ecomare, Oscar Bos

    There is an enormous variation of plants and animals living in the sea. Animals cannot live without carbohydrates, proteins and fats. Nor are they capable of producing these building materials themselves. However, plants, and thereby phytoplankton, can. Directly or indirectly, marine animals get their basic elements by consuming phytoplankton, algae or algae-feeders. Some zooplankton eat phytoplankton. All other zooplankton, benthic animals and fish, in turn, eat the zooplankton, and thereby come indirectly in contact with their building materials. And they, in turn, are eaten by larger fish, seabirds and marine mammals. This series of eat and be eaten is called a food chain. The relationship is usually more complicated than a simple chain and is then referred to as a food web.

  • Larvae from marine animals

    Not only animals that spend their entire life drifting in the water belong to the zooplankton. Many marine animals live a stage of their life as plankton. Several examples include eggs and larvae of fish, lobsters and crabs, starfish, sea urchins, worms and shellfish. Most fascinating are the shapes many planktonic larvae have and the incredible difference with their parents. However, many worms and shellfish have the same basic form in the earliest larva stage: a kind of pea with a fringe of short sweeping hairs around their middle and a brush of feelers on the crown of their head.