ZOOPLANKTON

All of the major phyla of animals are represented in the plankton All of the major phyla of animals are represented in the plankton. Remember: In the sea, microscopic plantlike organisms form the base of the food chain.

This means most of the organisms that feed on these plant like organisms are also small.

With such a wealth of microscopic food, organisms take advantage of this rich food supply either as a larval form or as an adult.

Some of these organisms are herbivores feeding on the microscopic plants. Some are predaceous carnivores which feed on the herbivores or other carnivores.

Those organisms that spend their entire life as plankton are called holoplankton.

Those organisms that spend part of their life as plankton are called meroplankton.

We'll take a look at a few examples from each group!

Holoplankton

Copepods

Copepods are the most abundant of the animal plankton types Copepods are the most abundant of the animal plankton types. They are Crustaceans that feed near the base of the food chain on diatoms.

Copepods posses a single, small compound eye and two jointed antennae.

Copepods also have a larval stage that can be very common in plankton samples. In Fact, many of the marine crustaceans have a planktonic Nauplius stage.

Shrimp Like Plankton Two shrimp like organisms are often found in plankton samples… Mysids and Euphausids.

Shrimp Like Plankton Large swarms of euphausids or Krill are often found in nutrient rich cold waters. There, they form an important part of the food chain.

Cladocera This group of planktonic Crustaceans is best known by the Daphnia or water flea.

Cladocera Cladocera have a large anterior, compound eye and an enlarged carapace.

Cladocera As can be seen here, the female carries the fertilized eggs within the carapace.

Ostracods These Crustaceans have a large, bivalve carapace. They are often nick named “bean clams” or “mussel shrimp”.

Coelenterata (Cnidaria) Many forms of coelenterates can be found in the plankton. In fact, all three classes have some representation.

Coelenterata (Cnidaria) Although large, most consider the macroscopic jellyfish to be part of the plankton.

Coelenterata (Cnidaria) The Box jellyfish or Sea Wasp is found off the Northern Coast of Australia. It is the most dangerous jellyfish!

Coelenterata (Cnidaria) There are many microscopic forms in the plankton. These are often juveniles of larger organisms.

Coelenterata (Cnidaria) You might also find this Planula which is the larval form of members in this phylum. This example has been stained.

Ctenophora Known as the comb jellies, these macroscopic organisms swim with their 8 cilliated rows. Many, are about the size of a baseball.

Arrow-worms Chaetognaths or arrow-worms are often found in plankton samples. About 1-5 mm in size, they are predaceous carnivores.

Protists There are protists that can be found in plankton samples. The three most common all have shells. Their shells are often found in marine sediments.

Tintinnids Tintinnids are marine protists that secrete a vase-like shell or lorica that protects them when they withdraw.They are filter feeders.

Foraminifera Forams are amoeba like organisms that have a calcareous shell. These shells are very important in marine sediments.

Radiolaria Radiolarians are amoeba like organisms with a skeleton formed of silica. They are common in colder waters where there shells can dominate the sediments.

Meroplankton

Meroplankton Remember, these are organisms that only spend part of their lives in the plankton. Most become benthic or bottom dwelling organisms as adults.

Barnacle Nauplii Barnacles spend their adult life firmly attached to an object in the water. They kick food in their mouths that’s filtered out of the water.

Barnacle nauplii are often common in inshore samples. Barnacles go from this stage to a settling cypris stage and then to the sessile adult stage.

Veliger Larvae Mollusks have several planktonic stages. They have a ball like trochophore stage and a veliger stage.

Veliger Larvae Often, you can find the shells of these immature mollusks in plankton samples.

Zoea Crabs, like many Crustaceans have a larval stage known as a zoea. At certain times of the year it can dominate the samples!

Megalopa The megalopa stage resembles the adult and will soon settle to the bottom.

Echinoderm Larvae Echinoderms have a bilateral larval stage that can often be found in near shore samples of plankton.

Fish Larvae Larval fish are often part of the plankton.

Even fish eggs can be found in the plankton samples.