Zooplankton

Heterotrophs— consume organic matter rather than manufacturing it, as do autotrophs.

Zooplankton can be:

herbivores

carnivores (several levels)

detritus feeders

omnivores

Zooplankton, in addition to being much smaller than familiar land animals, have shorter generation times and grow more rapidly (in terms of % of body wt / day).

 1. Crustaceans- include shrimp, copepods, euphausiids (“krill”)

Characteristics: Copepods, euphausiids and shrimp superficially resemble one another. All have:

• exoskeletons of chitin
• jointed appendages
• 2 pair of antennae
• complex body structure, with well developed internal organs and sensory organs

Habitats: Ubiquitous.

• Euphausiids predominate in the Antarctic Ocean, but are common in most temperate and polar oceans.
• Copepods are found everywhere, but are less important in low-productivity areas of the ocean - the “central ocean gyres”. They are found at all depths but are more abundant near the surface.
• Role in food webs:
•  Euphausiids and copepods are filter-feeders. Copepods are usually herbivores, while the larger euphausiids consume both phytoplankton and other zooplankton.

Shrimp are usually carnivores or scavengers.

2. Chaetognaths - (“Arrow worms”)

Characteristics:

• 2-3 cm long
• wormlike, but non-segmented
• no appendages (legs or antennae)
• complex body structure with internal organs

Habitat: Ubiquitous

Role in food web: Carnivore feeding on small zooplankton such as copepods.

 

 

 

3. Protozoan - Include foraminifera, radiolarians, tintinnids and “microflagellates” ca. 0.002 mm

Characteristics:

• Single-celled animals.
• Forams have calcareous shell
• Radiolarians have siliceous shell.
• Both Forams and Radiolarians have spines.

Habitat: Ubiquitous

• Radiolarians are especially abundant in the Pacific equatorial upwelling region.
• Protozoa are especially important components of the food web in low-productivity ocean areas.
• Both are found in sediments as well as in the water column.

Role in food web: Feed on small phytoplankton, bacterioplankton, and other protozoans. They can be bacteriovores, herbivores, or carnivores.

4. Gelatinous Zooplankton: includes a variety of fragile, jelly-like organisms which are not closely related taxonomically.

Cnidarians: jellyfish

Characteristics:

Very simple body structure, with 3 layers: inner membrane, jelly,

• and outer membrane.
• No internal organs but have a digestive cavity.
• Have stinging cells on their tentacles called nematocysts.

Habitat: Found everywhere and at all depths. More abundant in surface waters.

Role in food web: Carnivores, trap prey in tentacles.

Ctenophores: “comb jellies”.

Characteristics:

• Also have a simple body structure without internal organs.
• Move by means of cilia,.
• Sometimes have 2 long tentacles.
• Are often bioluminescent.

Habitat: Found everywhere

Role in food web: Carnivores, predators.

Salps: A type of tunicate.

Characteristics:

• Members of the phylum Chordata.
• Have a complex body structure including internal organs and a nervous system as larvae but are “degenerate” as adults.
• May be solitary or colonial.

Habitat: Warm surface waters. Rare at high latitudes.

Role in food web: Largely feeds on phytoplankton. A “ciliary-mucous” filter feeder.

Overall: Gelatinous zooplankton are very important, but little-studied because of sampling problems; they often disintegrate in nets or other sampling devices.

5. Pteropods

 Characteristics:

• Mollusks related to snails.
• Small, ~ 1 cm long.
• May or may not have a conical shell.
• Move by means of “wings” (modified foot).

Habitat: Found everywhere

Role in food web: May be herbivores or carnivores. Filter-feed using a “mucous net”.

6. Meroplankton

Meroplankton are organisms which are part of the plankton for only part of their life cycle, usually an early, larval stage.

As adults the meroplankton are benthos (including intertidal organisms) or nekton.

The meroplankton often do not resemble the adult forms, to the extent that some were once thought to be separate species.

Meroplanktonic larvae promote survival of the species:

• Currents carry the offspring to new areas, especially important for sessile (immobile) benthic animals. Thus, the offspring do not compete with the parents for scarce resources such as food or space. Also, local “disasters” will not wipe out all close relatives.
• Meroplankton live in surface waters where food is abundant. Sometimes, the habitat of the adult would not have enough food, especially for a very small organism that could not effectively use the feeding strategy (for example, predation, filter feeding) of the adult.

Meroplanktonic larvae also have disadvantages:

• Often, reproduction occurs to coincide with the spring bloom and abundant food. If the spring bloom is not “on time”, meroplankton may starve.
• Meroplankton are food for the many predators on plankton.
• The currents may not carry the meroplankton to an area that provides suitable conditions for adults.

Therefore, organisms which have meroplanktonic larvae usually produce hundreds or thousands of eggs, so that a few will survive.

 

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