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Holoplankton are organisms that are plankton (they live in the water column and cannot swim against a current) for their entire life cycle. Holoplankton can be contrasted with meroplankton, which are planktic organisms that spend part of their life cycle in the benthic zone. Examples of holoplankton include some , , some , foraminifera, , krill, , and , as well as some gastropod mollusk species. Holoplankton dwell in the pelagic zone as opposed to the benthic zone. Holoplankton include both phytoplankton and zooplankton and vary in size. The most common plankton are .
Diatoms are single celled phytoplankton that can occur as individuals or as long chains. They can reproduce sexually and asexually. Diatoms are important oxygen producers and are usually the first step in the food chain.
Copepods are small holoplanktonic crustaceans that swim using their hind legs and antennae.
Zooplankton have adapted by developing transparent bodies, bright colors, bad tastes and cyclomorphosis (seasonal changes in body shape). When predators release a chemical in the water to signal zooplankton; cyclomorphosis allows holoplankton to increase their spines and protective shields. Studies have shown that although small in size certain gelatinous zooplankton are rich in protein and lipid. "Many holoplankton seem to have very little visible defense mechanisms; therefore, it is hypothesized that a chemical defense may be possible. Pelagic (jellyfish and related species) have on their tentacles that eject a coiled microscopic thread very rapidly. These threads penetrate the surface of their target and release a series of complicated, biologically advanced venoms. Their stings can be very dangerous, due in part to the number of vital systems affected.
Defenses
Because of their small size and sluggish swimming abilities, holoplanktonic species have made certain specialized adaptations and in some cases are equipped with special defenses. Adaptations include flat bodies, lateral spines, oil droplets, floats filled with gases, sheaths made of gel-like substances, and ion replacement.
Sexual holoplankton
See also
Sources
Australian Museum Online
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