Links with the substratum
Planktonic organisms often have strong links with the substratum. For example, most jellyfishes start life on the substratum. The fertilised eggs of jellyfishes develop into 'hairy' planula, which settle on the substratum and grow into polyps. The polyps grow and bud into other polyps. When conditions in the water column are suitable the polyps 'strobilate' (i.e. bud) and release tiny (~2 mm) 'ephyrae' into the water column. They grow into tiny jellyfish and grow rapidly to adult form. For example, the polyps of stingers are found in estuaries and the release of small jellyfish is triggered by the input of freshwater.
In temperate waters copepods often release eggs into the water column that sink to the substratum. Egg reserves on the substratum can be great and development is usually arrested (they 'aestivate') until conditions in the plankton are suitable for hatching; this is probably not true in the tropics. Some copepods spend time in the plankton and on the substratum. The har-pacticoid Euterpina, for example, shows great variation in abundance in the plankton and this is partly related to seasonal use of the substratum.
There is great variation in tropical plankton between day and night. This is largely the result of emergence of plankters from the substratum at night. Invertebrates that include copepods, amphipods, mysids (23 species on GBR), isopods and polychaete worms migrate from the substratum after dusk. Nocturnal movements are probably for the purposes of feeding or dispersal. Demersal plankton provides a rich source of food for invertebrates and fishes and this can be of greater importance as an energy source than the import of plankton from inter-reefal waters. The feeding polyps of many corals only emerge at night and fishes that include cardinal fishes and squirrel fishes feed on large nocturnal zooplankton. Historically, the importance of demersal plankton was not understood, but is now viewed as a critical component of the food chain of reefs.
Plankton of inter-reefal waters, on the continental shelf and beyond often undertake nocturnal vertical migrations. A Russian scientist, Vinogradov, referred to a 'ladder' of migrations where phytoplankton and zooplankton migrate from different daytime residence depths toward the surface. Phytoplankton may alter depth of residence by secreting and absorbing gas, or by varying their oil or salt content. Phytoplankton often migrate to deeper, more nutrient rich waters during the night and back to the sunlit surface waters at the onset of the day. This migration pattern is known as reversed diel migration. Zooplankton and larger flagellated phytoplankton migrate by swimming. In other parts of the world, copepods often swim from depths of 100 m or greater toward food rich waters at the surface at night, but there are few data on this for the GBR. Visual predators are active in the photic zone during the day and it is thought that gloomy waters at depth provide refugia for zooplankton. Some studies have demonstrated that copepods will return to deep water at night if they detect the predators through chemical or tactile senses. The timing of some nocturnal migrations varies by reproductive state.
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