Science

zooplankton

Thesaurus

The name zooplankton is derived from the Greek work planktos meaning to wander and refers to the small size and weak swimming movements of organisms in this category. These characteristics seem to predispose them to floating at the mercy of the ocean currents...

It is not entirely true that zooplankton are at the mercy of the ocean currents. Many organisms in this catergory undergo vertical migrations over the course of every 24 hour period (diel vertical migration). The most common pattern is to migrate deeper in the water column during daytime and ascend towards the surface at night. The most likely explanation for this behaviour is to escape predators feeding in the upper lighted layers during daytime and exploit the food sources which are most abundant near the surface when it is too dark for successful capture by visual predators. However, vertical migration may also remove organisms from faster moving currents near the surface into deeper slower currents that may even be travelling in the reverse direction. Thus some degree of control over distribution is possible by varying the time spent at specific depths.

As well as being small and migrating vertically, they have adopted a range of other protective strategies, e.g. being transparent and agregating into schools or swarms.

Guide to Zooplankton of south eastern Australia
Ritz, Swadling, Hosie, Cazassus
University of Tasmania, 2003 p.1

Zooplankton constitutes the fascinating, diverse and abundant group of animals living in the upper layers of the seas around Australia. Zooplankton contains representatives of all the major invertebrate phyla including some that can only be found in plankton. Since they are the principal diet of most of the larger pelagic animals, including commercially important fish, their study is an essential component in forming a more complete understanding of the functioning of marine ecosystems.

Kim Evans
Forward, Ritz, Swadling, Hosie, Cazassus
Tasmania 2003