Creature of the week
Phytoplankton and Zooplankton
From the Greek -
Phyto = plant. Zoo = animal, plagtos =wandering.6````
Microscopic sizes
As seen
from
space
Green
Phytoplankton
bloom
between
NZ and
Chathams.
Various zooplankton
Phytoplankton and Zooplankton
Floating pastures
The open oceans around New Zealand are home to a great variety of plants and animals, from plankton to
whales and sharks. Floating pastures of tiny plants with chlorophyll called phytoplankton sustain this ocean
life. Where phytoplankton flourish, bigger creatures come to feed. When the sea is green and visibility is
low, usually there are large quantities of phytoplankton. When the sea is deep blue and we can see a long
way under water, phytoplankton is sparse.
The growth of plankton is governed by the seasons. Phytoplankton convert energy from sunlight into food
for tiny animals called zooplankton. These in turn become food for larger animals. Phytoplankton need
sunlight and nutrients, but for much of the year one or the other is in short supply.
All life in the open ocean ultimately depends on the growth of microscopic phytoplankton. But these tiny
plants are too small to be eaten by larger marine animals. While small creatures eat the phytoplankton, larger
ones are mixed feeders on plant and small animal plankton, or they are carnivores that prey on animal
plankton. Moon jellies and salps are examples of mixed feeders, and arrow worms are carnivores.
Moon jellyfish look like transparent bells. Tiny beating hairs direct food into their
mouths. Jellyfish have a stalked (polyp) phase, when they are attached to coastal
reefs, and a jellyfish (medusa) phase, when they float among the plankton. The
medusa is the reproductive stage; their eggs are fertilised internally and develop into
free-swimming larvae. After a brief period floating about in surface waters, the larvae
settle to the sea floor, attaching at one end. There they develop into polyps and begin
to feed and grow. In spring, some of the polyps start to bud off immature jellyfish.
These grow into mature jellyfish
. Salps have hollow, tubular bodies and sometimes
travel in chains, which can be as long as a whale.
They have been called the vacuum cleaners of
the ocean because they
.
suck plankton from the water.
Arrow worms dart at their food. They can eat up to a third of their body
weight each year.
Neocalanus tonsus is one of thousands of zooplankton crustaceans called
copepods. It has a life cycle that includes a number of active growing stages
and a long resting period. It breeds in deep water at the end of winter. The
eggs hatch into free-swimming larvae (nauplii) that bear little resemblance to
adults. They feed, grow and start to make their way up to surface waters.
They moult six times during their upward journey, developing into advanced
larvae (copepodites) with elongated segmented bodies. They feed in surface
waters during spring and summer, moulting another four times. At the end of
summer the copepodites descend between 500 and 1,300 metres, where they
enter a resting stage. They eat nothing for six or so months during autumn
and winter. At the end of winter they moult a final time and emerge as adults, ready to breed.
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