Z
ZOOPLANKTON
Dennis M. Allen
Baruch Marine Field Laboratory, University of South
Carolina, Georgetown, SC, USA
Definition
Zooplankton are small animals suspended in the water that
are neither attached to the bottom nor able to swim
effectively against most currents. Holoplankton taxa
spend all life stages in the plankton. Examples of
permanent members of the zooplankton include copepods,
rotifers, cladocerans, ctenophores, and chaetognaths.
Meroplankton include taxa in which only certain life
stages are planktonic (e.g., larvae of crustaceans,
annelids, molluscs, tunicates, and fishes or adults such as
medusa).
Characteristics
Zooplankton are ubiquitous and omnipresent in estuaries
worldwide. A short, small mesh zooplankton collection
in an estuary can yield many thousands of individuals
representing dozens to hundreds of species and life stages,
many of which have not yet been described (Johnson and
Allen, 2012). Abundances tend to be highest in temperate
estuaries, in intermediate to high salinity zones, and
during the warmest months of the year. Higher diversity
and abundance occur near structured habitats such as sea
grass beds, mangroves, salt marshes, and oyster reefs.
Many commercially important molluscs, crustaceans,
and fishes depend on the completion of planktonic stages
of development within estuaries.
The size range of zooplankton spans more than four
orders of magnitude. The smallest size group includes
heterotrophic flagellates and rotifers. Midsized zooplankton (0.2-2 mm) include copepods and larvae of familiar
groups of invertebrates (e.g., barnacles, polychaetes,
bivalves, and echinoderms). Larger zooplankton
(2-20 mm) include shrimp and crab larvae, peracarid
crustaceans (e.g., mysids, amphipods), chaetognaths, and
larval fishes. The largest members, large jellyfishes
(scyphomedusae) and siphonophores, can exceed 1 m.
Motility in zooplankton is usually underestimated, but,
when considered relative to their size, swimming speeds
often exceed those of vertebrates. Small ciliated forms
move slowly. Crustaceans (including copepods) with
jointed appendages, chaetognaths and medusae with
simple muscular contractions and larval fishes with fins
can attack prey or change positions in the water column.
In estuaries, some zooplankton use behavioral responses
to light, salinity, and tidal currents to move and maximize
retention in or export from estuaries. Sophisticated escape
responses allow many species to avoid predators or
collection with towed nets. Additional antipredator
defenses include transparency, protective spines, bioluminescence, distasteful or toxic chemicals, diel activity
patterns, and synchronized mass spawning.
Zooplankton play a central role in estuarine trophic
dynamics, providing a critical link between phytoplankton
and bacterial productivity as they transport energy to
higher trophic levels including some large fishes. Some
also consume detritus (often from land-based sources)
and help incorporate this energy source into the food
web. Estuarine zooplankton-based energy can also be
exported to the ocean.
M.J. Kennish (ed.), Encyclopedia of Estuaries, DOI 10.1007/978-94-017-8801-4,
© Springer Science+Business Media Dordrecht 2016
ZOOPLANKTON
Dennis M. Allen
Baruch Marine Field Laboratory, University of South
Carolina, Georgetown, SC, USA
Definition
Zooplankton are small animals suspended in the water that
are neither attached to the bottom nor able to swim
effectively against most currents. Holoplankton taxa
spend all life stages in the plankton. Examples of
permanent members of the zooplankton include copepods,
rotifers, cladocerans, ctenophores, and chaetognaths.
Meroplankton include taxa in which only certain life
stages are planktonic (e.g., larvae of crustaceans,
annelids, molluscs, tunicates, and fishes or adults such as
medusa).
Characteristics
Zooplankton are ubiquitous and omnipresent in estuaries
worldwide. A short, small mesh zooplankton collection
in an estuary can yield many thousands of individuals
representing dozens to hundreds of species and life stages,
many of which have not yet been described (Johnson and
Allen, 2012). Abundances tend to be highest in temperate
estuaries, in intermediate to high salinity zones, and
during the warmest months of the year. Higher diversity
and abundance occur near structured habitats such as sea
grass beds, mangroves, salt marshes, and oyster reefs.
Many commercially important molluscs, crustaceans,
and fishes depend on the completion of planktonic stages
of development within estuaries.
The size range of zooplankton spans more than four
orders of magnitude. The smallest size group includes
heterotrophic flagellates and rotifers. Midsized zooplankton (0.2-2 mm) include copepods and larvae of familiar
groups of invertebrates (e.g., barnacles, polychaetes,
bivalves, and echinoderms). Larger zooplankton
(2-20 mm) include shrimp and crab larvae, peracarid
crustaceans (e.g., mysids, amphipods), chaetognaths, and
larval fishes. The largest members, large jellyfishes
(scyphomedusae) and siphonophores, can exceed 1 m.
Motility in zooplankton is usually underestimated, but,
when considered relative to their size, swimming speeds
often exceed those of vertebrates. Small ciliated forms
move slowly. Crustaceans (including copepods) with
jointed appendages, chaetognaths and medusae with
simple muscular contractions and larval fishes with fins
can attack prey or change positions in the water column.
In estuaries, some zooplankton use behavioral responses
to light, salinity, and tidal currents to move and maximize
retention in or export from estuaries. Sophisticated escape
responses allow many species to avoid predators or
collection with towed nets. Additional antipredator
defenses include transparency, protective spines, bioluminescence, distasteful or toxic chemicals, diel activity
patterns, and synchronized mass spawning.
Zooplankton play a central role in estuarine trophic
dynamics, providing a critical link between phytoplankton
and bacterial productivity as they transport energy to
higher trophic levels including some large fishes. Some
also consume detritus (often from land-based sources)
and help incorporate this energy source into the food
web. Estuarine zooplankton-based energy can also be
exported to the ocean.
M.J. Kennish (ed.), Encyclopedia of Estuaries, DOI 10.1007/978-94-017-8801-4,
© Springer Science+Business Media Dordrecht 2016