Zooplankton (multiple phyla)
A female copepod Clausocalanus pavo found in temperate surface oceanic waters. Original painting Wilhelm Giesbrecht.
Janet Grieve
General Description
Zooplankton is the collective name for a wide range of organisms that need to ingest food to grow and reproduce and are specially adapted in a variety of ways to a floating existence. They are unable to swim against currents. Zooplankton are either permanently in the plankton or are in the water column for short periods during larval development. This floating existence makes the members of some types of animals almost unrecognisable compared with their heavier relatives that live on the sea floor. For example, planktonic molluscs, related to snails, have a foot that is extremely expanded and is used to "fly" in the water while their shell is very tiny and light.
Zooplankton live in the water column, from the sea surface to the deepest trenches. Each species has a preferred depth range. Some live in the surface 200–500 metres, whereas other species live at great depths (greater than 1000 metres), may have bizarre forms, and be brightly coloured. Some species also prefer the turbid water column just above the sea floor, while other species live at intermediate depths. New Zealand zooplanktonic species are typical of the populations that range around the globe in the southern hemisphere, although there are a few species that are known to occur only in New Zealand or Australasian coastal waters.
Zooplankton species that live permanently in the water column have a wide range of sizes: from a millimetre or two to the size of dinner plates. They include various types of single-celled organisms (Foraminifera, Radiolaria, Tintinnidae, heterotrophic flagellates, etc), jellyfish (Medusae (Hydrozoa, Scyphozoa etc), Siphonophora), comb jellies (Ctenophora), arrow worms (Chaetognatha), segmented worms (Polychaeta), other worms (Nemertea), Appendicularia and salps (Tunicata), molluscs (Gastropoda (Pteropoda, Heteropoda) and Octopoda), crustaceans (Copepoda, Cladocera, Ostrocoda, Amphipoda, Euphausiacea (krill), Isopoda, Mysidacea (possum shrimp), penaeid and carid prawns), and the more mobile lantern, angler and hatchet fish (Myctophidae, Gonostomatidae, Sternoptychidae).
As well as these animal groups, temporary members of the zooplankton include the larvae of an even wider range of animals: starfish, cushion stars, sea cucumbers, and sea eggs (Echinodermata); Bryozoa, marine worms, snails, clams and oysters, Crustacea of many types (crabs, mantis shrimps, lobsters, barnacles, parasites etc), fish, octopus, and squid.
Copepod crustacea of the order Calanoida are the most common members of marine zooplankton with 251 known species in the New Zealand region. The name "Copepoda" is derived from two Greek words (kope, oar, and podos, foot), hence oar-footed. Planktonic copepods are typically small, mostly in the range 0.5–5.0 mm with a few being a remarkable 28 mm long. In the marine plankton, calanoid copepods are extremely abundant. Some typical New Zealand coastal examples are Acartia ensifera, Calanus australis, Centropages aucklandicus, Temora turbinate, and Paracalanus indicus. Very few planktonic species are endemic to New Zealand. The main reason for this is that most species are oceanic and are relatively widespread in a global sense, ranging from circumglobal subantarctic and Indo-Pacific to distributions encompassing all the worlds' oceans. Only a few coastal calanoid species are endemic to New Zealand waters (Acartia ensifera, A. jilletti, A. simplex, Centropages aucklandicus), whereas Gladioferens pectinatus, Labodocera cervi, and Sulcanus conflictus are confined to Australia and New Zealand. Calanus australis is found in New Zealand, southeastern Australian and South American coastal waters where it is essentially restricted to the continental shelf. What we call Paracalanus indicus is restricted to coastal waters with maximum concentrations occurring close to shore. This common New Zealand species may turn out to be restricted to New Zealand waters and may have to be given its own name. Clausocalanus jobei and Temora turbinata also have a tropical/subtropical distributions whereas Drepanopus pectinatus has a coastal distribution around subantarctic islands.
Another important planktonic group are the salps; transparent, tube-like animals that belong to the class Thaliacea. These play a very important role in marine ecosystems because they feed on very small particles, have an extraordinary capacity for rapid growth, and can rapidly transport primary production from the upper water layers to the sea floor. Where they occur, they may be present in huge densities. For example, in New Zealand waters, salps may dominate the surface summer zooplankton in warm-core eddies off the east coast of North Island and in the greater Cook Strait region. Salps swim using a type of jet propulsion where muscles surrounding the tube-like body contract and relax, drawing water in one end and ejecting it out the opposite aperture. Not only is the flow of water through the organism used as a means of propulsion, but it is also an essential part of the feeding process. Salps have very high growth rates and to support this have very high feeding rates on particles 1 mm to 1/1000 mm in size. Aggregates of the common New Zealand species Thalia democractica eat up to 59% of their body weight in a day. Therefore, salps can respond almost instantaneously to favourable feeding conditions to grow very rapidly.
Arrow worms (phylum Chaetognatha) are important zooplankton predators. Their scientific name means literally "hairy jaw", referring to the moveable hooks with which these animals catch their food. The mouth is located on the underside of the rounded head that usually carries a pair of dorsal eyes. Arrow worms are usually transparent, elongate, and with lateral fins (one or two pairs) and a tail fin. They are 0.3–15 cm in length and are hermaphroditic. Most species are permanently pelagic, and all are carnivorous, feeding on other planktonic organisms, particularly copepods, and are even cannibals. They find food by detecting the vibrations arising from the movements of their prey. It appears that they paralyse their prey using venom that is produced by bacteria at an unknown location. Prey is swallowed whole. Arrow worms are capable of eating animals as big as themselves and may consume up to approximately one-third of their body weight in a day. The key predators of arrow worms are larval and juvenile fish, and other chaetognaths.
The moon jelly, Aurelia, is a very common sight floating in New Zealand coastal waters in summer. It is 1–40 cm across and is in the shape of an umbrella, mostly transparent, but with four purple, oval rings on the top. These are its gonads. Underneath the umbrella are four arms that extend from the mouth out to the edges of the umbrella. This species feeds primarily on plankton that includes diatoms, mollusc and worm larvae, tunicate larvae, copepods, protozoa, eggs, fish eggs, and other small jellies. Larvae of Aurelia have special stinging cells to capture prey and also to protect themselves from predators. The food becomes tangled in mucus on the outside of the umbrella and is then passed to the edge of the umbrella by the action of fine hairs. Here the four arms on the underneath lick the food off the edge of the umbrella and pass it into the mouth. Aurelia has two stages to its life history: the floating sexual phase, and the asexual bottom-living phase. The male and female floating forms reproduce to produce very tiny larva that settles on the sea floor. Here it forms a small, polyp-like animal that reproduces asexually by budding off small juveniles that grow into the floating sexual phase. It has recently been shown that there are several very similar species of Aurelia worldwide and that the form in New Zealand and Tasmanian waters will need to be given their own name.
Status / Significance
The significance of zooplankton is found in their role in transferring biological production from phytoplankton to large organisms in the marine food web (eg, whales, seals, seabirds, fish, and squid) and to the sea floor. Most grazing on phytoplankton is carried out by microscopic protozoa, tunicates, copepods, and other Crustacea. These in turn become food for other animals further along the food web. Therefore, variability in the reproduction of copepods would affect the survival of young fish that depend on them. For example, the very young larvae of hoki depend mainly on small copepod of the genus Calocalanus in New Zealand waters.
Key Locations
Zooplankton is everywhere in the marine environment, although communities tend to be associated with specific water masses (subtropical or subantarctic), specific depths, or brackish, inshore or offshore waters. Zooplankton is more plentiful where there are dense populations of phytoplankton, in parts of the ocean or at times of the year when surface waters are rich in nitrates and other essential organic nutrients for phytoplankton.
Summary of Threats
Zooplankton are not threatened as species. Nevertheless, we expect to see changes in biomass and distribution with climate variability and climate change. Also, acidification of the oceans with the rapid addition of more CO2 could be catastrophic to surface living plankton species of all types. Coastal zooplankton are potentially threatened by the introduction of foreign species from other parts of the world. Experience of Asian zooplankton overtaking native populations in San Francisco Bay and other US west coast embayments gives us a hint of what might be possible.
Typical Habitats
Zooplankton occurs in the water column everywhere but certain species are much more common in particular water masses, at specific depths or at certain times of year. Some species such as the pelagic nudibranch Glaucus marinus are highly specialised to living right at the water surface while other species are found only at great depth. Many species undertake daily vertical migrations of tens to hundreds of meters, typically travelling towards the surface to feed at night and descending to deeper waters during the day.
State of Information
Not all New Zealand planktonic species are known and our knowledge of larval forms is particularly poor. Those species that have widespread distributions are the best known. Through genetic analysis, we are becoming increasingly aware of the level of cryptic speciation in the upper layers of the ocean that is, as yet, unaccounted for in the classification system. Deep-living, bathypelagic and benthopelagic populations are least well-known because sampling them is difficult. The possibility of invasive zooplankton species being here has not yet been studied, nor is the long-term impact of invaders known even where they have been recorded. We have little long-term data on the levels of variability in zooplankton biomass in space and time. The potential direct and indirect impacts of the man-made addition of CO2 to the atmosphere have received scant attention.
Significance to Māori
No specific use or significance of zooplankton to Māori is known.
Key References
Caldeira, K and Wickett, M E. 2003. Anthropogenic carbon and ocean pH. Nature 425: 365.
Gordon, D P (ed). In press: The New Zealand Inventory of Biodiversity. Volume 1. Kingdom Animalia, Radiata, Lophotrochozoa, and Deuterostomia. Canterbury University Press, Christchurch (and references therein).
Gordon, D P (ed). In press: The New Zealand Inventory of Biodiversity. Volume 2. Kingdom Animalia. Chaetognatha, Ecdysozoa, and Ichnofossils. Canterbury University Press, Christchurch (and references therein).
Hardy, A. 1956. The Open Sea, its Natural History: Part 1, The World of Plankton. London, Collins, 335pp.
Todd, C D, Laverack, M S and G Boxshall. 1996. Coastal Marine Zooplankton: A Practical Manual for Students. Cambridge, University Press. Second edition.
Table 32 Numbers of permanent zooplankton species in New Zealand marine waters. SpC = Super-class, SbC = Sub-class
| Higher TaxaPhylum | Class | Order | Endemic species | Total species |
|---|---|---|---|---|
| EUKARYOTA | ||||
| Kingdom Protozoa | ||||
| Choanoza | 0 | 10? | ||
| Foraminifera | Globigerinida | 0 | 28 | |
| Cercozoa | (phaeodarians) | 0 | 5 | |
| Radiozoa | (radiolarian) | 0 | 183 | |
| Myzozoa | Dinophyceae | 0 | 234 | |
| Ciliophora | (ciliates) | 0 | 71 | |
| 0 | 531 | |||
| Kingdom Chromista | ||||
| Heliozoa | (sun protists) | |||
| 0 | 1 | |||
| Kingdom Animalia | ||||
| Cnidaria | Scyphozoa | 2 | 15 | |
| SpC: Cubozoa | 0 | 1 | ||
| SpC: Hydrozoa | 0 | 2 | ||
| Ctenophora | 5? | 19 | ||
| Nemertina | 0 | 0? | ||
| Annelida | Polychaeta | 0 | 10 | |
| Arthropoda | 0 | |||
| Sub-phylum: Crustacea | Branchiopoda | Ctenopoda | 0 | 0 |
| Onychopoda | 0 | 4 | ||
| Maxillopoda SbC: Ostracoda | 12 | |||
| SbC: Copepoda | Calanoida | 5 | 251 | |
| Harpacticoida | 0 | 4 | ||
| Cyclopoida | 0 | 4 | ||
| Poecilostomatoida | 0 | 39 | ||
| Mormonilloida | 0 | 1 | ||
| SbC: Malacostraca | Amphipoda | |||
| SO: Gammaridea | 0 | 6 | ||
| SO: Hyperiidea | 0 | 72 | ||
| Mysidacea | 11 | 23 | ||
| Euphausiacea | 0 | 20 | ||
| Decapoda | 0 | ? | ||
| Isopoda | 0 | 4 | ||
| Mollusca | Gastropoda (Heteropoda) | Neotaenioglossa | 0 | 3 |
| (Pteropoda) | Thecosomata | 0 | 32 | |
| Gymnosomata | 0 | 13 | ||
| Octopoda | ? | ? | ||
| Nudibranchia | 0 | 1 | ||
| Chaetognatha | Sagittoidea | 0 | 14 | |
| Chordata | Urochordata (Tunicata) | Appendicularia | 0 | 5 |
| Thaliacea (Pyrosomes, salps, doliolids) | 0 | 19 | ||
| 23 | 574 | |||
| Grand Total | 23 | 1096 |