What are they?
Copepods are one of the most abundant forms of life on Earth. They are a form of crustacean, a major group of invertebrates which have an external skeleton (exoskeleton), however due to their small size, it often appears transparent. Other examples of crustaceans include crabs, lobsters, shrimp and crayfish. Within the crustaceans, copepods are one of the most diverse groups. To date, approximately 11,145 species of copepods have been described. Their habitats range from freshwater to hypersaline (very salty) environments. They can be found in oceans, seas, lakes, wet organic soils, subterranean caves and amazingly even at hydrothermal vents.In the open ocean, copepods often dominate the plankton community in terms of numbers and biomass. As members of the plankton community, they drift with the ocean currents. They range in size from less than a millimetre to several centimetres, although adults are typically 1 – 2 mm. Copepods can be free-living, symbiotic (having an interdependent relationship with another organism) or internal/external parasites. The most common copepods found in Irish waters are the calanoid copepods, the cyclopoid copepods and harpacticoid copepods.
Ecologically, copepods are one of the most important primary consumers in the ocean ecosystems. They can be considered a keystone spcies as they are the link that transfers energy from the primary producers such as phytoplankton, to the higher trophic levels, such as fish larvae, jellyfish, seabirds and marine mammals. Beyond supplying energy to higher trophic levels, they are also an important part of the cycling of carbon in the ocean. When they shed their exoskeleton (moult), it sinks to the bottom of the ocean along with faecal pellets in a phenomenon known as ‘marine snow’. This brings essential nutrients like carbon and nitrogen to the ocean depths and indirectly supports organisms of the deep sea. Furthermore, copepods engage in the one of the largest daily migrations on earth: they spend the daytime at depth to avoid being seen by predators, and at night they migrate to the surface so they can feed on the phytoplankton before descending again at dawn. This daily migration is important for mixing the top layers of the ocean, as well as transferring nutrients from the surface to depth and vice versa.
Additionally, copepods could serve as potential biological indicators of climate change. As they are ‘cold-blooded’ (ectothermic), they are very sensitive to any changes in the water temperature. Furthermore, they are not commercially fished so changes in their distribution and abundance are mostly attributed to changes in climate. An example of looking at changes in copepod distribution as an indicator of climate change can be seen with the calanoid copepods Calanus finmarchius and Calanus helgolandicus found in the North Atlantic Ocean and off the coast of Ireland. Calanus finmarchicus is a more northerly, sub-artic species of copepod and Calanus helgolandicus is a temperate species of copepod. The distribution of both species is moving in a more northerly, poleward direction. These changes are believed to be an effect of warming ocean temperatures. This change in distribution could influence the migration of some of the most commercially important fish species, such as Atlantic Herring and Mackerel because Calanus finmarchicus is the primary food source for their juvenile stages.
The copepod lifecycle begins with an egg. When this egg hatches, a nauplius emerges, which is the typical larval stage amongst crustaceans. This larva is planktonic and has an external skeleton that they will shed (known as moulting) between each stage. The nauplius, in most free-living copepods, goes through six developmental stages. The sixth and final nauplius stage is the one that will develop into the first copepodite stage. The majority of planktonic copepods will go through six copepodite developmental stages, the sixth being the final adult stage. Some parasitic copepods go through fewer developmental stages.
Did you know?
Copepods can go through a period of dormancy referred to as ‘overwintering’ and can be found at depths of up to 1500 metres.