Biology of Copepods: An Introduction

by Johannes Dürbaum & Thorsten-D. Künnemann


Habitats

Marine Habitats

Although copepods can be found almost everywhere where water is available most of the more than 12.000 known species live in the sea. As they are the biggest biomass in the oceans some call them the insects of the sea. They roam the free water, burrow through the sediment at the bottom of the seas, are found on tidal flats and in the deep sea trenches.
At least one third of all species live as associates, commensals or parasites on invertebrates and fishes. One of the hotspots of species diversity are the tropical coral reefs in the Indopacific. Some coral species are hosts to up to 8 copepod species. Like the tidal flats the mangroves teem with copepod life (picture).

Freshwater Habitats

Species of the Calanoida, Cyclopoida and Harpacticoida have successfully colonised all kinds of freshwater habitats from little creeks to glacier lakes high up in the Himalaya. Although the species diversity in freshwater is not as high as in the sea copepod abundance may sometimes be great enough to stain the water. Even in the groundwater a specialised copepod fauna has evolved.

Some copepod species can be found in the leaf fall of wet forests or in a wet compost heap, sometimes in rather high densities. Others live in peat moss or even in the phytothelmata (little pools formed in the leave axils of plants) of bromeliads and other plants.


Locomotion

In the free water

The realm of the calanoid copepods. Their long and feathered antennae are ideal for drifting in the free water. Some species show daily migrations, ascending to the surface water during the night and descending to several hundred meters depth at daytime.
These tiny creatures (1-2 mm length) reach a speed of up to 90 meters per hour (this is about
45.000 times the body length per hour and would
equal a speed of 81 km/h for a human of 1,80 m height!!).
The propulsion is provided by the movement of the
mouth appendages, for faster movements
and flight the swimming legs are used.

On the sea-bottom

The locomotion of species that live on the sea-bottom or on water plants is different. The four first swimming legs are primarily used for a kind of swimming-crawling. Their cylindrical body wriggles around between obstacles or over the substrate.


Nutrition

Particle feeders and Aufwuchs feeders

Calanoida are typical particle feeders. Their mouth appendages produce a water current from anterior to posterior. As soon as food receptors detect the approach of a suitable algal cell the maxillae are opened. Water with the cell is sucked into the chamber between the maxillae. When the chamber is closed the water is pressed out again. The algal cell is trapped between the bristles of the maxillae. In this chamber particles of 5 µm diameter can be retained.
Many Harpacticoida feed on algae or microbes that cover the substrate. Some species live inside of macroalgae inducing the growth of galls in which their larvae develop. On tidal flats many harpacticoid copepods graze on diatoms. Take a closer look at the head appendages of a harpacticoid copepod.

Predators and Parasites

Most predatory copepods can be found in the Calanoida and Cyclopoida. Some of the latter are able to tear pieces out of the body of their victims (mosquito larvae, small fishes) with their strong mandibles. Parasitic copepods like the Siphonostomatoida and the Poecilostomatoida feed on skin particles of their hosts, others suck blood.
The body of the adult stages of some species is extremely modified in adaptation to a parasitic life. The typical body segmentation may be totally lost. An extreme example is Herpyllobius spec. (picture below). This species lives on Antarctic scale worms. Part of the parasite is inserted into the host and the whole body is reduced to a pea-like ball. Stomach and intestine are missing. Nutrients of the host are most likely absorbed through the body surface of the parasite.

Herpyllobius spec. (little ball in the middle with an eggsack)

Development

Mating

As there is no special copulatory organ for an internal fertilization the term copulation is used for the attachment of a spermatophore to the genital field of the female. A spermatophore is a container filled with sperm and various secretions. It is produced internally by the male and expelled during copulation. The reproductive behaviour of copepods is very diverse. In some species adult males clasp juvenile females already to be able to copulate straight after the final moult of the female (precopula, see picture at right, pairs and detail ). This behaviour may be interpreted as a consequence of competition between many males for few females. In other species the males guard their females at least for the time necessary for the spermatophore to discharge its contents into the female. This guarding has the effect of securing paternity (postcopula). Sometimes a complex mating behaviour precedes copulation. Females in such cases may be endowed with effective mechanisms to keep off males from copulatory attempts.

Eggs

A few hours or days after copulation eggsacks are formed by the female. Most species produce paired eggsacks. These sacks are carried outside the body under the abdomen and consist of eggs embedded into a mass of secretions. Depending on size and life style a few to several dozen eggs develop inside their protective cover. Some parasites produce several thousand eggs. The eggs are probably still nourished by the females. After a few days the larvae hatch and the eggsack is cast off.

Larvae

The first larvae of copepods are called nauplii (picture). They are very small (sometimes 20 µm) and like the adults are found in very different habitats. Usually they pass six naupliar stages, which are separated by a moult. The first stages have only three pairs of appendages that are responsible for locomotion and feeding. The older nauplii already show buds of further mouth appendages and swimming legs.


The 6th naupliar stage moults into the first copepodid. This moult is accompanied by important morphological changes. The emerging copepodid stage resembles the adult already more or less. With the increasing number of body segments more appendages become functional. After the fifth moult adulthood is reached and reproduction can take place.


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