Small pelagic invertebrates are near the base of many food chains.
There are well over ten thousand species of copepod. Most are all small (typically one or two millimetres long), with a few giant species (up to one centimetre long) found in cold arctic waters. The smaller ones feed on phytoplankton and small particles, while the larger ones are often predatory. They all tend to store surplus food as oil droplets, making them a very nutritious fish-food.
Copepod Anatomy
The size and shape of their legs and antennae is variable (see image below), but the main part of a copepod is its torpedo-shaped body (prosome), with a short tail behind (urosome). Their first antennae are often very long and used for rapid jerky swimming through the water, while the feeding legs on the body are generally used for a slower and less erratic form of movement – as well as creating water currents that bring food past the mouth. Females of some species carry two egg-sacs near their tail (enlarge the article image of Sapphirinia darwinii below).
Copepod Ecology
Because copepods that live in the open ocean are so abundant (they are among the most numerous animals on the planet), and because the reproduce so quickly and efficiently, pelagic copepods are extremely important as food for other marine animals. Those that eat phytoplankton directly (secondary producers) convert enormous amounts of plant food into nutritious, bite-sized, prey for other planktonic animals and fish fry. The phytoplankton absorb carbon dioxide from the atmosphere (about two billion tonnes of carbon a year), and most of this passes into copepods when the plants are eaten. It is true that the copepods need to respire (and therefore release carbon dioxide which goes back into the atmosphere), but much of the carbon ends up in moulted skins (exoskeletons) and excreta which sinks to the bottom of the ocean.
Copepods as Food
The fats and oils that copepods use as their own food store makes them excellent food for other animals. We are included in that food chain, and a healthy diet with ‘oily fish’ in it owes much to the activity of the copepods out at sea. They either fed the fish as fry or fed the animals that the adult fish ate.
Copepods in the Aquarium
Many hobbyists feed their marine fish copepods occasionally. They are the preferred food of some of the smaller-mouthed fish, and enthusiastically eaten by many others. Provided there are sufficient places for some copepods to lurk – out of danger – they can become established as residents of the aquarium helping to maintain water quality as well as providing the odd snack. Some go even further and attempt to raise copepods and algae in their own ‘Biotope Tank’.
See blog – ‘Growing Plankton in Biotope Tanks’.
Other articles by John Blatchford
