Binocular Microscopes
Observe Zooplankton with a Low Power Stereomicroscope
The most common mistake made by amateurs is to purchase a microscope which magnifies too much. For looking at zooplankton a low power binocular microscope (X10 or X20) is optimal.
Binocular Microscopes
Using both eyes reduces the eye-strain considerably, and the three-dimensional image (which is the right way up and has left where it should be!) makes manipulating or dissecting specimens easy. The two optical pathways each give a slightly different image to the eyes, and the brain converts this to show a solid object magnified a few times.
Capturing and Observing Zooplankton
Towing a plankton net behind a boat, or simply throwing it into the sea and hauling it ashore will provide a sample of marine zooplankton. Several hauls can be combined in a single jar, and then individual specimens can easily be pipetted into a suitable container. A small ‘Petri-dish’ is convenient, but any small glass or plastic container will do. If the animals are still alive they will jump about a bit, and maybe a small piece or cotton wool will keep them in the field of view. For detailed observation they must be killed. Dead specimens can be preserved with formalin to be studied at leisure.
Identifying Zooplankton
This is where the fun really begins! Many different phyla have zooplanktonic representatives, and a single sample might contain dozens of different species. Some of the easiest to recognise, and usually the most abundant, are the copepods. To identify each species it will be necessary to use a specialist key, or search online.
Some Common Zooplankton
Copepods - One of the most abundant animals on the planet is Calanus finmarchicus, and all species have a similar body plan.
Other Crustaceans – Most of the larger crustaceans (crabs and lobsters and their allies) have small planktonic larvae. These pass through a number of stages – looking a bit different each time – and they are quite difficult to identify to species level.
Jellyfish – many small medusae are active predators of the plankton. They will easily be recognised as jellyfish, but deciding on a name is a challenge.
Arrow Worms – these voracious carnivores are abundant and have a phylum (Chaetonatha) all to themselves. They are found in all oceans and eat anything they can catch.
Fish Eggs and Larvae – It is a bit ironic, but huge numbers of small fish are gobbled up by the zooplankton before they reach maturity and begin to feed on zooplankton. Fish eggs and developing fry are a regular, if seasonal, component of plankton samples.
