Plankton are an incredibly important part of the ocean. Phytoplanton form the base of the marine food web and zooplankton include an incredible variety of organisms, from curious critters called copepods and chaetognaths to young jellyfish, larval crustaceans and miniature molluscs. A while back, Alexis introduced these creatures as aliens of the sea and they are indeed alien looking, but despite their strange appearance, our eyes are incredibly good at distinguishing between different groups of plankton – far better than a computer in fact.

There are many ways of classifying plankton . The simplest is by size, where the smallest plankton (femtoplankton) are less than 0.2 micrometres across and the largest (megaplankton) can extend to several metres!

A more useful way of grouping plankton is by their shape. Species that are closely related and those that have similar lifestyles often have similar body shapes. For example, young jellyfish, known as medusae, all have the characteristic jellyfish bell and tentacles, though their shape, and both the number and length of tentacles vary between species.

So how do we get a good look at these plankton?

The longest running plankton survey is carried out using the Continuous Plankton Recorder, which has been capturing plankton since the 1930s. As it is towed through the water, it filters plankton onto a long band of silk to create a plankton silk sandwich that can be studied back in the lab.

More recently, a team from the University of Miami developed an underwater robot to rapidly capture images of plankton in the ocean. The robot, known as the In Situ Ichthyoplankton Imaging System (ISIIS) scans seawater as it moves through the ocean, recording the shadows cast by plankton using a very high resolution digital sensor. These shadows let scientists identify different plankton groups with great ease - take a look

In just three days ISIIS captured so many plankton on camera that it would take the team years to identify them all. This is where you and I come in. All the images captured during those three days are available online in an incredibly cool site called the Plankton Portal. After perusing the field guide, you'll have no problem telling apart your ctenophores from your copepods and you can have a go at classifying plankton yourself.

Every plankton you classify is saved for science (the aim of the project is to better understand plankton in the ocean). What are you waiting for?

The Plankton Portal is a great way to get to grips with what's in our ocean – take a look at their education section for more resources to use in the classroom. There's also some excellent information on classifying plankton available here.

References

Plankton Portal, accessed September 2013.

Sir Alister Hardy Foundation for Ocean Science (SAHFOS), accessed September 2013.

Sieburth, J. McN., Smetacek, V. and Lenz, J., Pelagic Ecosystem Structure: Heterotrophic Compartments of the Plankton and Their Relationship to Plankton Size Fractions Limnology and Oceanography 23 (6) 1256-1263 (1978).

Images

1) A larval ctenophore, or comb jelly. Beating cillia cause the colours to flash down the sides of the ctenophore (Credit: Alvaro E. Migotto ).