15 - Imaging twilight critters

Imaging the plankton

The traditional way to collect the small, drifting animals in the sea that are called zooplankton is to use plankton nets, and then analyze the tiny organisms under microscopes in the lab. That way you get a close-up view of the plankton, you can watch their movements (unless you preserve them at once!) and observe them from all sides. This makes it easy to determine the species of these animals, which range in size from less than a millimeter to a few centimeters. Unfortunately, many of the fragile forms are destroyed in the sampling process, and exist only as fragments in your sample.

The Video Plankton Recorder (VPR)
coming back from the ocean’s depth.
Image credit© Fredrika Norrbin

The Video Plankton Recorder (VPR) is a complimentary tool to the plankton nets and marine snow collecting systems used during this cruise. It is an underwater digital video camera with a macro lens and a flashing strobe for illumination, which is lowered into the water and towed up and down several times. It takes about 20 little pictures per second, and can be towed at a speed of a meter per second or more. The VPR lets you observe images of undisturbed, living plankton and particles (“marine snow”) in the water column and knowing exactly at what depth and temperature individual plankton are observed.

Here are some images of plankton collected with the VPR during this cruise.

Photo credit© Fredrika Norrbin

Appendicularians have a spinal cord, which make them our closest relatives among the invertebrates. They look a little like tadpole larvae, and are therefore sometimes called Larvaceans. Appendicularians build their own intricate “houses” with feeding nets to collect the smallest particles in the sea. However, these animals only live in their houses for a few hours before leaving them and building another. The discarded houses collapse and form a large part of the marine snow.

Photo credit© Fredrika Norrbin

Salps are closely related to appendicularians, and are also producers of marine snow, but in this case it’s not in the form of houses but poo – scientifically named “faecalpellets”.

Photo credit© Fredrika Norrbin

Copepods are one of the most abundant and wide spread animals groups in the ocean and also some of the most numerous animals on Earth. You can find them everywhere, eating phytoplankton, other copepods and even marine snow! Their poo is also a part of the marine snow.

Photo credit© Fredrika Norrbin

Siphonophores are strange kinds of jellyfish. living in colonies where each individual has its specialized job. Some have tentacles to catch and sedate prey, others digest it, and others again help the whole colony swim and float. You may have heard of the infamous siphonophore “Portugese man-of-war” - but most are quite harmless to humans.

By Fredrika Norrbin

Deploying the VPR at sunset. Image credit© Chris Lindemann