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Photo: Larry Madin
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The role of the salps in the Southern Ocean

The 26 February at 4 am the pelagic trawl, with five separate nets fishing from 0-750 m depth, entered the trawl deck on board G.O. Sars packed with gelatinous organisms, including species of salps. There were thousands of them in the nets.

Solitary stage of the salp Salpa thompsoni. Mouth is at the left, stomach is the brown ball at the right, while a chain of baby salps is seen underneath. Small shrimps visible inside their body are parasites. Photo by Larry Madin.

We also found some impressive jellyfish. One of them had a diameter of 1 m, weighing more than 20 kg. There were also some interesting mid-water and deep-water fish species, krill and amphipods in the samples. Everybody was busy sorting, identifying and measuring species, and dissecting a few salps still alive.

Vacuum cleaners of the ocean

You are probably wondering what kind of creatures salps are and what they look like. Well, they are almost like vacuum cleaners of the ocean. In general they are like a cylinder with muscle bands contracting along their body with a size of a few centimeters. The contraction provides a pulse of water through their body, producing a jet propulsion for the salp, which is a similar principle found in squid.

The distribution of these muscle bands is very useful to identify different salp species. Each species consist of two forms; the solitary and the aggregate. The solitary form as the name indicates lives on its own (Figure 1), whereas aggregate forms lives in chains consisting of 100–150 members (Figure 2). They can form large swarms consisting of thousands of individuals.

Photo: Larry Madin
Chain of an aggregate stage of Salpa thompsoni. Every salp is a separate individual, but they are elegantly connected into a coordinated chain of animals. Salps produce one offspring each. Photo by Larry Madin.

The salps are important and efficient plankton eaters, feeding on a wide size range of plankton. Salps produce a mucous sheet that serves as a net to capture all the tiny organisms present in the water. They can then filter particles smaller than 1µm, and retain particles up to 1mm, due to these special traits.

Salps and krill

Previous studies have suggested that salps and krill have a tight relationship, since they consume similar food organisms and represent potential competitors for plankton. However, salps tolerate higher temperatures than krill. In the winter, when the sea ice is formed, the krill in Antarctica eats algae growing underneath it. The sea ice will decrease if temperatures of the Southern Ocean continue to increase. Consequently, the ice algae, a valuable food resource for the krill particularly in winter, will be less abundant and affect krill survival. Since salps are able to survive at higher temperatures than the Antarctic krill, the salps may then favour a warmer climate and gradually dominate over krill as a major species in the Southern Ocean.

Written by

Paola G. Batta Lona
University of Connecticut, USA