World Ocean Census - Extract 9 - Collecting specimen

22 February 2010

Ninth amazing extract from The World Ocean Census: A global survey of marine life: Expanding the use of technology - Collecting specimen.

Trawling nets have long been used in oceanographic research, beginning with early studies of oceanic biodiversity, and Census projects use them extensively. Trawls are specialized large nets, akin to those used by fishers, in a variety of forms depending on the organisms of interest. Benthic trawls are used along the ocean floor, while pelagic trawls are used down to depths as great as 5,000 meters (3 miles). Some trawls sample in a series at different water depths to study the movement of organisms in the water column. Plankton nets are modified trawls used to collect intact planktonic organisms of nearly any size. Towed by an RV, plankton nets have a long funnel shape that terminates in a collection cylinder called a cod-end.

During a research cruise in 2006 on the RV Ron Brown to explore diversity in the deep Sargasso Sea, the Census of Marine Zooplankton field project successfully used three multiple opening/closing net and environmental sensing systems (MOCNESS) to trawl for zooplankton in deeper water than previously explored for these critters – 5,000 meters (3 miles). These newly designed trawl nets were fabricated from very fine (335 microns) nylon mesh, and the collected samples yielded a wealth of biodiversity from the deep water, including 13 species of cephalopods. Of these, three were octopods (Cirrothauma murrayi, Bolitaena pygmaea and Tremoctopus violaceus), one was a vampyromorph (Vampyroteuthis infernalis) and the remaining nine were squids belonging to at least five major groups (bathyteuthids, chiroteuthids, cranchids, histioteuthids and enoploteuthids). The new 335-micron mesh net yielded specimens that were in immaculate condition, greatly simplifying taxonomic analyses.

Though trawls are useful in the study of marine biodiversity, they have their limitations. Many animals are very good at avoiding capture in the nets, and other species can be easily damaged or destroyed in the process, especially those from great depths and soft-bodied creatures such as jellyfish. Therefore trawls are often used in combination with other research tools such as video plankton recorders, acoustic technologies and larger imaging equipment.

Text and images reprinted with permission from World Ocean Census: A Global Survey of Marine Life by Darlene Trew Crist, Gail Scowcroft and James M. Harding Jr., Firefly Books, 2009.

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