DTVs are towed behind a research vessel as it traverses a plotted path across the ocean. They are more simple than Remotely Operated Vehicles and Autonomous Underwater Vehicles, but are useful as platforms for a variety of different oceanographic instruments that measure biological, chemical, and physical aspects of the ocean. There are many different types of towed vehicles, such as the Moving Vessel Profiler (MVP) that can house a video plankton counter or similar device. At the same time, several external sensors can be carried that record various physical qualities such as temperature and current speed. The DTV BRIDGET, used by ChEss, moves up and down near the ocean floor, like a yo-yo, to study plumes associated with hydrothermal vents.
BRIDGET , a Deep-Towed Vehicle, being recovered here, carries instruments designed specifically to study hydrothermal vents. (Southampton Oceanography Centre)
Deep-Towed Vehicles, such as TOBI  pictured here, can carry many different types of instruments. (Biogeography of Deep-Water Chemosynthetic Ecosystems Project - ChEss)
The SEASOAR , an English-made Deep-Towed Vehicle, being readied for use. (Southampton Oceanography Centre)