From physiology to marine ecology
Hensen studied medicine in Würzburg, Berlin and Kiel and received his doctorate in 1859. His early work concerned the anatomy and physiology of sensory organs. With Karl Möbius and Heinrich Adolph Meyer, he investigated the ecology of Kiel Fjord and the neighbouring bight.
Counting plankton
Hensen used plankton for organisms drifting or suspended in water. Fine-meshed silk nets collected organisms from a defined water column, and repeated samples supported estimates of abundance. His investigations thus asked how many organisms occupied the water, alongside identifying which species were present.
References: [1]
The 1889 Atlantic expedition
In July 1889 Hensen led the plankton expedition aboard National from Kiel into subpolar and tropical Atlantic waters. Biological collections were accompanied by chemical sampling and physical measurements, allowing plankton observations to be compared with their surrounding marine conditions.