  The first prototype of a photo-detection unit of the future KM3NeT neutrino
telescope has been deployed in the deep waters of the Mediterranean Sea. This
digital optical module has a novel design with a very large photocathode area
segmented by the use of 31 three inch photomultiplier tubes. It has been
integrated in the ANTARES detector for in-situ testing and validation. This
paper reports on the first months of data taking and rate measurements. The
analysis results highlight the capabilities of the new module design in terms
of background suppression and signal recognition. The directionality of the
optical module enables the recognition of multiple Cherenkov photons from the
same $^{40}$K decay and the localization bioluminescent activity in the
neighbourhood. The single unit can cleanly identify atmospheric muons and
provide sensitivity to the muon arrival directions.
