  High-energy neutrinos could be produced in the interaction of charged cosmic
rays with matter or radiation surrounding astrophysical sources. To look for
transient sources associated with neutrino emission, a follow-up program of
neutrino alerts has been operating within the ANTARES Collaboration since 2009.
This program, named TAToO, has triggered robotic optical telescopes (MASTER,
TAROT, ROTSE and the SVOM ground based telescopes) immediately after the
detection of any relevant neutrino candidate and scheduled several observations
in the weeks following the detection. A subset of ANTARES events with highest
probabilities of being of cosmic origin has also been followed by the Swift and
the INTEGRAL satellites, the Murchison Widefield Array radio telescope and the
H.E.S.S. high-energy gamma-ray telescope. The results of twelve years of
observations are reported. No optical counterpart has been significantly
associated with an ANTARES candidate neutrino signal during image analysis.
Constraints on transient neutrino emission have been set. In September 2015,
ANTARES issued a neutrino alert and during the follow-up, a potential transient
counterpart was identified by Swift and MASTER. A multi-wavelength follow-up
campaign has allowed to identify the nature of this source and has proven its
fortuitous association with the neutrino. The return of experience is
particularly important for the design of the alert system of KM3NeT, the next
generation neutrino telescope in the Mediterranean Sea.
