  The deep-water fauna of the Mediterranean is characterized by an absence of
distinctive characteristics and by a relative impoverishment. Both are a result
of events after the Messinian salinity crisis (Late Miocene). The three main
classes of phenomena involved in producing or recording these effects are
analysed and discussed: - Historical: Sequential faunal changes during the
Pliocene and thereafter in particular those during the Quaternary glaciations
and still in progress. - Bathymetric: Changes in the vertical aspects of the
Bathyal and Abyssal zones that took place under peculiar conditions, i.e.
homothermy, a relative oligotrophy, the barrier of the Gibraltar sill, and
water mass movement. The deeper the habitat of a species in the Mediterranean,
the more extensive is its distribution elsewhere. - Geographical: There are
strong affinities and relationships between Mediterranean and Atlantic faunas.
Endemic species remain a biogeographical problem. Species always become smaller
in size eastward where they occupy a progressively deeper habitat. Thus, the
existing deep Mediterranean Sea appears to be younger than any other deep-sea
constituent of the World Ocean.
