All the arbitrary limits in the deep-sea do not fit with the reality or
rarely. The shelf break varies according to the seas and oceans between
about 70 to 150 m (about 100 m in the Mediterranean see Emig C. C., 1997.
Bathyal zones on the Mediterranean continental slope: An attempt. Publ.
Espec. Inst. Esp. Oceanogr., 23, 23-33.)
The famous 200 m is that of the econonomical zone - a political decision -
nothing to do with a scientific limit.
About the photosynthesis, according to the clarity of the waters the limit
can be deeper than 300 m in the benthic realm !
The use of arbitrary limits in fauna distribution does not allow to use
these data for later comparisons because bottom conditions and ecological
requirements are prevailing, in this way depth is not a factor but the
third information after latitude and longitude in 3D !!!