Why does the fish shape in The Rarely Glimpsed Junkfish function as negative space?

a)  The artist left the space blank and used ink and charcoal on the background.
b)  The artist used white paint.
c)  The fish shape is smaller than the background.
d)  The fish shape is recognizable from the mouth and eye.

a)  The artist left the space blank and used ink and charcoal on the background.