In great contrast to the narrow, crowded, difficult conditions of the
shore-haunt (littoral area) are the spacious, bountiful, and relatively
easygoing conditions of the open sea (pelagic area), which means the
well-lighted surface waters quite away from land. Many small organisms
have their maximum abundance at about fifty fathoms, so that the word
"surface" is to be taken generously. The light becomes very dim at 250
fathoms, and the open sea, as a zoological haunt, stops with the light.
It is hardly necessary to say that the pelagic plants are more abundant
near the surface, and that below a certain depth the population consists
almost exclusively of animals. Not a few of the animals sink and rise in
the water periodically; there are some that come near the surface by
day, and others that come near the surface by night. Of great interest
is the habit of the extremely delicate Ctenophores or
"sea-gooseberries," which the splash of a wave would tear into shreds.
Whenever there is any hint of a storm they sink beyond its reach, and
the ocean's surface must have remained flat as a mirror for many hours
before they can be lured upwards from the calm of their deep retreat.