By the shore of the sea the zoologist means much more than the narrow
zone between tide-marks; he means the whole of the relatively shallow,
well-illumined, seaweed-growing shelf around the continents and
continental islands. Technically, this is called the littoral area, and
it is divisible into zones, each with its characteristic population. It
may be noted that the green seaweeds are highest up on the shore; the
brown ones come next; the beautiful red ones are lowest. All of them
have got green chlorophyll, which enables them to utilise the sun's rays
in photosynthesis (i.e. building up carbon compounds from air, water,
and salts), but in the brown and red seaweeds the green pigment is
masked by others. It is maintained by some botanists that these other
pigments enable their possessors to make more of the scantier light in
the deeper waters. However this may be, we must always think of the
shore-haunt as the seaweed-growing area. Directly and indirectly the
life of the shore animals is closely wrapped up with the seaweeds, which
afford food and foothold, and temper the force of the waves. The minute
fragments broken off from seaweeds and from the sea-grass (a flowering
plant called Zostera) form a sort of nutritive sea-dust which is swept
slowly down the slope from the shore, to form a very useful deposit in
the quietness of deepish water. It is often found in the stomachs of
marine animals living a long way offshore.