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For many organisms in the ocean, especially those living near the shoreline, tides are very important.
At low tide, these organisms are exposed to the air, and perhaps fresh water.
At high tide, shoreline organisms are bathed in sea water which brings with it both nutrients and food organisms, and removes wastes and products of reproduction.
The daily rhythm of many organisms is based on the rhythm of the tides.
The tides are due to the moon's gravitational attraction of the Earth and its oceans, so that a bulge of water is formed on the side of the Earth facing the moon, and on the opposite side as well.
As the Earth rotates, each place on the waterfront experiences two high
tides, and two low tides roughly every 24 hrs.
Since the moon is also revolving around the Earth, the period between
high tides is actually about 12 h 26 min.
The moon is not the only heavenly body which affects the tides.
The sun also has a role.
Neap tides (with lower than normal tides)and
Spring
tides with higher than usual tides occur due changes in the relative
alignment of the sun, moon and earth.
Animations.
For an animation showing how Spring and Neap tides occur, go
to this web site.
For a simpler animation showing how the tidal bulge follows the moon
in its orbit go
here.