Re: What living things does an ocean food chain begin with?

In the open ocean, far away from land, there are no large plants for fish and other animals to eat. There must be some other type of life that uses energy from the Sun to grow and provide food for animals.

That type of life is phytoplankton. Phytoplankton are tiny organisms made of a single cell. They are only about 50 microns across -- that's about the size of the red blood cells in your body.

The word "phytoplankton" means "light-drifter": the name refers to any creature that drifts around in the ocean and uses the energy from sunlight to grow. There are many different sorts of phytoplankton: two of the most common are diatoms and dinoflagellates

diatoms are one-celled algae -- they're closely related to seaweeds. They protect themselves with a pair of hard shells made of "silicate" (glass, basically). The shells are often arranged like a petri dish and its cover, or a pot with a lid on top. They make more of themselves by splitting in two: one keeps the old "pot" and grows a new "lid", while the other keeps the old "lid" and grows a new "pot". Diatoms are full of chlorophyll, the same green stuff that trees and grass use to turn sunlight into food ("photosynthesis").
dinoflagellates are alveolates, which isn't a helpful definition -- they're not closely related to anything you have ever seen before. They also have chlorophyll to turn sunlight into food; unlike diatoms, however, they have a very unusual asymmetric shape, and have a pair of whiplike tails which they beat in the water to push themselves forward. The name "dinoflagellate" means "spinning whip". Isn't that strange? A "plant" that's able to swim! They also have hard shells, but these shells are made of cellulose (the same stuff in wood and paper). Dinoflagellates are especially important to humans because every once in a while, the ocean becomes absolutely full of a very poisonous variety of them. When this happens, they color the water bright red (a "red tide"): their poison can kill fish and other sea animals, and is very dangerous to humans as well.

Both diatoms and dinoflagellates are neither plants nor animals nor fungi: they're protists, which is basically a catch-all group for everything that isn't a plant, animal, or fungus. Diatoms behave a lot like plants, and dinoflagellates are a lot like animals, but they're neither.

What eats these guys? There are larger animals of a variety of sorts, many of which look like shrimps or crabs (but which are still too small to see) which eat the phytoplankton. These are called "zooplankton" (animal-drifters). Small fish eat the zooplankton, and so on.

For more information on phytoplankton, see
http://seagrant.gso.uri.edu/G_Bay/phytoplankton.html
http://research.nwfsc.noaa.gov/ec/tox/Harmful_Algae.htm
http://fig.cox.miami.edu/~161hon3/temp4.htm