The word "plankton" comes from a German word meaning "drifters." This is what plankton do, drift as opposed to swim. For the most part they are microscopic and get around with the movement of the ocean currents. They are also typically found at or near the surface of the water. There are several major classifications of plankton.
Phytoplankton are the photosynthetic plankton or the producers. Notice I did not say the plants, because although it is easy to think of them as plants, they are not plants as defined by biologists. Included in the phytoplankton are some members of the Kingdom Monera (remember that in biology organisms are classified in Kingdom, Phylum, Class, Order, Family, Genus, and Species). These are the most primitive and simplest single-celled organisms on earth and include bacteria and blue-green algae. These are especially important as decomposers. Because they are only a few microns in size, although they are very abundant, they are not a major food source except around hydrothermal vents (obviously the bottom dwelling versions do not photosynthesize!).
Most Phytoplankton is a member of the Kingdom Protista. These are also mostly microscopic and all photosynthesize, but they are made of slightly more complex cells that the Monera (eukaryotes vs. prokaryotes if you know about those). These are the common marine algaes and are responsible for nearly all the primary productivity in the ocean. The Protists come in several main varieties (there are pictures of these below):
Coccolithophores (coccoliths for short) have tiny calcite exoskeletons (shells, in other words). They are found mostly in tropical waters and aside from being important primary producers, they have served scientists well. Since they like the tropics and their little shells get deposited in the sediments on the ocean floor, they can be used to look back in time at the earth's climate. For example, if coccolith shells were found buried in the sediment off the coast of Alaska, this would indicate that at some point the climate there was tropical. They make for very reliable little fossils!
Silicoflagellates are similar to coccoliths except their skeletons are glass. They're found in cooler water. Calcium tends to dissolve in cooler water so you can understand why these glass critters would tend to replace the coccoliths where it's cold. A flagella is simply a tail-like structure and both these plankters have those. This gives them a little bit of mobility and since they are so small, they are very slow to sink. This is obviously a very important characteristic for a photosynthesizer and in the tropics, especially, where there is not much else (remember productivity is generally low in the tropical open oceans) coccoliths do most of the producing.
Diatoms are bigger (we're still only talking tens of microns here) and thus more important producers in the ocean. They do well where nutrient levels are high--continental shelves, upwellings, higher latitudes. Partly this is because they need a high concentration of nutrients to support their larger size. Also in these areas the water tends to be better mixed (that's partly why there are more nutrients) and the mixing in important in keeping these bigger things up high in the water where the sun is. In order to increase their surface to volume ratio and float better, they often group together in chains which are less prone to sinking. Diatoms come in two main designs: the centric diatoms look just like petri dishes. The pennate diatoms look like skinny footballs. In both cases the shell is in two halves and so to divide the cell merely splits into the two halves and then has to grow another half shell. This means they can multiply or bloom very quickly. "Diatomaceous earth" is mined from ocean deposits (the White Cliffs of Dover in England are diatoms) and is sold as an organic pesticide. For something like a slug, it's like walking across broken glass--in fact that's exactly what it is! This also has lots of commercial uses in abrasives like cleaners and polishes including toothpaste, paint removers, and fine filters.
Dinoflagellates have two flagella or tails and have skeletons of cellulose. This makes them more buoyant and motile than diatoms and so they do not need that well mixed water and have the advantage where it is calm. These are what makes the ocean glow or bioluminesce at night when it is disturbed by waves, boats, and either resident or visiting swimmers. Dinoflagellates can also reproduce very quickly when conditions are good. This is not surprising since they do well at low nutrient levels to begin with. Give them extra and they go nuts. These can become what we call red tides or other types of toxic blooms.
These are diatoms and dinoflagellates:
If you are a microscopic drifter who is an animal, you are zooplankton. These tiny animals eat phytoplankton by using various adaptations to move water around them and then filter out the algae cells. Most of these are tiny arthropods or animals with jointed feet. "Bugs" on land are arthropods and just like on land, arthropods are the most diverse and numerous animals in the ocean. The smallest and most abundant of these are the copepods. These are shown below.
These are copepods:
The next most abundant are krill which you may have heard of. These look like small shrimp and are the base or the Arctic and Antarctic food webs. There are also various small worms and jellyfish. All these are considered holoplankton. That is, they are born plankton and die plankton. That's all they do. A lot of the zooplankton is made up of meroplankton. This is plankton that is only planktonic some of the time. Generally when they hatch out, they are planktonic and at some point in their life cycle become members of the nekton (the swimmers) or the benthos (the bottom dwellers). Nearly all shellfish, things like starfish and anemones, and a lot of finfish ("regular" fish) have this lifecycle. Two of these are shown below:
These animals need to stay up in the surface water where their food is, so they have various adaptations to improve their buoyancy. Most of them can swim up and down, although they can't fight currents. Many contain oil globules which help them float. They also tend to have various antennae and hairs and projections which help them catch food and move, but also increase that all important surface to volume ratio. Consider that when you want to float, it helps you to spread out your arms. Same idea here.
The Benthos is the seafloor and the organisms that live there. It includes ocean bottom in very shallow coastal water as well as very deep water. Organisms that live on the bottom are benthic. Let's start with the algaes.
Macroalgae which you know as seaweed is a Protist just like the microscopic algae in the phytoplankton. In other words, it's not biologically a plant, but it photosynthesizes. In fact, these big algaes were probably the precursers to our land plants. Remember that for various reasons, the ocean is really a more easily habitable place than land, and so it makes sense that life would have started there and later moved onto land. How exactly are seaweeds different from plants? Well, consider their challenges compared to land plants. They have all the water and nutrients they need all around them. They don't need roots. Instead, seaweeds have holdfasts which serve no other function than to glue the seaweed into place. They have water to support them, so they need not worry about gravity. Instead of trunks and leaves, seaweeds have stipes or flimsy stems and fronds. Some varieties have air pockets in their fronds to help them stay near the surface.
These algaes come in Green, Brown, and Red varieties and are obviously all found in shallow water since they need sun. Besides eating them directly, we use them in a lot of things requiring a creamy texture: milkshakes, ice cream, frozen cream pies, paint, make-up, and shampoos, for example. Look for the ingredients algin or carageen and you've got seaweed.
Since there is a huge variety of benthic habitats, mud, sand, rocks, shallow, deep, there is a huge variety of benthic organisms. What they mostly have in common is that they don't swim, at least not much. Nearly every category of animal is included in the benthos. There are the sponges. These animals are porous just like your kitchen sponge and that's what they feel like. Before we knew how to manufacture plastic sponges, we harvested marine sponges. They are filter feeders, filtering small food particules out of the water that passes through their pores. There are worms of every description, from microscopic to several meters (relax---those big ones are only found in the deep sea around hydrothermal vents---so far!),
The benthos includes many of the mollusks. These all have soft bodies, many of which are protected by calcareous (made of calcium) shells. These include the gastropods which is all the snails you're probably familiar with. The periwinkle snails we saw in the marsh and the whelks and conchs you find on the beach. These animals go along grazing on algae. If you have a fish tank, they're good at keeping the glass clean. A lot of the shells your average beach comber collects are from the gastropods and if they aren't that, they're probably bivalves. These are animals with shells in two parts: clams, oysters, scallops. These animals are filter feeders. They suck in water, sieve out the good stuff, and spit the cleaned water back out. Oysters, in fact, filter several gallons of water an hour this way. The fact that only 10% of the original oyster population present in the Chesapeake Bay still exists goes a long way toward explaining the water quality problems there. (On the other hand, the water quality problems probably go a long way toward explaining the lack of oysters .these type of questions keep marine biologists off the streets!). Some gastropods lack shells completely. The nudibranchs are an example. These are brightly colored, rather large, sea slugs. They are much more interesting than your standard garden variety slug, but they are close cousins. The figure below shows a pair of them:
The cephalopods are the most advanced mollusks and include the octopus, squid, and cuttlefish. Of these, only the octopus is really considered benthic. Other than the cuttlefish, these do not appear to have shells. In fact, both squid and octopi have a beak made of shell material. If you try, you can get a squid to bite you. A giant squid is definitely not benthic, although perhaps I should not say that because we don't know much about them except that they can be at least 15 meters long and sperm whales eat them. We've never seen one alive.
As I said above, arthropods are the largest group of animals on earth, both in terms of variety and numbers. Over 75% of all animals belong to this group (and you thought your Raid was going to solve your problems). Not only are they the most abundant form of zooplankon, they show up all over the benthos, too. In fact, as I mentioned, many of them are both, starting out life as zooplankton and at some point settling down to the bottom. The most prominent arthropods in the ocean are the crustaceans. (On land, as you probably know, it's the insects.) These include the forementioned copepods and krill. Small insect-like critters called amphidpods and isopods frequent the benthos. The isopods look a lot like pill bugs, rolly-polies, or potato bugs (which of these means something to you depends on where you are from). Barnacles are another common arthropod. The part you see is actually the shell and a little crustacean lives inside. If you catch them feeding you can see that the top of the shell opens and the animal waves feathery appendages which trap other zoo- and phytoplankon. This is shown in the figure below. Lobsters, crabs, and shrimp are all crustaceans also. Lobsters and crabs will eat pretty much anything., shredding it with those claws. If you've ever gone crabbing, you know the older and more repulsive your bait, the better. And humans just love to eat these things! I've always wondered who tried it first. Shrimp are not considered benthic.
The echinoderms are another major group of benthic animals. "Echinoderm" means "spiny skin" and this group is also radially symmetrical. This means they are formed around a central disk---starfish, sea urchins, and sand dollars. Sand dollars are filter feeders, but urchins and starfish are typically aggressive predators. A starfish uses its suction cup feet to latch onto a bivalve for example, and just pulls until the hapless clam gives up. Mind you, this can take several hours. Then the starfish sticks its stomach into the shell of the bivalve, digests the animal, and pulls its stomach back in. Neat, eh?
There are, of course, benthic vertebrates, namely fish, but I think this is enough for one day.