Sea hares are members of the class Gastropoda in the phylum Mollusca. Known for their use of ink as a defense mechanism, sea hares are closely related to sea slugs and nudibranchs. They have numerous species, the most well known being Aplysia californica. Sea hares are found throughout the globe and are important to both the environment and scientific study.
"Then God commanded, 'Let the water be filled with many kinds of living beings...' So God created the great sea monsters, all kinds of creatures that live in the water... And God was pleased with what He saw. He blessed them all and told the creatures that live in the water to reproduce and fill the sea." - Genesis 1:20-22 (NASB)
Sea hares receive their name from the pair of rhinophores on their head. These "ears" give the appearance of a rabbit, or hare, hence the name "Sea Hare." Along with the rhinophores, sea hares have two oral tentacles on either side of the mouth.  Both the rhinophores and oral tentacles have nerve receptors that detect chemicals in the water and react to stimuli.  The eyes of sea hares are simple, only able to detect light from dark. Most species have a thin, internal shell, protecting the gill and heart. The mantle cavity is covered by large flaps called parapodia.  These are easily identified by the "ruffles" on the backs of sea hares.
As gastropods, sea hares have one foot that they move around on. To eat, they use jaws and a radula (A flexible tongue-like organ in certain mollusks, having rows of horny teeth on the surface).   Sea hares don't have a brain, but rather groups of nerve cells called ganglia. They also have an open circulatory system, where the blood is pumped into sinuses (large, sac-like spaces) rather than in a network of blood vessels. (Miller & Levine pg 695,704) The color of a sea hare is directly related to the algae that they eat. Whatever color the algae is, that is the color the sea hare will be as well. 
Sea hares are hermaphroditic, meaning that they are both male and female. In mating, three or more pile up into lines. Each sea hare is female to the one behind and male to the one in front. In this way, a sea hare breeds internally with two other sea hares at the same time.  Once the eggs are fertilized, they are laid in long ribbons or masses, similar to strings of spaghetti. The species Aplysia californica can lay up to 86 million eggs during a single period.
The eggs can form in masses as a large as a grapefruit's diameter! These masses are usually attached to eelgrass, and hatch in 10-12 days. The larvae live as plankton for around 30 days. The larvae then settle and eat massive quantities of red algae for growth. Sea hares have a lifespan of about one year, dying shortly after laying their eggs.  Their lives are sometimes longer if cool temperatures prevent spawning.  When they die, they die in mass numbers, washing ashore by the dozen. This is an odd, but natural phenomenon that occurs annually throughout the world. 
Sea hares are herbivores, feeding on various types of algae and eelgrass. As larvae, they eat primarily on red algae, transitioning to green and brown algae as they mature. Predators of sea hares include: Anthopleura xanthogrammica-- a large green sea anemone, Melo amphora-- a bailer shell (gastropod), Coscinasterias calamaria-- giant starfish, and humphead wrasse.  If given the opportunity, many other animals will feed on sea hares as well. Most predators tend to prey on their egg masses more than anything. Favorinus japonicus, a nudibranch, especially feeds on sea hare egg masses.  Pycnogonids, sea spiders, feed primarily on juvenile sea hares.
Sea hares depend on camouflage and ink for defense. Like squid and octopi, sea hares can release a purple ink that acts as a screen to confuse predators and give the sea hare time to escape.  Sea hares are found in waters all around the world. The Aplysia punctata for example, can be found anywhere from Greenland waters to the Mediterranean Sea.  Over all, sea hares seem to prefer sheltered coastal waters that contain mass amounts of vegetation. Adult sea hares live in shallow lower or middle tidal zones, while the juveniles will live as deep as 60 feet! 
In recent years, sea hares have become very important in neurology. Scientists have discovered that the nerve cells in a sea hare's ganglia are ten to fifty times larger than those found in mammals. They are also brightly colored. For both of these reasons, neurologists are using sea hare nerve cells to study the basic nervous system of an organism and its reaction to stimuli.  In fact, there are 10,000 easily identified neurons within a single sea hare. With this information, scientists are learning more about how memory and various genes function within the brain. 
Another scientific discovery involving sea hares is the presence of Escapin (an anti-bacterial protein) in their ink. Escapin is chemically similar to toxins found in some poisonous snakes, and prevents bacteria from forming. Researchers are hoping to produce Escapin commercially as a prevention against biofilm formation on marine products, ship hulls, and fishermen's nets or traps. This could provide an environmentally friendly alternative to using toxic metals such as copper to control bacteria. 
Aplysia fasciata, found in the Red Sea and Mediterranean; Species Fasciata 
Aplysia punctata, found from northeast Atlantic to Greenland to the Mediterranean; Species Punctata 
- What are sea hares? by W.B. Rudman. Sea Slug Forum.
- We need information on Sea Slugs (Hares) by W.B. Rudman. Sea Slug Forum
- Sea Hare (Aplysia californica) Monterey Bay Aquarium
- California Brown Sea Hare Aquarium of the Pacific
- Miller; Levine Prentice Hall Biology, pg 695,705
- Sea Hares Scuba Travel
- How many eggs can a sea hare lay? W.B. Rudman. Sea Slug Forum
- Sea Hares saltcorner.com
- Mass mortality W.B. Rudman. Sea Slug Forum
- What preys on sea hares? W.B. Rudman. Sea Slug Forum
- Humphead Wrasse--Ecology & Habitat www.panda.org
- Favorinus japonicus W.B. Rudman. Sea Slug Forum
- Sea slug predators - Pycnogonids (Sea Spiders) W.B. Rudman. Sea Slug Forum
- Sea Hare or Sea Slug www.thejump.net
- Aplysia punctata W.B. Rudman. Sea Slug Forum
- Aplysia, the Hare of the Ocean William H. Calvin williamcalvin.com
- How Many Genes Does It Take To Learn? Lessons From Sea Slugs www.sciencedaily.com
- Sea slug protein could benefit marine industry Poul Olson. www2.gsu.edu
- Aplysia fasciata W.B. Rudman. Sea Slug Forum
- Animal Diversity Web animaldiversity.ummz.umich.edu
- Definition of Radula www.dictionary.com
- Multiple authors Bible (TEV), Genesis 1:20-22