When I was in high school I started to adventure in tide pools with my Aunt and Uncle in Bandon, Oregon. That is where I fell in love with the wonderful slugs of the sea: Nudibranchs. These creatures are the most beautiful marine organism that I have ever seen. My favorite of all time is the Hermissenda crassicornis:
These beautiful creatures are extremely abundant in California but are found on areas of the West Coast of North America all the way down to Mexico. They have an elongated body between 30 and 80 mm that is translucent blue with orange stripes; fingerlike cerata that are ranges of orange with white tips. Because these organisms lack a shell, they need a different way of protecting themselves from predators: cerata. These cerata carry nematocysts: stinging cells obtained from anemones and other animals in its diet that irritate the skin animals waiting to prey on these beautiful creatures. Also because they do not have a shell, they have increased motility that helps them to move away from predators faster than those organisms with shells.
Hermissenda crassicornis are usually found in a lower intertidal zone or subtidal zones on hydroids since those are their main food source but are also found around anemones, sea pens, and small crustaceans. They are also found in tidepools, rocks, pier pilings, and mudflats up to 110 feet deep. They are hermaphroditic which means that they possess both male and female organs but self-fertilization is very rare. Most Hermissenda crassicornis mate and then lay their eggs, from a few to a million, next to a food source. These organisms have shown to produce longterm planktotrophic veliger larvae which means that they spend a large amount of time in the plankton stage before settlement can occur. Some of these larvae take three weeks to reach competence to be able to metamorphose and settle.
These organisms are very small (only a few millimeters to an inch or so); the following picture shows a nudibranch next to a key that represents the size.