In researching last month’s article on dorids, I finally got around to sorting my more than 370 photos of nudibranchs according to their classifications. I also looked for images for that article (on Doridacea) and this one (on Aeolidacea). The majority of nudibranchs are dorids and when the sorting was complete, that was overwhelmingly evident. I had loads of dorid images and they were of several different species. My many images of aeolids, however, were of just four species. 

California Diving News
Aeolidacea are named for the Greek god Aeolis (god of the wind) and commonly called aeolids (pronounced a-O-lid). While dorids have branchial plumes on their posteriors, aeolids have cerata, arranged on their bodies in clusters, groups or rows. Cerata can be one of several shapes. Those of the nudibranchs described here have long, narrow cerata. They are multi-purpose organs, used not only in respiration but also digestion and defense. In a bad-ass example of “you are what you eat,” nematocysts (stinging cells) from the sea anemones and hydroids that aeolids eat travel through their digestive systems and end up at the tips of their cerata, where they serve as weapons to fend off predators. Talk about a “power lunch.” Unfortunately, even when armed with stinging cells aeolids are eaten by sea stars and various other opisthobranchs, including aeolids.
Aeolids’  rhinophores are nonretractile and can be simple, annulate or lamellate. Two of the animals shown here have rhinophores with rings (annulate) and two have rhinophores with horizontal flaps (lamellate). 
The elongated aeolid foot resembles that of their land slug cousins. They have tiny and often invisible eyespots behind the rhinophores that only sense light and dark. The oral tentacles, much longer in aeolids than in dorids, are sensitive to touch, taste and smell. They help the animal make its way along the substrate as well as find food and perhaps a mate. 
Aeolids have strong jaws and feed primarily on hydroids, sea anemones, bryozoans, gorgonians and the eggs of other opisthobranchs. 
In case you missed last month’s article, “Those Darling Dorids: SoCal Nudes Exposed,” I’d like to point out again that all nudibranchs are hermaphrodites. The male organ is stored in a sac on the right side of the head, next to a rhinophore. The female port is just above the mid point on the right side. The animals position themselves right-side to right-side and head to tail so the male organs align with the female ports. After mating, both animals lay egg strings that are unique to their species. Shelled veligers hatch from the eggs and drift with the currents until they settle down on the substrate and morph into adult form, losing their shells in the process.
The four nudibranchs described here are members of two Suborders: Flabellinidae (F. iodinea and F. trilineata) and Glaucidae (Hermissenda crassicornis and Phidiana hiltoni).
Some nudibranchs have more than one common name and others have none at all. Since guidebooks use only scientific names, we will, too.
Flabellina iodinea ranges from the Galapagos Islands to British Columbia and is abundant off the California coast. This is a gorgeous animal with a bright purple body, yellowish-orange cerata and bright orange, lamellate rhinophores. To escape predators, F. iodinea is able to let go of the substrate and “dance,” i.e., wiggle or undulate, through the water. I have several photos of it standing on its tail. Is the animal preparing to flee or just practicing an evasive maneuver? F. iodinea can be two and three-quarters inches long and is found from intertidal waters to about 130 feet. Its three colors come from eating its favorite hydroid, which contains a carotenoid pigment. Besides eating this hydroid, Flabellina iodinea deposits its pink egg strings on it. 
Flabellina trilineata ranges from Alaska to Baja and gets its name from the three white lines on the translucent white body of the adults. This is a small nudibranch, less than an inch and a half long. While the one shown here has red cerata with orange tips, yellow rhinophores (annulate) and yellow tentacles, the colors vary from animal to animal and can be much paler. I have photographed just one of these aeolids since I took up underwater photography thirty years ago. 
Hermissenda crassicornis has the largest range of the four animals described here. It is found from Alaska to Baja as well as off Japan and South Korea. It lays white egg strings and has annulate rhinophores. The body is translucent white with white or bluish-white lines. The tips of the tentacles, cerata and rhinophores are white. There is a bright blue or white line from the tip of each tentacle to the head and a bright yellow, white or red median line from the mouth to the final group of cerata. There is also a white line running along the edge of the foot. The colors of this species vary. It can be two inches long and has a varied diet that includes ascidians and other aeolids.
Phidiana hiltoni is found from California’s Central Coast to Baja and can be two inches long. It has lamellate rhinophores. Among its prey are hydroids. It is cannibalistic and in a macabre twist, may start consuming its partner while they are mating. This is a combative nudibranch and when it meets another head to head, it will fight. Whoever gets the first bite usually wins the battle, and has the loser for lunch. However, if one escapes with only the loss of cerata, not to worry, they will be regenerated in a few days. 
Aeolid Stats
Phylum: Mollusca
Class: Gastropoda
Subclass: Opisthobranchia
Order: Nudibranchia
Suborder: Aeolidina
Superfamily: Aeolidiodea
Family: Flabellinidae: F. iodinea and F. trilineata, 
Glaucidae: Hermissenda crassicornis and Phidiana hiltoni
Reproduction: Sexual
Shearwater TERN