Many underwater photographers love nudibranchs and I am not the only one with numerous images of these colorful creatures. We Southern California divers are lucky; of the more than 170 species found off the Pacific Coast, a large number live in our part of the ocean. 

Most nudibranchs range in size from less than an inch to several inches long and can have unusual shapes. While a few have colors that serve as camouflage, most are flamboyant eye-catchers. They can afford to be highly visible because most don’t taste good and some are toxic. Their close cousin, the three- to eight-inch long Navanax inermis, eats them anyway.
A spectacular cousin of the land slug, nudibranchs (the name means “‘naked” gills) are members of the Phylum Mollusca, Class Gastropoda, Subclass Opisthobranchia and Order Nudibranchia. There are four Suborders: Doridacea, Dendronotacea, Arminacea and Aeolidacea. The majority of nudibranchs are classified as Doridacea and are commonly called dorids. This article focuses on four of them.
Dorids are identified by several different features. Their branchial plumes (gills) circle the anus on the animal’s back and are one of three types: unipinnate, shielded or tripinnate. The plumes further lead to classification as cryptobranchia (retractile gills), phanerobranchia (rigid, non retractile gills) or porostomata (retractile gills, small head with no tentacles, radula or jaws).
Dorids typically have a mantle, a foot, oral tentacles and rhinophores. They also have tiny and often invisible eyespots, which are behind the rhinophores. Since the eyespots only sense light and dark it is the oral tentacles (sensitive to touch, taste and smell) that help the animal make its way along the substrate as well as find food and perhaps a mate. 
There are several types of chemical-sensing rhinophores and they differ with the species. Those of the four dorids pictured here have horizontal flaps called lamellae. Some dorids can retract their rhinophores to protect them. The pair of mating Diaulula sandiegensis shown in the photo have retracted their gills and rhinophores. 
Nudibranchs are carnivores. Most opisthobranchs and all dorids except the porostomata have a ribbon of chitinous teeth called a radula. The shape and number of rows of teeth on the radula are specific for each family/genus and are adapted for grazing on a particular prey. Most dorids seems to prefer sponges though at least one genus (Triopha) eats bryozoans.
Many marine creatures are hermaphrodites and nudibranchs are no exception. Nudis only live about a year and having both sexes in one body increases their chances of reproducing because they can mate with any nudibranch of their species they come across. The male organ is stored in a sac on the right side of a nudi’s head, next to a rhinophore. The female port is just above the mid point on the right side. Nudis truly “‘hook up.” They position themselves right-side to right-side and head to tail so that the male organs align with the female ports. After mating, both animals lay eggs, usually in a spiral ribbon that has one edge attached to the substrate. Each species’ egg ribbon is unique.
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 all belong to the Suborder Doridina. One is classified as a member of the Superfamily Anadoridoidea, while the other three are Eudoridoidea. 
Common names are considered too confusing and guidebooks use only scientific names, a practice that continues here. 
Chromodoris macfarlandi: A tiny dorid, C. macfarlandi is more elongate than most of its close cousins and only attains a maximum of two inches. It is easily identified by its bright purple gills, rhinophores and mantle. The three yellow stripes on its back are another important characteristic. These are not always solid; they can be broken. Some C. macfarlandi also have a white band on the edge of the mantle, as does the one in the photograph. The gills are retractile. Chromodoris macfarlandi is found from Monterey Bay to Baja and its favorite meal is the sponge Aplysilla polyraphis.
Diaulula sandiegensis: This dorid attains a maximum length of four inches and can be white, pale yellow or pale brown. The number, shape and placement of the brown rings or blotches varies. The animal looks velvety because the mantle is covered with tiny, sensory, retractile tubercules that are surrounded by tiny, sharp pointed structures. The retractile gills are white and the rhinophores match the color of the mantle. D. sandiegensis ranges from Alaska to Baja and has also been found off Japan. It dines on sponges.
Peltodoris nobilis: One of the Pacific Coast’s largest nudibranchs and those found in deep water can be more than seven inches long. It was formerly classified as Anisodoris nobilis, then Diaulula nobilis. The mantle is covered with yellow tubercules and black or brown blotches that are among, but not on, the tubercules. The retractile gills have white tips. P. nobilis is known as the lemon nudibranch because it exudes a lemon scent (said to be “‘quite pleasant”) when it is handled. It is found from Alaska to Baja, in intertidal waters down to 750 feet.
Triopha maculata: This is one of my favorite nudibranchs and I have photos of several different species. They remind me of circus clowns. The main color can be pale yellow, orange, red or dark brown. The mantle is covered with raised white to bluish spots. On the mantle and along the front edge of the head are branched appendages called processes. These are bright orange in all of my Triopha photos, as are the rigid, nonretractile gills. T. maculata can grow to lengths of seven inches. These especially colorful sea slugs are found from Vancouver, Canada, to Baja. Most of my Triopha encounters have occurred off the nudibranch lover’s paradise of San Miguel Island or nearby Wilson’s Rock.
Nudibranch Stats
Phylum:  Mollusca
Class: Gastropoda
Subclass: Opisthobranchia
Order: Nudibranchia
Suborder: Doridina
Anadoridoidea (Triopha maculata)
Eudoridoidea (Chromodoris macfarlandi, Diaulula sandiegensis, Peltodoris nobilis)
Dorididae (Diaulula sandiegensis, Peltodoris nobilis)
Polyceridae (Triopha maculata)
(Chromodoris macfarlandi)
Reproduction: Sexual