Some of the prettiest creatures in the sea belong to the phylum Mollusca, most specifically the subclass Opisthobranchia, which includes sea slugs, sea butterflies, sea angels and sea hares, along with sacoglossans. Though they may have shells when they are embryos, most have lost them by the time they are adults.

Shearwater TERN
Never heard of sea butterflies or sea angels or know what sacoglossans are? Me, either! So pardon a short digression. While you may be envisioning ethereal creatures, sea angels and sea butterflies don’t look anything like their namesakes. Sea angels (Gymnosomata) are small, pelagic sea slugs about the size of a bean. While most slugs have a foot, sea angels have flaps called parapodia that allow them to swim through the sea. They have a head, no shell and eat just one thing — sea butterflies (Thecosomata) — one of the world’s most abundant gastropod species. These tiny pelagic swimming sea snails have a shell and eat both plants and animals. 
Sacoglossa, commonly known as the sacoglossans, are very small sea slugs that eat the insides of algae. Some digest their meals, while others retain the algae’s living chloroplasts in their tissues where they continue to photosynthesize and feed their hosts. This process makes sacoglassans unique among animals and is why they are known as the “solar-powered sea slugs.” While sacoglossans are found in SoCal waters I don’t think I’ve ever seen one. Those listed in creature ID books are less than an inch long. I have, however photographed members of this order in tropical waters, most especially the lettuce sea slug, Elysia crispate, which is solar powered. 
Opisthobranch (pronounced o-pis-tow-brank), which means, “gills behind and to the right of the heart,‚” is a large (about nine orders) group of marine gastropods with one or two pairs of tentacles. The eyes are at the base of the rear-most tentacles. These mostly soft-bodied animals may or may not have a shell and if there is one it may be small or internal. Unlike many of their gastropod cousins, they don’t have an operculum, which is used to seal a shell’s opening and protect its inhabitant. Not all opisthobranchs have gills. If they don’t, respiration takes place through the skin. 
Opisthobranchs may eat plants, detritus or flesh. Those that eat bryozoans, cnidarians and sponges absorb their toxins and become toxic themselves. This is why so many can afford to flaunt flamboyant colors, which practically shout, “I am inedible.‚” Their predators include other opisthobranchs and such toxin-resistant creatures as sea spiders. Those that aren’t toxic may be so well camouflaged that they escape notice. 
Opisthobranchs are hermaphrodites. When they mate, each animal transfers sperm to the other and each lays a species-unique egg ribbon. 
Opisthobranchs belonging to the order Nudibranchia include dorids and aeolids, which are abundant off our coast. There are other, less common nudis found in our waters, however, and we picture and describe two here. I have seen only two Dendronotus albus, both on the same dive off San Miguel Island. Tritonia festiva seems to be much more common and I have photographed this nudi several times off various Channel Islands. 
Besides the two nudibranchs, I have chosen two other uncommon opisthobranchs, sea slugs Berthella strongi and Tylodina fungina, to feature here. Each is the only member of its order I have ever happened upon. 
According to David W. Behrens” Pacific Coast Nudibranchs, the rare side-gilled sea slug Berthella strongi lives on tunicates in intertidal waters. The Sea Slug Forum says it ranges from Central California to Baja. These tiny, delicate looking creatures are less than an inch long. I have only seen one. There is a similar species known as B. californica, which has small, rounded protuberances on its back and a thin white line along the mantle margin. Both have rolled rhinophores and a branchial plume on the right side. B. strongi lays cylindrical egg ribbons while B. californica‘s are wide and flat.
Named for its mushroom-like shape and considered rare, Tylodina fungina‘s range is Central California to the Galapagos Islands. To me, the animal looks like a slug wearing a Chinese peasant hat. A large one would be just more than an inch long. I have photographed T. fungina off the islands of San Clemente, Santa Barbara and Catalina. They are said to feed exclusively on the yellow sulfur sponge. According to Pacific Coast Nudibranchs, T. fungina has “an external, cap shaped shell covered with brownish bristles.” The creature has rolled rhinophores and the gill is on its right side. There are eyes at the base of the rhinophores but I don’t know how sophisticated they are.
Tritonia festiva and the other nudibranch described here, Dendronotus albus, are both members of the same superfamily, Tritonioidea. D. albus ranges from Alaska to Baja. The translucent body can be either white or pink to pale violet. The colorful nudis were once considered a different species than the white but the consensus now seems to be they are just a color variation. These nudibranchs can be 2.75 inches long and eat hydroids. A much larger cousin, D. iris, also found in SoCal waters, can be nearly eight inches long.
Tritonia festiva is the only one of the four creatures described here that has a common name, the festive triton. This nudibranch ranges from south-central Alaska to northern Baja as well as Japan. It can be nearly one and one-half inches long and eats the extended polyps of octocorals such as sea pens and gorgonians. The polyps are eventually regenerated by most of these corals.
T. festiva in turn is preyed upon by certain seastars but, like the aeolid known as the purple nudibranch, can escape by letting go of the substrate and swimming away. The egg ribbon of a Tritonia festiva is described on the Sea Slug Forum as “a thin delicate rosette.” Unlike those of most of its close relatives, the eggs “are not embedded in a jelly matrix but float free inside the fluid filled cord,” according to the Sea Slug Forum. The larvae that hatch from the eggs drift with the currents and eat plankton. 
Opisthobranch Stats
Phylum: Mollusca
Class: Gastropoda
Subclass: Opisthobranchia
Order: Nudibranchia (Dendronotus albus, Tritonia festiva)
Pleurobranchomorpha (Berthella strongi)
Umbraculida (Tylodina fungina)
Superfamily: Tritonioidea (Dendronotus albus, Tritonia festiva)
Pleurobranchoidea: (Berthella strongi)
Umbraculoidea (Tylodina fungina
Family: Dendronotidae (Dendronotus albus)
Tritoniidae (Tritonia festiva)
Pleurobranchidae (Berthella strongi)
Tylodinidae (Tylodina fungina)
Reproduction: Sexual
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