Giant Nudibranch (Dendronotus iris)

By Scott Boyd

The Pacific Northwest is home to more then 200 species of nudibranchs, which are essentially snails without shells.  In most species, the flamboyantly colorful gills are prominently displayed along their dorsal surface. Nudibranchs are benthic organisms that can be found crawling over rocks, seaweed, sponges, mud and many other substrates throughout the world.

The Giant Dendronotid (Dendronotus iris) is one of the largest and most varied of sea slugs, found in many different colors including white, grey, orange and red.  They can grow up to 12” long but are most typically found at about 4” in length.  When under attack, the Giant Nudibranch has the ability to swim away by undulating its body or it can let the predator get a taste of its toxic gill tufts (the frilly white tipped members are actually its gills).  The Cerata ( gill structures ) are easily cast off and regenerated by the nudibranch, if needed.  They live in sand and mud up to a depth of about 600 feet and can be found from Alaska to Baja Mexico.   Dendronotus Iris typically comes up into shallower depths to mate in July and August, which is when most divers will spot them.

They feed almost exclusively on tube dwelling anemones.  When feeding, individuals have been observed to be pulled completely inside the anemones tube, as the anemone retreats with the nudibranch hanging on with a mouth full of tentacles.  These anemones rarely die after predation by a Giant Dendornotid, so the nudibranchs can often be found to be visiting the same areas, year after year.

Nudibranchs are unusual in that they are simultaneous hermaphrodites, which mean that they possess both male and female sex organs at the same time. This strategy increases the probability of finding a mate, since every mature individual of the same species is a potential partner (self-fertilization is very rare).  After mating, giant nudibranchs lay their egg masses in delicate looking lace coils (see photo), often underneath an overhanging rock ledge. The eggs will hatch in 30 – 45 days, and it is not uncommon to revisit the site of an egg coil after a few months and find the bottom literally covered with hundreds of tiny nudibranchs.