Giant Nudibranch, Dendronotus iris

Giant Nudibranch, Dendronotus iris. Underwater photographs taken in the coastal waters off Monterey, California, February 2022. Photographs and identification courtesy of Bob Hillis, Ivins, Utah.

Phylogeny:  The Giant Nudibranch, Dendronotus iris J. G. Cooper, 1863 is a member of the Dendronotidae Family of Nudibranchs. The genus Dendronotus is one of three genera in this family, and there are thirty-seven species in this genus. This species is also known as the Rainbow Nudibranch, the Giant- frond aeolis, the Barber Nudibranch, and the Giant Dendronotus and in Mexico as dedronotus de arco iris. The specific name iris comes from the mythological Greek goddess of the rainbow.

Morphology: Giant Nudibranchs appear to be shaggy nudibranchs because of their branched gills and cerata. The bushy cerata are arranged into two longitudinal dorsal rows. The oral veil has four pairs of papillae. The rear edge of the rhinophores have 3 to 6 branched processes. They vary in color from translucent white or gray, to orange or red. They generally have a white edge around the foot, though this may be absent. The branched gills may be tipped with orange, yellow, white, or purple. These are large nudibranchs, reaching 20 cm (7.9 inches), but most specimens are about half that size.

Habitat and Distribution: Giant Nudibranchs are found on mud or sand bottoms from the lower intertidal zone to depths of 200 m (660 feet). In Mexican waters they are found along the entire west coast of the Baja Peninsula. They have a limited, and poorly documented, distribution in the Gulf of California.

Diet: Giant Nudibranchs are predators, feeding on the Tube-dwelling Anemone Pachycerianthus fimbriatus. They graze over the tentacles of the anemone, leaving the anemone alive.  This “tentacle haircut” is what gives them the common name Barber Nudibranch. On occasion, they are so committed to feeding on the tentacles that they are pulled down into the anemone’s tube, as the anemone retracts. The nudibranchs retain the toxin of the anemones, incorporating it for their own defense.  Giant Nudibranchs are also known to eat hydroids, nemateran worms, and scyphistomae  jellyfish.

Predators:  The Giant Nudibranchs possess toxic tipped cerata and they can rapidly swim through the water by quickly bending and straightening their body to avoid predation. They are preyed upon by crabs, polychaetes, sea spiders and sea stars including the  Sunflower Star, Pycnopodia helianthoides.

Reproduction:  Giant Nudibranchs are simultaneous hermaphrodites ( having both male and female reproductive organs). They reproduce sexually. Copulation requires two individuals to face opposite directions, with their right sides touching. They pass sperm packets through a tube called the copulatory apparatus. After being fertilized internally, the eggs are laid in white, gelatinous, strings, often on the tubes of their prey. These egg masses can contain millions of eggs. After 4-7 weeks, the eggs hatch into planktonic larva.

Ecosystem Interactions: The parasitic or commensal relationships of the Giant Nudibranch has not been documented.

Human Interactions: Giant Nudibranchs have no direct impact on human activities. From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern

Synonyms:  None