Rainbow Nudibranch    Dendronotus iris

This weird, pink, bushy thing, swimming near the surface at the Boston Harbor Marina is a rainbow nudibranch about 6 to 8 inches long.  It is one of the largest species of a group of animals called sea slugs or nudibranchs (pronounced nudibrank).  Highly variable in color, it can grow up to 12 inches in length. The head of the animal is at the top left.  The bushy projections are sensory receptors above the head and oxygen exchange plumes covering the rest of the body.

Swimming is only done by a few nudibranch species and is thought to be an escape reaction to predators.  Normally the animal glides slowly along on its foot hunting the tube anemone (Pachycerianthus fimbriatus) which lives in the sediment in the shallow subtidal zone.  When startled it brings its foot together along the mid-line and then with a swaying motion launches its body into the currents.  See a movie of a hunting rainbow nudibranch at http://www.metridium.com/movies/iris.html.

Nudibranchs are shell-less relatives of snails.  Mainly predatory, they feed mostly on attached animals such as sponges, bryozoans, and anemones but some pursue other nudibranchs.  They occur world wide from cold oceans to tropical reefs.  

A variety of nudibranch species occur throughout our area.  Many are extremely colorful.  Some biologists have referred to them as marine butterflies.  The vivid colors are meant as a warning to would-be predators that they are toxic.  They can have a bad taste due to chemicals released from special glands or from stinging cells taken from their prey.