Nudibranchs

Picasso Critters on the Reef

The naked gills of a nudibranch look like feathers

Nudibranchs, or nudis as they are affectionately called, are some of our favorite sea critters: colorful, bizarre, and endlessly challenging to find.  Their small size (often less than 1 inch!) makes them a real challenge for our age-hindered eyes!

In layman terms, "nudibranch" is sort of a catch-all name for a whole group of generally small, colorful sea critters that are often oblong or cylindrical and maybe only an inch or 2 long (though the flamboyant Spanish Dancer nudibranch can attain 18 inches!). Many people lump almost any soft-bodied, colorful critter into the "nudibranch" category.

Nudibranchs, aka Sea Slugs, belong to the Phylum Mollusca, a huge Phyllum of animals that includes all the sea shells and bivalves like clams, cephalopods like squid and octopus. There are literally tens of thousands of species of Mollusks in the world, and most live in the ocean.

More specifically, nudibranchs are part of the Class Gastropoda, Subclass Opistobranchia. Sea slugs generally have soft bodies (no external shells) and their mantles are brilliantly colored and cover their backs.  Nudibranch literally means "naked gills" and most of these creatures have feathery masses on their bodies that are their naked, external gills. They are of many shapes and colors, but most all have two rhinophores, or thick sensory tentacles near the head. Some nudibranchs have hidden gills on the sides and some have a mass of "hairy looking" cerata on their backs which are actually part of their respiratory system.

We only started finding nudibranchs in 2015 while diving with Triton Bay Divers in Papua, Indonesia.  The dive guides there are great at locating these beautiful little critters, and now we're getting better at it ourselves.  Unfortunately as we get older and our eyes get worse it gets harder and harder and to see them.  The key is to look for bright contrasting colors of an oval or oblong shape.  You pretty much have to ignore all the busy fish moving around to do this!  Luckily, most nudibranchs move very slowly so they are not hard to photograph if you have a light and the right lens.  That being siad, we have encountered nudis that can move their full body length in a minute or less, creeping around the edge of a bommie going out of sight.  Many nudibranchs hang out under overhangs and in crevices, so you have to look closely, or turn upside down maybe to find them.

A note on identification:  Of the hundreds of nudibranchs that have been "found" and photographed, a relatively small number have actually been scientifically "described".  For an animal to be described it has to be captured and studied and dissected and have its DNA analyzed.  Given that Genus is often determined by physiological likeness to other creatures in that genus, scientists can often give a pretty good guess as to the genus of an undescribed nudibranch, but until more studies are done, this is only a temporary classification.  New species are being discovered every year and more and more species are getting themselves an official scientific name every year.  But there are so many creatures in the sea that we still don't know about.  That's one thing that makes this amateur "animal hunt" so exciting.

Here are a few of the amazing nudibranchs we have discovered while diving and snorkeling.  Don't forget to click on a picture to see a bigger version!

Unless otherwise noted, all specimens were photographed in eastern Indonesia, in either Raja Ampat or Triton Bay, areas renowned for their multitude of small sea critters.

The simple, but beautiful Funeral Jorunna

The spotted hypselodoris is an elegant nudibranch

At left, the Spotted Hypselodoris H. maculosa, ispart of a large genus of nudibranchs, all with elongate bodies and many with a combination of stripes and spots. On this speiceis the rhoniphores are banded with brown-red and white.

At right, the beautiful white and black nudibranch is in the Genus Jorunna, in which there are at present only a few named species.  This one is the Funeral Jorunna, J. funebris and is characterized by the black circular markings which appear fuzzy due to fine, erect papillae.

At right is the slightly warty Geometric Chromodoris with white tubercles surrounding a lavender dorsum (back).The white rhinophores with green tips are characteristic. This was found deep under a ledge and we had to practically stand on our heads while scuba diving to photograph it.  That our friend Dave saw it to begin with was amazing!

At right is a Blue Dragon Pteraeolidia ianthina whcih can be a variety of colors from this dark blue to a pale blue, yellow green or tan. The swirly things on its back are cerata.  All Blue Dragons have blue or violet bands on their rhinophores and oral tentacles. THese can be up to 4 inches long and we have seen them several times.

The Genera Phyllidia and Phyllidiella together account for dozens of species, each with very noticeable "warty" appearance.  To tell one species from another it's necessary to study the pattern of lines and "islands" of color or tubercles to see the differences.  These are all pretty common on the tropical Pacific reefs.

On the left is a P. varicosa, or, in common terms a Swollen Phyllidia.  In the enlarged photos (click on the small one) you can see small red crabs, maybe on the dorsum, something that we didn't see until after the photo was taken!

At right are a pair of P. pustulosa ("postulosa" meaning bumpy, which could apply to most any of this genus!)  The English common name is Pimpled Phyllidiella.

At right is a hunched up picture of a Shreens Phyllidiopsis, a close relative of the Phyllidia and Phullidiella genera.  The pink rhinophores of this Shreens are not visible in the picture.

At right we're out of the Phyllydia genus and their cousins. This beauty  is probably in the Chromodoris Genus, as it has many similar looking critters in our book.  But the closest, the C. Anna's describes the animal as NOT having a complete central stripe and the blue mantle having black speckles, which this specimen does not.

(Misool, Raja Ampat Photo by Sue Hacking)

At right Mushroom Coral Reticulada, Reticulada fungia is strange with its very clear ridges that give it a stiff, rather than soft-looking back. Size, about 1.5 inches.