Nudibranchs, aka Sea Slugs

Facts, Habitat, Diet, Underwater Photography Tips and Dive Sites

By Scott Gietler

Nudibranchs are the underwater macro subject of choice for many underwater photographers. Sea slugs move slowly, are very colorful, are just the right size for compact and dSLR photographers, and are plentiful at many popular dive spots.  

Nudibranch Underwater Photography

Nudibranchs are great subjects because they usually stay fairly still, and they can be very colorful. Always get the rhinophores in focus. The rhinophores resemble two "antennae" that stick up from the front of their head.

Shoot low, from the front or the side, not from above. For dorids, it is important to wait until the gills come out. Try to get the food source in the photo. Pay careful attention to the background so the nudibranch looks separated from the rest of the photo. Play with the lighting. Gills out, gills & rhinophores in focus is usually considered a great shot, but it can be difficult to get both into the depth of field, and other compositions work well also.

In this shot of 2 Cerastoma nudibranchs, I was able to get all 4 rhinophores in focus. F18, 60mm lens. With a compact camera, shoot at F18 for maximum depth of field.

Here's a "classic" nudibranch composition - get down low, shoot the nudibranch at an angle, get the rhinophores in focus. Photo by Kevin Lee.

Getting low can be difficult, but it pays off. You usually have to find a nudibranch that's on a little hill, like this one, and then lay right on the ground. F18, 1/125th, 105mm lens.

Best dive sites for Nudibranchs

"Nudibranch hot spots"

Here are some excellent dive locations for sea slugs and nudibranchs.

• Anilao, Philippines - Nudibranch capital of the world
• Puerto Galera and Dumaguete, Philippines also are good for sea slugs
• Lembeh straits, Sulawesi - you'll find an excellent assortment of sea slugs here
• Bali, Indonesia - especially Jepun, Biaha, Seraya
• Loloata Island, Papau New Guinea
• Southern California - especially La Jolla shores, Palos Verdes and the northern Channel Islands
• La Bufadora, Mexico
• Puerto Vallarta, Mexico
• Any of the muck diving locations in Asia will be excellent for nudibranchs
• Raja Ampat, Indonesia - diver Kevin Lee recently found 90 species here on a 10 day trip, almost as many as in Anilao
• Nelson Bay, Australia - just north of Sydney (see all the species here)
• Byron Bay, Australia, near Julian Rocks (just south of Brisbane)
• Old woman island, 100km north of Brisbane. Garry Cobb does lots of branching here
• Australia has more great nudibranch sites. I saw quite a few at Edithburgh Jetty near Adelaide.
• Nha Trang, Vietnam
• Diving in Kenya, Africa. Read Kevin Lee's article on all the amazing nudibranchs you can find in Kenya.

• Contact me if you know of other nudibranch "hot spots".

Nudibranch photography tips

• Get low, get the rhinophores in focus. If the gills have retracted, be patient and wait until the gills come out.
• Get close and try to fill the frame
• Understand how your aperture controls your depth of field
• Think about the kind of background you want - black background, background sharply in focus, or a background nicely blurred. All choices can make great underwater photographs.
• Learn how to position your strobes for front-lighting and side-lighting. Different sea slugs look better in different kinds of light.
• Use your histogram to avoid blowing out colors and highlights. View your histogram as a 3 color histogram if possible.
• To identify a nudibranch, try to get the gills, rhinophores, oral tentacles, etc. in sharp detail - see the anatomy section below.
• If your camera allows you to move your focus points, choose spot-focus, compose your photo, and move the focus point until it lies over the rhinophores of the nudibranch.
• Read about supermacro underwater photography, to photograph very small nudibranchs.
• Please don't harm a sea slug just so you can get a better photograph, or move them from their environment. Sea slugs feed on very specific food sources.
• Compact camera users - read the lens selection section below, to understand how zooming in and zooming out affects your composition and background.

Janolus nudibranch with a black background,  Catalina island, California. 60mm lens, F18, ISO 250, 1/250th

Nudibranch Anatomy & Taxonomy

Nudibranchs are animals in the phylum Mollusca, class Gastropoda - which means, like snails, they are Molluscs and Gastropods. Technically, Nudibranchs are a sub-set of animals called Opisthobranchs (sometimes call Sea slugs). Which means all nudibranchs are sea slugs, but not all sea slugs are nudibranchs. Got it?

Most, but not all, Nudibranchs can be classified as Aeolids or Dorids.

Dorids have 2 rhinophores and plume-like gills. The gills are also known as a branchial plume.

The 2 rhinophores of this white Dorid nudibranch are on the left, sticking up like antennae. The gills are on the right, and will retract if the animal is startled. The skin covering the top of the dorid is called the mantle, which is this case is covered with small bumps.

Aeolids have oral tentacles, rhinophores and cerata. Some Aeolids can store nematocysts in their cerata, after they eat a Cnidarian such as a hydroid or anemone.

In this Aeolid, you can see the two large oral tentacles in the front, and the two smaller rhinophores sticking straight up on top of the head. The brown cerata cover the rest of the body, flowing backwards in this photo. Some nudibranchs can lose their cerata when harassed.

Nudibranch Natural History

Most nudibranchs feed on only a particular hydroid, anemone, sponge, bryozoan, or tunicate. Underwater photographers often find a nudibranch by locating it's food source. Some nudibranchs that eat hydroids or anemones can store the neumatocysts of their prey, using it for defense later on. Nudibranchs generally crawl around looking for food, but some can be surprising mobile and launch themselves into the water column when their feel threatened.

Some nudibranchs, like some Phyllodesmium species, are solar-powered, getting energy from symbiotic zooxanthellae that they absorb, much like corals do.

Nudibranchs are hermaphrodites, willing to mate with any other member of it's species, usually. Nudibranchs almost always begin their life as free-swimming planktonic larvae, and their lifespan is between 2 months and 1 year for most species.

Lens selection for Nudibranch photography

• Compact camera users - should zoom out to get the effect of the 60mm lens listed below. Zoom in all the way to get the affect of the 105mm lens. Wet lenses can give you additional macro capability for smaller nudibranchs.
• 60mm lens on a cropped sensor camera (90mm in 35mm equivalent) is good for photographing nudibranchs in their habitats, in-situ photographs. Auto-focus is fast, night photography is easy, and you can get close to the subject. Also good for low-visibility dives. If you are using a compact camera, simply zoom out all the way.
• 60mm lens + 1.4x teleconverter. Another excellent choice, especially if you think you might see very small nudibranchs. Slower auto-focus, but still useable on a night dive.
• 100mm or 105mm lens - great for isolating a nudibranch, filling the frame and blurring out the background. Also good for nudibranchs deep in reefs, cracks, crevices, etc - due to the increased working distance. Can be more difficult to use on night dives. If you are using a compact camera, zoom in all the way to isolate the nudibranch.
• Fisheye lens - this can be used if you get very, very close to large nudibranchs, for unique photographs that capture expansive backgrounds in the photograph.
• Further reading on lens selection - supermacro photography, lens selection and composition, 60mm vs 100mm macro lenses

Dirona nudibranch photographed with my 60mm lens, F20

Dendronotus albus nudibranch, Nikon 105mm lens, F16

Chromodoris coi, Nikon 60mm + 1.4x Teleconverter, F22

Janolus nudibranch, head on composition. You need to get very low to get this composition, and alighn the nudibranch carefully. Using the 105mm lens helps to isolate the subject. Shot at F13, photo by Mike Bartick. Mike used a custom +10 diopter for this shot, and waited for the nudibranch to crawl towards him.

Further Reading

Best dive destinations for underwater photography

Muck and macro diving critter list

Super macro underwater photography

Macro underwater photography

Underwater settings for macro

Advanced underwater composition (with some good ideas for Nudibranchs)

Nudibranch resources

Sea slug forum

Slugsite

Nudibranch ID forum

Inspirational Nudibranch photos by Kevin Lee

Marli Wakeling's Nudibranchs of the world

Jim Anderson's Scottish Nudibranch Page

Nudibranchs of the Mediterranean sea

Other Marine Life Articles

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Bobbit Worm - ambush predator

Frogfish - Camouflaged ambush predators

Black Sea Bass - Gentle giants of California