Nudibranchs (sea slugs)

What to look for

These animals are tiny. Most specimens measure between 0.5 cm and 5 cm in length. You must look closely at the substrate. They often hide on sponges or coral rubble.

Look for bright colors. Many species use these patterns because they warn predators of their toxicity. I remember seeing a bright blue Chromodoris in 2018 near Koh Tao. It sat on a sponge at 12 m depth.

Focus your light. Small macro life requires steady hands and good lighting. Use a torch to find them.

Behaviour you’ll observe

They move slowly. Most nudibranchs crawl along surfaces to find specific food sources like sponges or hydroids. They do not swim well.

They are mostly solitary. You might see two individuals mating if they meet on a reef slope. This happens often in calm waters.

Watch their gills. The namesake “naked gills” sit on their backs and move slightly as they breathe. They stay very still.

Where and when

Macro diving is a year-round activity. You can find them in every month from January to December. However, visibility changes with the seasons.

Anilao is a top spot. I spent two weeks there in 2019 searching for rare species. The muck diving around /dive-sites/anilao/ offers high sighting probabilities. You will likely see one on 80% of your dives if you look at the rubble.

Koh Tao is also good. Many divers visit /dive-sites/koh-tao/ for easy macro encounters. You might find a specimen in 1 in 4 dives during the dry season.

Check the depth. Most species live between 5 m and 25 m. Deep reef walls like Richelieu Rock host larger varieties. They thrive where there is constant nutrient flow.

Diver etiquette

Do not touch them. Their skin is extremely delicate. A single touch can remove their protective mucus layer.

Avoid chasing them. If you move too fast, they will hide in the sand or under rocks. This makes them impossible to photograph.

Maintain buoyancy. Use your fins carefully so you do not kick up silt. Silt covers the small creatures you want to see.

Stay close to the bottom. Keep a steady hover at 1 m above the reef. This prevents accidental contact with the substrate.

Conservation status

Data is quite limited. Most nudibranch species are not currently listed on the IUCN Red List. This is because they are too small for easy population counts.

Local habitats matter most. If the sponges die, the slugs die too. They rely on specific food sources to survive.

Climate change is a risk. Warming oceans affect the delicate balance of reef ecosystems. We must protect the reefs they inhabit.

Finding the best shots

Macro photography is hard. You need a macro lens and a steady hand. Use a stabilizer if you can.

Focus on the eyes. Even though they lack complex eyes, the head area provides a focal point. Position your light at a 45 degree angle. This prevents harsh shadows on the reef.

Watch your air. Macro diving requires staying very still for long periods. You might use 50% more air because of the constant hovering. Keep an eye on your pressure gauge.

Check your gear. Ensure your macro lens is clean before you enter the water. Small particles can ruin a close-up shot.

Bring a macro torch. A dedicated light helps reveal the true colors of the specimen. Practice your buoyancy in shallow water first. If you want easy macro, start with a muck diving course in Anilao.