Characteristics of Indo-Pacific Nudibranchs

If there was a fashion show on the ocean floor, the most extravagant outfits would belong to nudibranchs. In the Indo-Pacific region, nudibranchs run the gamut, from the sleek to shaggy, to the downright whimsical.

The Indo-Pacific speaks of the vast expanse of tropical oceans in the world, stretching from the Indian Ocean coast of southern Africa and the Red Sea to the Pacific of the Hawaiian Islands, Easter Island, and the Marquesas. The area supports the world's leading marine wildlife with a wide range of species. Opisthobranchs (nudibranchs and other sea slugs) are no different from the show. These are a diverse and fascinating group of marine gastropods which are often a favorite among underwater photographers. There are about 3,000 species of nudibranchs discovered in the world and at least 40% of these are found only in the tropical Indo-Pacific. In this article, you will find some interesting facts about the characteristics of Indo-Pacific nudibranchs.

Indo-Pacific Nudibranchs can survive at all depths, from the wetlands to depths over 700m. The most remarkable variation of nudibranchs is seen in warm, shallow rivers, although one species of Nudibranch was found in quantities close to 2,500m.

Just like good fashion breaks all the rules, so does the taxonomy of nudibranchs. However, the four main types of nudibranchs which inhabit the Indo-Pacific region have a few signature features.

Dorids and Dendronotids

The word nudibranch means “naked gills”, gills that help the nudibranch breathe. Most nudibranchs in the dorid group rock these feathery tufts with ease. Sea Bunnies are one of the cutest looking nudibranchs in the Indo-Pacific oceans. The fur coat we see on the sea bunny is definitely not fur. They are actually groups of small rods known as caryophyllidia, which cover their back. They’re arranged around small black knobs that give it a spotted look. Most experts believe that these organs play sensory roles. The two ears that make the creatures look like bunnies are rhinophores. They help them to identify chemicals in the water that allow them to find food and mates. The sea bunny has both male and female reproductive organs. Predators stay away from these cute little slugs because they are incredibly toxic. Sea bunnies often eat sponges that contain toxins. The average lifespan of a sea bunny is between a couple of months and a year.

Then there are the more avant-garde groupings, like the dendronotids. Rather than a small tuft, giant dendronotids can let their gills fly and can grow up to a foot long. The dendronotid is a fierce predator.

Aeolids and Arminina

Various species of Aeolids and Arminina are found in the Indo-Pacific region. The backs of these nudibranchs are covered in fabulous finger-like adornments called cerata. Cerata are where fashions meet function. They’re used to breathe, as a place to digest food, and for defense.

The Coryphellina rubrolineata is one of the most attractive aeolids found in the Indo-Pacific oceans. These aeolids love to eat one thing – stinging hydroids. They repurpose the hydroids’ stinging cells and store them in their cerata for their own defense.

Another beautiful nudibranch found in the Indo-Pacific region is the stunning Glaucus atlanticus. This nudibranch is known by many other names such as blue dragon, blue angel, and blue sea slug. These nudibranchs are pelagic and float upside down by using the surface tension of the water to stay up, where they are carried along by the winds and ocean currents.  The blue glaucus makes use of countershading, where the blue side of their body faces upwards blending in with the color of the water, while the silver side faces downwards, blending in with the sunlight reflecting on the ocean’s surface when viewed upwards from underwater. The blue sea slug grows up to a length of 3 cm. Even though they live in the open ocean, they sometimes accidentally wash up onto the shore, therefore they may be found on beaches.

Common Habits of Indo-Pacific Nudibranchs

Nudibranchs are carnivorous. A few eat sponges and others feed on hydroids or bryozoans. A few are cannibals who eat other sea slugs or occasionally members of their own species. Some groups feed on tunicates, barnacles, or anemones.

Most nudibranchs usually use radula, a toothed structure that they use to scrape their prey from clinging rocks; a few suck on the prey after scraping its tissues with selected proteins, like wasps.

Do not be fooled by their delicate looks - these soft-bodied creatures have come up with few and perhaps amazing ways to protect themselves from predators. Not only can they be tough, bumpy, and aggressive, most hold toxic chemicals and stinging cells. They actually steal the toxins from the food they eat to use as their own.

Like many other marine creatures, nudibranchs interact with chemical signals. For example, when they are in the larval phase and they move into the water, they do not change until they receive a flag from a certain hormone, which can be an indication that there is food within the region. They can feel this chemical rise with the help of their tentacles close to their mouth. These organs are called rhinophores.

For all their extravagant pageantry, one thing all these Indo-Pacific nudibranchs have in common – we just can’t get enough of them! 

To learn more about nudibranchs, please visit our website www.ilovenudis.com

References

• Holland, J. S. 2008. Living color: Toxic nudibranchs—soft, seagoing slugs—produce a brilliant defense National Geographic, June 2008.
• National Geographic Society. 2008. Nudibranch (Nudibranchia) National Geographic Society
• Turnbull, John (Spring 2016). "The Nudibranch - Creature Feature". Nature New South Wales.
• "Diving with Nudibranchs". https://www.dive-the-world.com/creatures-nudibranchs.php

Photo information in order of appearance:

• Chromodoris and Nembrotha Nudibranchs found in Indonesia by Kate and Peter Photography 
• Jorunna parva aka the Sea Bunny by Kiawmanas
• Coryphellina rubrolineata found in Tulamben, Indonesia by Oksana Maksymova
• Blue Dragon, Glaucus Atlanticus, Blue Sea Slug by Sahara Frost
• Photos courtesy of Shutterstock