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A nudibranch in Anilao, Batangas, Philippines

For most of the 24 years that I have been scuba diving, I have been on the lookout for big sea life like barracuda, jacks, sharks, sea turtles, moray eels and other reef fish.  It has only been in the past few years that I have discovered that there are so many  small  marine organisms that live on the reef bottom which are just as fascinating as the more well-known denizens of the reef.  One group of bottom-dwelling animals which fascinates me are nudibranchs (pronounced as “new-dee-branks”).  Nudibranchs are members of the mollusc family.  They are soft-bodied animals like their relatives the octopus, squid, clams, and sea snails.  Nudibranchs are also referred to as sea slugs.

While scuba diving at a depth of 4-5 meters in Twin Rocks (a dive site in Anilao in the province of Batangas in the Philippines), I was excited to find the nudibranch shown in the photo above (scientific name Jorunna funebris).  (You can see a short video clip of this nudibranch at the end of this post.)

Nudibranch means “naked gill”.  The part on its back which looks like black and white feathers are its gills.  Gills are organs which allow marine life such as fish, crabs, lobsters and sea snails to obtain oxygen from the water.  Nudibranchs are unusual because their gills are located outside their body.

Nudibranchs are usually only a few centimeters in length. Photo shows the size of Jorunna funebris in relation to my forefinger.

The appendages which look like a pair of white antennae with black tips are its sensory organs called rhinopores.  The part of the nudibranch’s body with the rhinopores is the head of the nudibranch.

The blue version of Hypselodoris bullockii, which also comes in purple or white.

Nudibranchs are sea snails which do not have a shell.  They may seem defenseless, but that is definitely not the case.  Some nudibranchs use stealth to hide from predators.  Their color and shape blend perfectly with their environment, providing them with perfect camouflage.  On the other hand, most nudibranchs have opted for a completely opposite strategy.  Instead of trying to hide, they use bold, eye-catching colors and designs to advertise themselves to would-be predators, saying “If you eat me, you’re going to get very sick, so stay away!”

Nudibranchs are not a popular item on the menu of predators due to the ability of nudibranchs to retaliate using chemical warfare.  When disturbed, some nudibranchs secrete acids which taste awful to predators.  Others secrete toxins which can kill, not only the attacker, but also other marine life in the immediate surroundings.

There are even nudibranchs with more extreme chemical defenses.  A group of nudibranchs (called aeolids) feed on sea anemones and hydroids.  Sea anemones and hydroids have stinging cells which inject a toxin into a victim.  Aeolids, however, are not harmed by these stinging cells.  When it eats a sea anemone or a hydroid, an aeolid is able to retain the stinging cells and place them in appendages called cerata.  When a predator bites the cerata, the predator gets a mouthful of stings!  Aeolids are a perfect example of the saying “You are what you eat.”

A nudibranch possibly from the Polyceridae family

Like other snails, nudibranchs eat through the use of a radula.  A radula looks like a miniature chainsaw.  The nudibranch moves its radula which cuts away at its prey.  Nudibranchs are carnivorous creatures.  Some feed on sponges and tunicates, while others prey on coral polyps, sea anemones and hydroids.  The sea tiger nudibranch, however, is very different:  it eats other nudibranchs.  When it finds its prey, it quickly brings out and expands its mouth (in exactly the same way as an activated airbag of a car).  In a few seconds, the mouth of the sea tiger envelops a part of its prey, and then the sea tiger uses its radula to pull in its victim.

Wart Slug

Nudibranchs come in all shapes and colors, with some already in the “What in the world is that?” category, like the one in the photo above.  Not surprisingly, this is called a wart slug (Phyllidia ocellata).  I took this photo during a twilight dive at a depth of 3-4 meters, also at the Twin Rocks dive site.  The wart slug does not have external gills like Jorunna funebris, but it also has rhinopores.  Can you determine where the head of the wart slug is?  Find  the rhinopores and then you will know where the head is.  The rhinopores are the pair of orange-colored appendages which look like antennae.  Were you able to find them?

(The rhinopores are towards the right side of the photo.)

A pair of nudibranchs (possibly of the Chromodorididae family) photographed in the Bajura dive site in Anilao.

If you want to see other amazing (and even bizarre) nudibranchs, then you should visit coral reefs of the Philippines.  Dr. Terry Gosliner of the California Academy of Sciences has been studying the nudibranchs of Anilao.  In his blog “Scientist at Work” last May 2010, he said “Since 1992, we have found 684 different species of nudibranchs and their relatives from this one area, making it the richest nudibranch fauna in the world.  Not only that, more than half the species we find are new species… This is a dream come true.  Where else can you find an average of one new species per dive?”

If you are a scuba diver, you can also find nudibranchs.  The method I recommend is:  look at pictures of nudibranchs so you will become familiar with their various shapes and designs.  See as many pictures as you can by surfing the internet.  When diving:  instead of passing quickly over a reef, slow down and stay in one spot for a few minutes.  Then you will be able to notice tiny movements on the reef bottom.  Take a closer look, and with some luck, you might find one!  If you are not yet a scuba diver, consider taking up the sport to have the chance to personally see the wonderful world of nudibranchs!