While some divers could care about slimy sea slugs, there are many more who devote a lot of time and effort into finding and identifying the colorful little creatures known as nudibranchs. For these divers, it's the underwater equivalent of butterfly collecting. In fact, nudibranchs are sometimes called the butterflies of the sea.
Like the winged insects that inspired this nickname, nudibranchs display a fascinating range of colors and patterns, and there are hundreds of varieties to identify and add to a photo album or a personal life list of discoveries. As many divers can attest, the more you know about nudibranchs, the more interesting they become.
Naked Snails
Nudibranchs are basically aquatic snails without shells. Unlike other members of the mollusk family, they don't keep their built-in body armor past the larval stage and live out their adult lives with their soft parts and frilly gills exposed to the elements. This trait has earned them the collective name of nudibranch, which comes from a combination of Latin nudus, (naked), and Greek brankhia (gills).
As you'd expect from a relative of the garden snail, nudibranchs aren't speed demons; they top out at just over a foot-an-hour as the make their way across the seabed. But that's probably a good thing because a fast-moving nudi would probably run into things, as they are nearly blind. They do sport a pair of eye spots, but these are good for little more than telling light from dark.
To make up for their poor vision, nudibranchs have excellent senses of smell, touch and taste. Most species have a pair of feather-like stalks atop their head known as rhinophores. These organs can detect tiny traces of chemicals in the water that might indicate food or the approach of a would-be predator. When it feels threatened, a nudibranch will retract its rhinophores, but can do little else to evade capture and consumption.
A Case of Bad Taste
The thing that keeps nudibranchs from becoming an easy snack for almost everything on the reef is what these slugs choose for their own meal plan. Since they have no chance of running down dinner, nudibranchs focus on things that can't move. They aren't big on algae and other grazing fodder, however, and are actually carnivores with hard tooth-like growths in their mouths that can crunch and crush things. Nudibranchs like to feed on certain types of corals, anemones, sponges and barnacles, they enjoy a batch of fish eggs when they come across one and aren't above eating each other when the opportunity arises.
Many of the things on the nudibranch diet are pretty noxious or even toxic. But these crafty little slugs actually use these bad tastes to their advantage. They absorb and assimilate the toxic residues from their meals, turning their flesh into an unpalatable option for most everything in the ocean. Some species are also able to manufacture their own acidic toxins to further increase the foul taste of their flesh. To advertise their bad taste, nudibranchs sport flamboyant color schemes that are nature's equivalent of a hazmat warning sign.
Some nudibranchs deploy additional defense mechanisms. After consuming stinging organisms such as jellyfish, anemones and hydroids they will store the stinging cells from their consumed prey in the tips of spike-like protrusions on their body. This prickly packaging ensures any nibbler gets a nasty mouthful of stingers. Some species of nudibranchs go one step further and turn these stingers into tiny missiles than can be launched at a would-be attacker.
Sex and the single Slug
Since they don't get around much, you might wonder how nudibranchs manage a hook-up. The answer is slime. As in the scented slime trails they leave behind as they slide along the seabed. When a nudibranch looking for love happens across a promising slime trail, it follows the smell at what passes for top speed at a slug's pace. If the pursuer manages to catch up to a potential partner, things get interesting, because these slimly slugs do it different.
Nudibranchs are hermaphrodites, sporting both male and female genitals. Not a problem. When two like-minded individuals get together, they often go tail-to-head for a double dose of mating that could last from minutes to hours. And should additional amorous nudibranchs catch the scent of copulation, they may join in to create a ring of procreation. Afterward, each slug leaves with a clutch of fertilized eggs, which will be excreted in long jelly-like ribbons. There's no need to hide these eggs from predators, as they are every bit as toxic as the nudibranchs themselves, and often equally as colorful.
Finding Nudibranchs
Nudibranchs live in most of the world's oceans but are most common in tropical waters — and especially in regions of the Indo-Pacific oceans such as Indonesia, Malaysia and the Philippines. Lembeh Strait in Northern Sulawesi is a favorite with nudibranch spotters, and the Anilao region of the Philippines boasts more than 600 known species, with more discovered each year.
While many varieties of nudibranch are easy to spot, making a positive identification can be a bit tricky, as there may be considerable variation in both color and shape within a given species. It's quite easy to photograph these slow-moving slugs, but divers should resist the temptation to touch or reposition a nudibranch for the sake of an image.
Nudibranchs live a slow life, but not a long one. While some species may have a life span of up to a year, others live for only a few weeks in adulthood. So should you come across one of these butterflies of the sea, realize that you are witnessing one of nature's most colorful but short-lived creations.