ea slugs may be nature's "best-dressed" marine animals, as the Red Sea creatures in the gallery above attest. Unlike their garden-pest cousins, they come in a dazzling array of shapes, sizes, colors and patterns, and they are definitely not slimy. In addition, they may offer scientists maps to new drugs through the chemical compounds they produce.

Sea slugs, scientifically known as opisthobranchs ("rear gills" in Latin), are highly evolved relatives of marine snails. Primitive sea slugs retain a thin external or internal shell, but advanced ones, like the nudibranchs, or "naked gills," pictured in this gallery, have none. Living without this protection means they have had to develop other methods of defense, expressed in a wide display of adaptations seen in sub-orders such as dorids (which have gills near the tail and two sensory horns near the head), dendronotids (those with tree-like gills in pairs along their backs) and aeolids (those covered with fingerlike projections). Their bright colors and bold patterns warn predators of their noxious taste.

Of the 4000-plus species of nudibranchs worldwide, more than 175 are found in the Red Sea and nearly one quarter of these live only there. This almost self-contained waterway—running some 2100 kilometers (1300 mi) from the Gulf of Suez in the north to the narrow Bab al-Mandab in the south—is a very special place, hosting endemic fish, sea urchins, worms, slugs and snails, and myriad other animals. It has been isolated for approximately five million years, and its fauna has suffered partial extinctions with several glaciations, the most recent 20,000 to 15,000 years ago, which substantially lowered sea levels.

The Red Sea's southern gateway opened around five million years ago—at the time land uplifts closed off its shallow link to the Mediterranean—and has never shut, so its fauna is Indo-Pacific. With the subsequent rise and fall of sea levels, the creatures of the Red Sea found themselves in a unique environment and either adapted, evolved into new species or died off. For example, the fossil record of shelled sea slugs shows that interglacial periods were times of high diversity through influx. During glacial periods of little contact with the oceans, as well as during post-glacial periods of stability, speciation occurred.

Despite much research into Red Sea fauna during the last 250 years, many sea slugs remain to be discovered and described. Indeed, 30 unidentified individuals are pictured in my book Sea Slugs of the Red Sea, and more than 20 are in bottles in my university lab.

My obsession with the Red Sea and its wildlife began in 1968 when I moved to Saudi Arabia as a child. The gift of a mask and snorkel from my father on a trip from Riyadh to Dhahran, near the Arabian Gulf, opened up another world. Months later, we made the long trek to Jiddah to go snorkeling, and so began my love of coral reefs, my father's shell collection and our forays into the challenges of identification.

In choosing a career, I was following in the footsteps of Petrus Forskål of the ill-fated Danish expedition to the Red Sea (1761-1767) and Jules César de Savigny, who was only 21 when he joined Napoleon's expedition to Egypt (1798-1801). These pioneers recorded a total of 18 opisthobranchs, including a number of nudibranchs. The Red Sea is an ideal location for these animals, with its calm, well-lit, warm and clear waters providing perfect conditions for the growth of coral reefs, an ecosystem that harbors one of the most diverse habitats in the world.

Nudibranchs don't just decorate the coral reef, however; they eat nearly every marine creature except fishes. Many restrict their diets to a single prey, most frequently a sponge. Some of these have evolved to resemble their quarry and even burrow into it and remain there. Other species go so far as to mimic the polyps, patterns and positioning of their prey. Those that feed on upright branching animals such as hydroids are often long and thin, making themselves nearly invisible by matching their dinner's shape and color. And there are sea slugs that eat their relatives: Gymnodoris impudica feeds on chromodorids, for example.

We have a great deal to learn from nudibranchs and their relatives. A number of species are able to toxify chemicals they "capture" from their prey, and some even make their own pharmacies of complex toxins to fend off predators or communicate alarm or reproductive readiness. These toxins are sometimes located in brightly colored body extensions that distract predators from vital organs and can be regenerated if bitten off, or in a series of glands located around the edges of the body.

The acids and chemical compounds found in sea slugs have attracted much recent research and may prove useful to the world at large. However, just as coral reefs are threatened today by man and by warming sea temperatures, so are their inhabitants. While some types of sea slugs may be tolerant of environmental changes and able to adapt, others may disappear before we even know they are there, along with their potentially life-saving pharmacies.

I have had the good fortune to spend most of my life identifying sea slugs, discovering and naming several. I hope these photographs convey some of my enduring fascination with these remarkable creatures.

	Nathalie Yonow, Ph.D., ([email protected]) is a marine biologist at Swansea University in Wales who lived in Saudi Arabia for 15 years. Her research focus is Red Sea opisthobranchs, but her expertise extends to sea slugs of the Indo-West Pacific oceans.
	Gordon T. Smith ([email protected]) has been diving and photographing in the waters of the Red Sea since he first moved from Scotland to Jiddah more than 20 years ago. He now resides in Dubai.