Nature's Best—Dressed

Sea slugs may be nature’s “best-dressed” marine animals, as the Red Sea creatures on article attest. Unlike their gardenpest 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 (thosewith 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.

The chromodorids are probably the most colorful and easiest sea slugs to recognize. Doris was a Greek goddess, the daughter of Oceanus, and chromo means color, so they are the colorful goddesses of the sea. Their colors and patterns are limitless, although their basic body plan remains the same. Chromodoris geminus Rudman, 1987 is found in the Red Sea and Indian Ocean and is truly beautiful underwater—its ocellated spots are almost luminescent. Like many spotted species, it flaps the edges of its mantle in a kind of rhythm, and the spots above and below glow.

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.

Another spotted species, Chromodoris charlottae (Schrödl, 1999), was first discovered by a British diplomat running the marine station in Suakin, Sudan, in 1911. Sir Charles Norton Edgecumbe Eliot thought it was a Red Sea variety of a western Pacific species and called it Chromodoris reticula var. In fact, Eliot’s species was different and was only described as a new species many years later. It is known only from the Red Sea. The describer’s name and the year are in parentheses because he originally described it in a different genus.

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 postglacial 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.

The lovely Chromodoris obsoleta (Rüppell & Leuckart, 1830), originally described and illustrated from the Red Sea 180 years ago, is quite common on reefs from north to south. Like many of the species illustrated, it has remained in the Red Sea, never colonizing the Indian Ocean.

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. 

One can only assume that the stripes of Chromodoris africana Eliot, 1904 work as camouflage or as an advertisement of toxicity. All chromodorids produce defensive secretions from a series of glands located around the edge of the mantle. This species is found only in the Red Sea and along the northeastern coast of Africa. A similar species, the Pyjama nudibranch (see table of contents), has blue stripes (instead of white) on its back.

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 hemselves 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 andcan be regenerated if bitten off, or in a series of glands located around the edges of the body.

This spotted species, Chromodoris annulata Eliot, 1904, occurs in the Red Sea, the Arabian Gulf and the western Indian Ocean. Typically, there are two purple rings on the dorsum, one around the rhinophores at the front and one around the group of gills at the rear. However, almost all specimens from the Gulf have either a line between the two rings, as in the upper photo, or the rings are broken into dots and dashes. The reasons for this are not known, as in all other respects they are identical. Recent research on a Sudanese reef demonstrated that individuals can travel up to 20 meters (65') in one day!

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.

Written by: Nathalie Yonow | Photographed by: Gordon T. Smith

Courtesy: Saudi Aramco World | Vol. 63 | No. 4