Nudibranch. The very word strikes terror into the hearts of sponges and anemones. Sadly, that very same word is mispronounced by humans, who don't realize its Greek roots: beta, rho, alpha, nu, chi, iota, alpha: brancia. What a heritage! Nudibranch means naked gills, and is pronounced with a hard "k" sound at the end of the word. Nudi-brank is how to say it. You knew the nudi part, now you know the rest of the story--branchia--with a k.
So much for English. Let's look at biology. These creatures are some of the most fascinating and intriguing life forms under the ocean's surface. They look like what they eat. You thought your child after a Burger King looked like catsup? You haven't seen anything. Protective coloration and camouflage are the rule among this group of molluscs. They have their food not only smeared on their face, it is literally and chemically inside and outside their body. Yellow nudibranchs are found on yellow sponges; imagine the rainbow, and you will have an idea of what these incredible sea slugs are all about. But underneath the vast waters of the ocean, colors don't show up! So here we have blue predator on blue prey, or yellow on yellow, and the fish (the next level up of carnivory) can't see it because below a few meters, colors disappear. The slug blends into its food, and is not munched off by the passing garibaldi or sheepshead.
To really understand nudibranchs (and their relatives, prey, and life style) there is a lot to explain. Let me begin with some basics to set the scene properly. Scientists use a hierarchical classification system to organize all life forms. It is based on a sequence of categories:
This hierarchical system was originally developed by Carolus Linnaeus (with the 1758 publication of his Latin treatise, Systema Naturae); since then it has become modified and adjusted to meet the demands of modern evolutionary biology.
The lower the level shared in common between two organisms, the more closely they are related. The biological definition of a species (proposed by Ernst Mayr of Harvard University) emphasizes genetic interaction--a species is a group of organisms that breed with members of their own species, but not with members of other species. A genus (plural is genera) consists of a group of species that have evolved from a common ancestor. And so on up the hierarchical ranks. The species is the basic biological, ecological, and physiological unit. Chemicals that may cure human diseases need to be identified by the species from which they originated. We identify a species by its binomial scientific name, two words given this reproductive group and used by all scientists around the world. Common names change from French, Chinese, Spanish or English; but the Latinized scientific name is universal. People often complain, "Why do we have to use these complicated long names for animals?" The answer is quite simple--they are universal, understood worldwide, and properly identify the basic biological unit.
Animal phyla are uniquely different. Each has a certain "life style" that we can recognize and that helps us to understand their evolutionary relationship. The phylum Mollusca consists of a huge group of soft-bodied animals, usually with a shell, and having a characteristic gill shape, which is located inside a structure called the mantle cavity. Many molluscs also have a feeding apparatus called the radula.
Major groups of Mollusca are clams (Class Bivalvia), squids and octopuses (Class Cephalopoda), and snails and slugs (Class Gastropoda). Using our hierarchy, the Class Gastropoda is divided into three subclasses: Prosobranchia, Opisthobranchia, and Pulmonata. Respectively these are the marine snails with shells, the marine slugs, and land snails and slugs.
Opisthobranch molluscs are Gastropods that exhibit a major evolutionary trend--the reduction or loss of a shell. Within this subclass, are five major orders. One can see varying degrees of shell loss throughout these groups. The more primitive forms have well-developed shells; the majority have no shell at all as adults, but retain a larval shell that will be lost when the animal metamorphoses from juvenile to adult.
Order Cephalaspidea are known as the bubble shells; some of them have a very thin, bulbous shell, yet others have only a small internal remnant or no shell at all. In local San Diego waters, one of the more spectacular Cephalaspidea is Navanax inermis. It is the bane of all nudibranchs, because it is one of the few known predators on this group of slugs. It is a giant vacuum cleaner, sucking up slugs as it crawls over the sand or rocks. The animal is tubular, about 50-100 millimeters long, with a black body on which are festooned varying styles of yellow or brilliant blue speckling or streaks. Its head appears almost catlike. There are lateral cephalic extensions that resemble cat's ears. The dorsal surface of the head consists of a flap of skin (called a head shield, from which its scientific name Cephalaspidea is derived) that extends back up over its body. It is basically an adaptation for rooting through sandy substrates, like a Caterpillar Tractor pushing and groveling its way through the muck. Only Navanax is in search of food. It follows the slime trail left behind by the slug, sneaks up onto its prey, and then extrudes its buccal tube (sort of lips from inside its mouth, which form an enclosed suction tube) onto the body of the slug. Silently, quickly, the slug is sucked down the throat of Navanax. This story is getting even more complicated--because I need to describe why there are so few predators on the nudibranchs. But first I must continue the description of some of the major groups before delving into their remarkable biological features.
Most dorids eat sponges (invertebrate Phylum Porifera). Once again we see evolutionary chemical warfare. The sponge has evolved distasteful or poisonous chemicals, but the nudibranchs have evolved the ability to like such foul toxins or to use them in their own body for their defense. Having lost the shell, protective mechanisms among the nudibranchs tend to revolve around chemical defenses. They have often incorporated chemicals from their prey items into their own bodies. Dr. D. John Faulkner of Scripps Institution, has identified a lot of these chemicals; he has even been able to isolate the chemical from the sponge and the nudibranch, and show the slightly modified configurations that the nudibranch's metabolism has altered. Twenty years ago I named a species of nudibranch that occurs in the Gulf of California to honor my major professor at UC Berkeley. Named Hypselodoris ghiselini, it is dark indigo purple with numerous small yellow spots covering its body. Dr. Michael Ghiselin was proud of the honor, but swelled out even more when he learned that the nudibranch secretes an obnoxious chemical, appropriately named ghiselinin.
Some of the more common dorids along our coast include Aegires albopunctatus, a small white, bumpy textured creature with minute salt and pepper dots over its body. This animal eats calcareous sponges such as Leucilla nuttingi. The spicules defending these small, tubular sponges are to no avail against Aegires. Rostanga pulchra and Aldisa sanguinea are both small red nudibranchs, that lay red egg masses, while camouflaged on their red prey sponge Ophlitaspongia. They graze on the sponge, while at the same time being protected from other predators by substances obtained from their prey.
In the tropics, some of the most brightly colored species belong to the genera Mexichromis, Chromodoris, or Hypselodoris. Some of their color patterns are clearly aposematic in nature--that is they are brilliant and showy, advertising their presence while at the same time warning predators that they taste bad. Others are probably cryptic, causing the animals to blend in with the variegated underwater substrates. Pinks or reds are especially cryptic, since these colors tend not to be visible below 10 or more meters. These wavelengths of light cannot penetrate very deeply into the water; so even though the nudibranch's pigment may pink or red, these colors are not seen because the color is not given the proper wavelength to reflect.
Eolid nudibranchs typically have an elongate body shape; there are two cephalic tentacles protruding from the front end, always wiggling around testing the substrate ahead of the animal. Right up on top of the head is the pair of rhinophores, further sniffing and tasting the waters. Then most curiously, extending down the back of the animal, are bunches of thin, tubular processes called cerata. This is where biology becomes more unbelievable than the life forms bellying up to any Star Trek Interplanetary Bar! Cerata are branching extensions of the digestive gland of the slug. It is sort of like having your liver branch out through your fingers. The skin is quite transparent, so inside each ceras (the singular!) can often be seen the thin, brownish gut contents. These animals have no gills, so oxygen exchange occurs across the skin surface. An area of high energy need (the digestive system) is therefore exposed closely to the external oxygen-laden seawaters. Also the small size of these structures greatly increases the animal's surface area (relative to its total volume), maximizing the amount of oxygen that can be absorbed.
But we're not done yet with the cerata. Eolids usually eat members of the Phylum Cnidaria. This primitive group of multicellular creatures are often bags of mucus, although some can secrete a calcium carbonate cup in which they grow and are partly protected. Cnidaria include sea anemones, corals, alcyonarians (the soft corals and gorgonians), jellyfish, and Portuguese man-of-war. Our theme of evolutionary counter-warfare takes an astounding leap in the cerata of eolids.
Cnidaria have a defensive structure called the nematocyst. It is a stinging cell which fires into or around potential prey or predators. Offensive and defensive in nature, many humans have felt their sting. I will never forget the row of welts on my arm one summer while doing research at Puerto Peņasco, in the northern Gulf of California. The long tentacle of a Portuguese man-of-war had wrapped itself around my arm, causing quite a bit of burning pain.
Eolids eat the tentacles of cnidarians; they swallow the stinging cells intact, that is, without firing them. Then they pass the stinging cell to the tips of the cerata, where the nudibranch uses the cnidarian's weapon for its own protective arsenal. Many eolids (including our local Cuthona divae which can be found in the La Jolla Canyon on its cnidarian prey item Hydractinia) have a small white dot at the tip of each ceras. This is the small pouch or pocket within which the stinging cell is carried. Dr. Tom Thompson reported that swimmers in Australia were being badly stung. It turns out they were touching nudibranchs that had eaten the Portuguese man-of-war.
The brilliant purple, white and pink mottled colors of Bajaeolis bertschi seem exuberant when captured by close-up underwater photography. However, in situ, on their prey hydroid cnidarian, and with their delicate pink string of eggs, they can be nearly invisible. By contrast, California's Flabellina iodinea is always a show, with its rich purple body, deep red rhinophores, and bright orange cerata.
Copulation in opisthobranchs is usually reciprocal (both function simultaneously as male and female, giving and receiving sperm), although there are a few variations on the theme.
Sea hares engage in group sex, forming a copulating chain. The local Aplysia californica can be observed in such groups at the Cabrillo Monument tide pools, or in shallow subtidal zones frequented by scuba divers.
Nudibranchs copulate only in pairs (usually). They position themselves side-by-side so that the genitalia are lined up. They must face in opposite directions so that the right sides of their bodies (where the copulatory apparatus is located) will be next to each other. Copulation can be brief or last several hours. I once watched two dorids remain in copulo for over 5 days.
Eggs are laid in masses of hundreds, thousands, or even millions. Mucus sheaths around the egg capsules serve to keep them together and attach the spawn to the substrate. They vary in size, shape and color, but some are species or generic specific and can be identified by a practiced naturalist. In Hawaii, the 12 inch nudibranch Hexabranchus sanguineus lays a bright pink or deep red egg mass; its protective coloration (remember the wavelength not penetrating below about 10 m?) and chemicals in the ribbon protect against predation.
Eggs can be small or big, few or numerous. Since most have a planktonic larval stage during which the juvenile is freely floating about in the water, most tend to have numerous small eggs. The planktonic gauntlet is a horrifying realm of currents and filter feeders. So the survival of the species depends upon quadzillions of larvae thrown out into the vagaries of the oceans, with the "hope" that a few make it to adulthood, and reproduction of the species.
The radula is a tongue-like ribbon on which are positioned rows of teeth. There is great variability in the numbers of rows of teeth and of teeth per row, and in their shapes. This variability is directly correlated with prey specificity. Those animals that feed on a particular type of food have a certain number (or range of numbers) of teeth, and a specific shape. Matching radular tooth patterns with prey items is the study of feeding functional morphology. Just like bats, cats and rats have different shaped teeth because they feed on different food items, so do that various groups of opisthobranchs. Sponge-feeding dorids and notaspideans have many rows of many scythe-like teeth. They rasp and scrape over the surface of a sponge, like so many garden rakes all lined up one behind the other. The teeth of eolids consist of a few rows, with only one or three teeth in a row. But they have very well developed jaws. So the slug holds onto the cnidarian prey with the jaws, and then plucks the tentacle flesh with its meat-hook-shaped teeth.
If you have to crawl on your stomach, and lack arms and legs, a good rasping tongue is a great adaptation to get food.