Sponges Get Respect

The next time you luxuriate in a tub with a piece of natural bath sponge resting on the soap dish nearby, consider this fact: One recently discovered species of sponge is a carnivore--adept at attacking, engulfing, killing and consuming flesh. Scientists discovered the species in 1994 in a Mediterranean marine cave about 12 miles from Marseille, France.

Asbestopluma hypogea has elongated filaments extending from a white oval body. Minute spikes of silica, called spicules, jut out from the filaments like tiny shards of glass. "The spicules act as hooks, so that small crustaceans are trapped as if the surface were Velcro," says marine biologist Jean Vacelet of the Marseille Oceanographic Center. Also, the sponge's cells can move around. And they do. "The cells of the sponge migrate as soon as the prey is trapped," Vacelet says. "After 24 hours, the prey is completely covered by sponge cells." The cells grab bits of meat, absorb them into their cytoplasm and move away to start digesting. The creature has no brain, no heart, no stomach, no muscles-- yet it is a voracious killer all the same. "The sponge is like a giant amoeba," adds Vacelet. But before you swear off scuba diving in the Mediterranean forever, be advised that he uses the word "giant" in relative terms: Asbestopluma is barely larger than your thumbnail. Scientists since Aristotle have wondered whether sponges were plants or animals. Indeed, if you don't look closely, you won't notice them doing much. "Sponges are the blobs of the animal world," says invertebrate biologist Sally Leys of the University of Queensland in Brisbane, Australia. "Every other animal has a nervous system." Not to mention muscles and a digestive tract. Even flatworms, clams and corals have these features, however rudimentary. Sponges, the oldest multi-celled animals on Earth, do not. Yet recent findings confirm not only that sponges are most assuredly animals but that they are quite accomplished ones at that. Though they occupy the bottom rung of the animal ladder, they can perform feats that would be amazing in higher animals, and scientists are now trying to understand how they do so. Like the "Blob" of the 1958 Steve McQueen movie, sponges can regenerate from small bits of tissue, even after being squeezed through a mesh. They can outcompete and outlive competitors among other inhabitants of rocky sea floors. They can brush off injury, send signals, shape-shift and produce the building blocks of possible anti-cancer drugs. Says Leys' Brisbane colleague and fellow sponge expert Mary Garson: "I think sponges deserve a lot more respect than they've gotten in the past." Close to 10,000 spe-cies of sponge populate the underwater world--in salt water and fresh water, in the tropics and off Antarctica, in the shallows of coral reefs and in trenches three miles down--though they are most plentiful and most colorful in shallows. Some are hollow globular structures big enough for a diver to hide in. Others are bouquets of hollow tubes. Some are mere incrustations on rocks, shells or blades of sea grass. A sponge's anatomy is unlike that of any other creature. Most sponges are covered by a slimy, leathery skin dotted with small pores that let in seawater--hence the phylum's scientific name, Porifera. The soft brown sponge we know from well-appointed kitchens and bathrooms is actually a chunk of collagenous skeleton, bereft of other tissue. (Don't be fooled. Cheap supermarket sponges are usually made of cellulose derived from plants.) Only certain kinds of sponge have skeletons this user-friendly. Most are embedded with spicules, which range from tiny chalky bars to glasslike needles and often give a dried skeleton the consistency of wall insulation. A distinguishing feature of the sponge's anatomy is its lack of internal organs. Everything that happens in a sponge--eating, breathing, moving, reproducing--happens at the cellular level. Sponges were once thought to be colonies of independent one-celled animals rather than single animals of many pieces, but no longer. "A sponge is an individual," says Leys. "It's made up of cells that are not yet really well understood." Like clams and other sedentary sea animals, most sponges are filter feeders. Their bodies are usually riddled with chambers and canals lined with cells that have tail-like structures called flagella. "These things whip their flagella and develop a low pressure, which pulls water through," says Leys. These cells also do the eating, a filter mesh on each sifting out tiny food particles. Other, free-ranging cells spread nutrients throughout the sponge, functioning something like a bloodstream. For that matter, the canal system itself acts in some ways like a circulatory system. Various cells also absorb oxygen from the passing stream. And still other specialized cells do jobs that might be performed by muscles in other animals, contracting or opening up to control the sizes of the outer pores and of the typical sponge's large central cavity. As a pump, a sponge is an amazingly efficient bit of bioengineering. In 24 hours, a large wool sponge can pump several hundred gallons of water through its filtration system. The sponge ejects the processed water, now containing waste, from its central cavity. Though sponges lack reproductive organs, they create offspring both asexually and sexually. In the former case, an extremity breaks off and, after adhering to a hard surface, keeps growing into a fully formed sponge. In the latter case, certain cells apparently transform into egg- or sperm-producing units. Precisely how remains a bit murky. A single sponge may produce both sperm and eggs, though probably not at the same time. When released, sperm are carried by water currents to an egg- containing sponge. Larvae later break loose from the parent, paddle to a nearby surface, propelled by hairlike cilia, and settle down to grow. And in some cases grow and grow: Some elephant-ear sponges are large enough to fill an average bedroom. A sponge's growth occurs at a leisurely pace, so big sponges are old sponges. They are, in fact, among the longest- lived of all animals. "In the genus I'm looking at," says Leys, referring to a stovepipe-shaped sponge called Rhabdocalyptus found off the coast of Vancouver Island, "the big guys might be 200 years old, or maybe 300, 400 or 500. They're fairly monstrous; they can be as big as you or I." Sponges are old in evolutionary terms as well. When they first appeared, the only other multi-celled life-forms were plants. "Of the sponges living today, the most ancient that we know is probably Geodia, which is approximately 700 million years old," says molecular biologist Werner Muller of the University of Mainz in Germany. Muller uses genetic analysis of modern sponges to trace their likely lineage. He calls Geodia--a pinkish, cake-shaped creature with spicules that fan out like rudimentary armor--"a kind of living fossil." Despite their antiquity and their simple body plan, sponges have many of the makings of higher animals. "We discovered in the past four years that sponges contain all the molecules typical of later multi-celled animals," explains Muller. Also, like higher animals, sponges can pick up and move. Using time-lapse photography, biologist Calhoun Bond at Greensboro College in North Carolina has discovered so far that 10 species have the ability to crawl. "Many, many cells will crawl together so that the whole margin of the sponge is moving," he says. Top speed for a sponge? A few millimeters a day. As it creeps, the creature rearranges its shape and internal structure, cell by cell. Spongiologists, being patient types, had noticed this cellular motion before, but most had assumed the sponge was simply growing new tissue, perhaps forming asexual buds. "There was still resistance to the idea of sponge locomotion until people saw my films, which just blew them away," Bond says. "Of course, I did speed it up 1,200 times." Locomotion offers several advantages to a sponge. It helps it compete with corals and other encrusting animals. "If a coral wants to move into an open space, it has to grow new polyps; all a sponge has to do is ooze its way over," says Bond. Another handy use of the crawling- cell trick is to repair injuries, he adds: "Wounded sponges just rearrange themselves. Sheets of cells moving together can close a wound in a matter of days." This is also why sponge tissue can survive being forced through silk or cheesecloth. Cells that remain intact will move around independently until they find and join up with other, similar cells, forming pancakelike clusters. Some then continue to grow. A sponge's predators include certain fish and shellfish but not as many as you might expect, considering what a tempting meal a soft, fleshy sponge appears to be. In fact, biologists have long observed fish and crabs avoiding certain sponges. "If you take a little piece of living sponge and pop it into an aquarium in which you've got fish, the fish tend to avoid it," says University of Queensland chemist Garson, who specializes in the biochemistry of Great Barrier Reef sponges. "If you put it in a tank with a sea slug, the sea slug may try to crawl out of the tank." The reason is that sponges, unable to hide from predators, keep them at bay by exuding noxious chemicals instead. The secretions may also help the sponge in turf battles with competitors and aid in keeping its own body free of harmful parasites. Medical researchers are now trying to understand the workings of these chemicals. Cell-killing secretions from sponges and other sedentary sea creatures, notably sea squirts and sea fans, are already being tested as anti-cancer and anti-viral treatments. They may soon be used, too, to keep protein deposits from building up on implants like pacemakers and catheters. "These molecules that sponges make are highly complex, both in their size and in their three- dimensional shape," says Garson. Not all marine animals find sponges to be inhospitable neighbors. The maze of small channels honeycombing a sponge sometimes resembles teeming aquaria. Tiny shrimp, polyps, shellfish and fish may pass their entire lives inside a sponge, feeding from the endless water flow while concealed from predators, apparently without harming their host. In one unusual arrangement, the sponge crab tears off a bit of sponge tissue and holds it against its hairy back until the remnant becomes fixed in place. In time, it grows larger and larger. Other creatures move in. For the crab, the advantage of hauling this ever-growing bio-appendage around is concealment, while the sponge gets a free ride and new feeding grounds. Sponges, passive though they seem from a distance, are proving capable not only of moving (albeit slowly) and eating meat (in very small bites, to be sure) but also of sending nervelike signals. In the mid-1970s divers noticed that when they swam close to glass sponges the sponges stopped pumping. "They would completely shut down," says Sally Leys. "But how could all the flagellated chambers stop beating at the same time? This seemed highly unusual, because sponges aren't supposed to have any ability to behave. They have no nerves." In addition to the huge Rhabdocalyptus, Leys has been studying the elegant, billowy cloud sponge, also found off the coast of Vancouver Island. Both are glass sponges, which often are striking in appearance, with latticework cylinders or funnels. Unlike other sponges, they consist mostly of a single enormous cell with countless nuclei that is stretched over the sponge's skeleton. "It's like an amoeba that someone pulled out like chewing gum in lots of different directions," Leys says. No cell membranes divide the interior to act as barriers. "It's just one continuous cytoplasm from one end of the sponge to the other." Such cells are not unusual in the animal world--the axon of the giant squid is a well-studied example. So are our own muscle cells. What is unusual is for an entire adult animal to be virtually a single cell with many nuclei. Leys and University of Victoria biologist George Mackie have found evidence of electrical signals moving along the overall cell membrane. The signals likely explain the way a disturbance at one end of a glass sponge--silt stirred up by a diver, for example--can cause a fast shutdown throughout its body. "Because it's all one cell, when you touch one end, you're touching the whole animal," Leys says. "These electrical signals are the sponge solution to nerves, because sponges evolved before animals had nervous systems." Or did they? Molecular biologist Muller recently found genes and proteins in sponges that are very similar to parts of mammals' nervous systems--more evidence that sponges likely will confound researchers for a long time to come.