Sponges: The World’s Simplest Multi-Cellular Creatures

This article is the second in a series that elaborates upon the marine animals you and your diving pals are likely to encounter when...

This article is the second in a series that elaborates upon the marine animals you and your diving pals are likely to encounter when you dive. The articles are being presented in order according to the taxonomic system used by scientists when they describe and classify plants and animals. An overview of the articles and the taxonomic system was provided in the February 2001 issue.

Early in your pursuit of scuba diving, you are likely to hear about Little Cayman’s famous Bloody Bay Wall. Descend down this wall, and you will be treated to a mosaic of brilliant hues of reds, yellows, oranges, greens, purples and browns. There is little question that one of the feature attractions of the underwater world is the amazing color we enjoy. So often the colors are associated with dazzling fishes that dart about the reef, but on this widely acclaimed wall, the colors are provided mostly by a stunning diversity of sponges.

Occurring in a wide variety of rich colors and captivating shapes, sponges are a key component of many of the most awe-inspiring seascapes in many parts of the world. This is especially true in the popular waters of the Caribbean, for nowhere in the world are sponges more spectacular or prominent. It is somewhat ironic to think that in a world of whales, sharks, sea lions, manta rays, turtles and more, sponges, the world’s simplest multicellular animals, can be such an attraction, but there is little question that they are.

Worldwide, there are approximately 5,000 species of currently living sponges, and there are another 5,000 or so known from fossil records. All sponges are described in a single phylum Porifera, and they are the only members of this phylum. The name Porifera (pore bearer) is derived from the Latin words porus, meaning “pore,” and ferro, meaning “to bear.” Certainly, the name is well-applied as it relates to the system of pores that perforate the bodies of all sponges. The passageways and tunnels of pores are quite obvious in sponges during their adult stage.

Scientists tell us that sponges are the most primitive of all multicellular animals. The body of a sponge essentially consists of masses

of cells, among which there is only a loose association and very little coordination. The cells that make up the organism are embedded in a single gelatinous mass that we refer to as a sponge. No matter how big, how small, how young or how long-lived, sponges lack specialized tissues, organs and muscles. However, there is just enough association and coordination between cells that specialists accept that sponges bridge the gap between colonies of single-celled animals and truly multicellular organisms, even though the contribution by individual cells to an entire sponge is thought to be a matter of coincidence. Indeed, it is rather astonishing that such simple animals can be so varied, so beautiful and so well-adapted for living in the marine environment.

The bodies of most sponges are quite flexible and resilient. As adults, they are usually attached to the reef, but in some instances they are also attached to the shells of decorator crabs, hermit crabs and other animals. Scientists believe that no other organisms have evolved as an offshoot of sponges, and thus suspect that sponges are a dead end on the evolutionary road. Considering the fact that sponges are believed to have occurred in the seas of planet Earth for in excess of 550 million years, this dead-end quality might seem surprising. But students of evolutionary studies are quick to point out that there are plenty of dead ends along the path of evolution.

Very small silica-based needles called spicules and stringy protein-based fibers known as spongin help to create an internal skeleton in sponges. The skeleton helps sponges develop and maintain their shape, as well as adding strength to the loosely associated mass of cells. Some species of sponges have both spicules and spongin, while others possess only one or the other.

If you examine a typical upright sponge, you will quickly notice that there is a single large opening. This opening is referred to as the osculum. The bodies of many encrusting species possess numerous oscula.

Sponges take on a variety of shapes, sizes and colors. Various species are generally shaped like barrels, baskets, vases, tubes, ropes, bowls or cups, or they are flattened and encrusted. Encrusting sponges often occur wedged between coral heads, below coral plates, and at or near the base of colonies of hard corals. Specialists believe that in some instances, encrusting sponges protect some species of corals by preventing boring organisms, such as worms and mollusks, from digging into and weakening the corals.

Other species of sponges, such as a variety of tube, vase, bowl, barrel, finger and rope sponges, have more consistent and easily identifiable shapes than those observed in encrusting sponges. Barrel sponges and tube sponges stand out prominently on many Caribbean reefs, while rope sponges adorn many walls. A variety of species of barrel sponges are among the largest of all sponges. They commonly reach heights of 5 feet (2 m) or taller. Larger specimens typically occur in deeper, quieter waters along the forward slope of a reef system, and many specimens are thought to live as long as several hundred years.

The common names given to sponges generally make some kind of reference to their shape or color, or some combination of both. As examples, some more prominent Caribbean species are commonly known to sport divers as red rope sponge, green rope sponge, red cup sponge, green finger sponge, yellow tube sponge, purple vase sponge, basket sponge, cup sponge and elephant ear sponge. In the temperate waters of California, commonly encountered sponges are known as urn sponges, moon sponges, crumb-of-bread sponges, orange puffball sponges and gray puffball sponges.

A word to the wise and the practical: Expecting to make positive identification of various species, especially of encrusting sponges, by looking for specific shapes and colors can prove tricky or impossible. This is because many species tend to take on different shapes depending upon the shape of whatever structure they are adhering to and the nature of nearby organisms they are competing with for space on the reef.

In addition, coloration can appear to vary dramatically according to depth. As a rule, the use of common names allows for casual conversation among divers without significant misunderstandings, but don’t bet too much when attempting to make positive identifications. Both color and shape can vary considerably between different specimens that prove to be the same species when examined under a microscope.

Without the use of a dive light during the day or a strobe when creating photographs, the colors of many sponges go unnoticed at depth. But a splash of light from an artificial light source can add striking colors to your dives and underwater images.

Interestingly, many specimens of the beautiful and relatively common Caribbean species known as yellow tube sponges appear to be an exception to this comment about the lack of color at depth, as this species often retains some degree of bright coloration due to fluorescent pigmentation in the cells. The pigments alter the wavelength of the sunlight that strikes them, and as a result these sponges often look yellow, even at depths when a large percentage of the yellow part of the spectrum has been filtered out by sea water. Yellow tube sponges commonly occur at medium depths, along the tops of walls, and along some drop-offs in some Caribbean waters.

The Role of Sponges in Reef Ecosystems

Sponges serve a variety of vital functions in reef ecosystems. With the exception of recently discovered predacious species, sponges feed by filtering tiny diatoms, bacteria, protozoans and other microscopically small organisms out of the water column. Studies have demonstrated that many sponges can easily filter 50 to 100 gallons of sea water in a single day. By filtering the water, sponges add to water clarity, especially in areas where sponges are prolific.

Studies have also shown that a high percentage of the food consumed by many sponges is so small that it can only be seen with the assistance of a microscope, and that only a small percentage of the food sources of sponges is preyed upon by other organisms. The bottom line here is that sponges appear to have discovered an environmental niche in which they have very little competition for food. The noncompetitive nature of this niche is of great value to sponges, because they are unable to move around in the pursuit of food.

Sponges feed by creating a current of water that flows through a series of tiny pores and canals. The current is generated by the seemingly frantic and ever-constant beating of numerous small, hairlike flagella that line the walls of the pores and canals. Despite the tiny nature of the flagella and the lack of coordination or synchrony in their efforts, in some species the incoming current can be detected by sensitive instruments from a distance of more than 3 feet (1 m) from the sponge.

The incoming current passes through the outside wall of the sponge as food (consisting primarily of plankton and organic debris) and oxygen are extracted. Once filtered, the water continues through another series of passageways. In some species, such as barrel, cup, tube and vase sponges, these passageways eventually merge to create a single, large excurrent opening known as an osculum, through which water filled with waste is eliminated. In other species, especially in many encrusting sponges, a series of excurrent openings (oscula) are present. The oscula are large enough to be seen with the naked eye. In some species, the oscula are distributed in a regular pattern, while in others they are irregular.

Some sponges benefit from the presence of small symbiotic algae known as zooxanthellae. The algae live in the tissues of the sponge, where they conduct photosynthesis, a process that helps provide oxygen and food for the sponges.

Sponges are also involved in a number of additional types of relationships with other organisms in reef communities. Many sponges provide superb hiding places and living quarters for tunicates, a wide variety of invertebrates, such as nudibranchs, shrimps, brittle stars, crabs, juvenile lobsters and more, as well as a number of species of small fishes, such as gobies and blennies.

If you slow down and take the time to closely examine sponges you encounter, you will often be rewarded with a good look at some creature that is resting on or moving slowly across the surface of a sponge. Some animals that live in or on sponges stand out prominently, while others are well-camouflaged. During daylight hours, if you look carefully at the base of a sponge or between the lobes of tube and branched sponges, you will often discover creatures that openly roam across the surface of the sponge and the nearby reef at night. A variety of brittle stars, hermit crabs and shrimps are especially common.

Some fishes, including a variety of angelfishes and filefishes, along with myriad invertebrates, such as sea slugs, nudibranchs, sea stars and bristle worms, feed on sponges. In most cases the predators are species-specific, meaning they are not generalists that feed on a wide range of sponges. However, most reef residents consider sponges to be repugnant or inedible. This is apparently because the spicules, the silica or calcium-based needles in the skeleton that help sponges maintain their amorphous shapes, taste bad to many organisms. The fact that sponges are avoided as a food source is a vitally important adaptation, since they have no other means of defense and cannot flee.

Some species of sponges are parasitic, boring their way into a variety of corals. This action weakens corals and can eventually lead to the demise of large coral heads. Other sponges help to protect corals by creating a protective encrusting barrier on the edge of coral heads that prevents boring sponges from working their way into the corals.

In Caribbean waters, a variety of gobies and blennies can often be seen swimming along the edges of tube and barrel sponges. From these vantage points, the fishes sometimes advertise their services as cleaners to others that need to be rid of ectoparasites. When frightened, the cleaners are often quick to seek safety inside the sponge. Other fishes, such as cardinalfishes, royal grammas and blackcap basslets, commonly hide in vase sponges.

Small animals known as zoanthids, which are described in the phylum Cnidaria, commonly occur on the surface of some sponges. For many years the zoanthids were thought to parasitize the tissue of the sponges. However, new findings have revealed the true nature of this relationship. Rather than being parasitic, the presence of the zoanthids might actually be beneficial to the sponges by deterring fishes from preying upon them. But some fishes, such as the Caribbean rock beauty, a member of the angelfish family, are not repelled by the presence of zoanthids.

How Sponges Reproduce

Like many invertebrates with little or no mobility, sponges are able to reproduce both asexually and sexually. Asexual reproduction is achieved by budding or breaking off small pieces capable of developing into complete sponges. The buds break away from the parent sponge and drift away in the current. Exactly where the buds settle is a matter of chance, but if bottom conditions are favorable, the bud can develop into a healthy, whole sponge.

Asexual reproduction results in genetic clones. Sexual reproduction produces a new organism via the genetic recombination of sperm and eggs from members of the same species. While cloning requires less energy expenditure, it produces offspring that are exactly alike, and a single environmental problem or disease can more easily and quickly wipe out an entire species. Genetic recombination takes more effort but provides a better chance that some offspring will survive if conditions change.

In some species of sponges, the sexes are separate, but many sponges are hermaphrodites, meaning that the same sponge has both male and female reproductive capabilities. Those sponges tend to produce only sperm or eggs, not both, during a given spawning event. Heavily preyed upon, sponge larvae are free-swimming members of the community of plankton. Those larvae that manage to survive long enough eventually settle to the sea floor, where they attach to solid substrate.

The majority of sponges reproduce through a process defined as “internal fertilization,” although the use of this term can prove confusing. The male does not release sperm inside the female as is the case with internal fertilization in sharks, rays, skates and marine mammals, but instead dense, milky clouds of sperm are released into the water column through the oscula of the donor sponge. Divers commonly refer to the donor sponges as “smoking” sponges due to the billowing clouds of sperm they release. If mating is to be successful, the sperm must be taken in by a recipient sponge (thus the term “internal fertilization”), where they are united with eggs.

Sometimes many sponges spawn simultaneously, causing water visibility to be greatly reduced by the dense concentration of released sperm.

Once I watched a group of underwater photographers swim right past a spawning sponge without giving it a second glance. Having seen a mass spawning before, I recognized what was happening and captured the event on film. The lasting lesson for me was that not all marine phenomena jump up, grab you and demand that you look, so the more you know about the marine environment, the better able you are to recognize and enjoy certain phenomena when they present themselves. This is as true for the simplest of multicellular animals, the sponges, as it is for the most complex.

Sponges Possess Remarkable Regenerative Powers

While scientists tell us that sponges are simple animals, they do possess some astonishing regenerative powers. This regenerative capability plays an important role in some species during asexual reproduction, as some sponges break off pieces of themselves to form new sponges. Equally as remarkable, experiments with one species have demonstrated that if an adult is broken up into very small pieces by being forced through a sieve made of a stretched silk cloth, soon after settling the tiny parts will reorganize themselves into a near-replica of the original sponge. This incredible feat is accomplished by an animal that lacks specialized tissues, and may cause you to reconsider the meaning of the term “simple animal.”

Look, Don’t Touch

While we generally admire sponges and seek them out, it is important to be aware that some sponges are toxic to humans. Merely rubbing or brushing against some more potent species can prove to be a painful mishap. The intensity of reactions varies, but severe rashes, burning sensations and itching are common.

By Marty Snyderman