Sponges: A Bunch of Holes Held Together by Some Cells by Ronald L. Shimek, Ph.D.

Although a few organisms in a couple of other small groups are good claimants to the title, sponges are widely regarded, and justifiably so, as the simplest animals (Barnes and Harrison, 1991). While it is tempting to dismiss such simple organisms as some sort of failure, such a dismissal would be based more on arrogance than on fact. In one way or another, within their particular habitat, all organisms have to be able to do the same kinds of tasks or overcome the same sorts of specific problems. Learning the different ways that various organisms accomplish these same tasks is, in a very real sense, the science of biology. While sponges are very unlike, in just about all properties, the readers of this column, both the readers and any sponges in their aquaria must perform the same basic tasks of life.

The ways that these problems are solved varies from organism to organism, and the sum of such "solutions" is unique to each organism. Those solutions define and describe the organisms, but more importantly for hobbyists, they provide a blueprint for the husbandry of those animals.

By using "molecular clocks," it has been possible to roughly deduce when the first organisms that we might call animals appeared in the evolutionary history of life on Earth. This rather momentous event probably occurred sometime between 800 and 1000 million years ago. These were small organisms, and left no fossils, so any discussion of their morphology is largely speculation. However, one thing is clear: they were unable to make their own food by photosynthesis. This lack of photosynthesis is the primary hallmark of being an animal. To obtain nutrition the first animals had to eat other organisms, such as bacteria, or to eat the byproducts of other organisms, such as sugars, mucus or their corpses.

Other organisms predated animals, of course, and the most highly developed of these were likely the animal-like protozoans. Although the name "protozoa" conjures up images of primitive "animal" life, in fact, these organisms, such as amoebas, ciliates and flagellates, represent a diverse array of decidedly non-animal life. Presently, animals are defined as multicellular; that is to say, built of many smaller component parts called "cells." Protozoans lack cells. Although such organisms may be thought of as having but one cell, it probably is better to consider them as lacking cells altogether. Many protozoans, for example, have multiple copies of internal cellular structures; they just seem to lack the internal division into cells. Good aquarium examples of such intracellular structural multiplicity may be found in Caulerpa and other similar algae. Each individual algal "plant" contains several thousand nuclei, but these nuclei are not found within individual cells. Consequently, each individual of Caulerpa may be considered to be either a single-celled organism with many nuclei, or a multinucleate organism without cells.

Some protozoans may be "colonial" organisms, having several almost independent "individuals" attached to a single stalk or growing from a common base. Colonial organisms are particularly common within the two groups of protozoans referred to as the ciliates and the flagellates. It is within the flagellates that are found the organisms that appear to be most similar to what is likely the ancestor of sponges and, by inference, the ancestor of all animals. These organisms are called choanoflagellates (2, 3, 4, 5). Colonial choanoflagellates are clusters of small, rather spherical cells, each of which has a structure resembling the top flaring part of a funnel on its surface. In the center of this funnel is a single microscopic hair-like flagellum. As the flagellum beats it creates minute water currents that push water up its length, out away from the cellular surface. This movement, in turn, draws water from the sides and directs it in toward the base of the flagellum. To get there, the water has to pass through gaps in the wall of the "funnel," which, in reality, is comprised of tiny finger-like projections referred to as microvilli. The microvilli trap small particulate materials, mostly bacteria, and transport them to the cell's surface where they are ingested. The shape and function of choanoflagellate cells is virtually identical to that of choanocytes (2, 3, 4), the most characteristic type of sponge cells, whose name literally means "funnel cells." Choanocytes were first seen in sponges in the early nineteenth century, and were considered to be unique to, and absolutely characteristic of, sponges. If you found a choanocyte in some unknown animal, it had to be a sponge, as no other groups had them. Then, starting in the late 1970s, cells of similar construction were found in many other animal groups, including vertebrates. This presence of choanocyte-like cells throughout much of the animal kingdom is considered to be one line of evidence for the common descent of all other animals from an ancestor containing these cells.

Figure 1. A diagrammatic view of a choanocyte. For images
of actual choanocytes, follow the links in the text.

Sponges don't have the simplest type of structure found in animals, but they are close to that limit, and their structure is neither difficult to discern nor to understand (Harrison and De Vos, 1991). These are animals that get their nutrition by filtering water through themselves. If that fact is kept in mind, their body form becomes quite explicable and reasonable. The simplest sponges are tubular animals that pull water through the sides of their body into the center of the tube and then blow it out both ends of the tube. Larger sponges have much more complicated water filtration pathways, but otherwise the sponges' body structure is relatively simple.

There really is nothing like a sponge throughout the rest of the animal kingdom. Most sponges, quite literally, suck water in through tiny holes covering their body's entire surface, filter it clean of all acceptable foods, and flush it out through large drainage pipes. Depending on the type of sponge, the tiny holes that allow water to pass internally may be so small that they pass through a single cell, or they may be somewhat larger, but still tiny, openings to small tubes. These passageways are pipes or conduits constructed of cells. The water is pulled into the sponge by way of the surface pores and is moved along inside the water channels by the beating action of the flagellated choanocytes lining "filtering chambers." Downstream of the filtering chambers the water channels get progressively larger until the water leaves the sponge through a large aperture, called an osculum. A large sponge may have millions of these small entry pores, each of which is called an ostium, and one to several hundred excurrent apertures.

When water is passed through the filtering chambers, it moves through the fine microvillar comb surrounding the bases of the choanocyte flagella and particulate materials, bacteria or phytoplankton get removed from the water and eaten by the choanocytes. When they have eaten enough food, the choanocytes are able to perform one of the better metamorphic feats in the animal kingdom. They may "reel in" their flagellum and turn into an amoeboid cell, called an archeocyte. These cells can wander all over and through the sponge. Archeocytes are totipotent and may perform any task within the sponge with the exception of becoming a gamete. Many different functional cell types have been described within sponges; these have been called choanocytes (filtering cells), myocytes (contractile cells), porocytes (cells having a pore in them) and pinacocytes (surface lining cell), to name but a few. Such names are illusory, however; the cells are named by their function at the moment of observation. In reality, all of them can revert to being an archeocyte and wander away to become something else.

Figure 2. A diagrammatic representation of a simple sponge showing the basic
features of sponge anatomy.

Given the mobility of the cells that constitute the living "stuff" of sponges, it is not surprising that sponges lack tissues. And, lacking tissues, they have neither organs nor organ systems. Most sponges can be thought of as a group of cells rather loosely working together. Unlike all other animals, in the sponges, the constituent cells are only casually connected to each other; in fact, with vigorous action some sponges can literally be shaken apart resulting in a slushy mixture of living cells and mesogleal components. "Sponge smoothie, anyone?" This can occur because sponges lack what biologists call "tight junctions" between the cells. Tight junctions are really the "glue" of animal life; they are fine molecular threads connecting the cell membranes of adjacent cells. In all animals except sponges, the cells ALWAYS are securely glued together by tight junctions. No other type of animal can simply be shaken apart; its cells would tear and be destroyed before they would separate.

Sponge cells are typically arranged in outer and inner layers living over, and lining, a middle skeletal layer called the mesoglea. The term mesoglea literally means "middle glue" and that name for the middle region of sponge's body wall approximates its function. The mesoglea is not sticky like any adhesive, but rather consists of the sponge's skeletal material, mostly spicules or protein, and is largely formed by the secretions of the cells surrounding it as well as of some cells that wander around within it. While not without living cells, the mesoglea doesn't have many cells in it relative to the number of cells in the sponge's inner and outer surface layers. The reproductive cells also generally reside within the mesoglea; presumably they get more protection in that location than they would on the body's surface.

One of the classical demonstrations of this "fragility" of sponge structure can be done by aquarists. You will need:

Fill the blender with about one cup (250 ml) of seawater. Add two small (1 cm x 1 cm x 1 cm or smaller) pieces of the two sponges. Turn the blender on and let it rip until you have a "sponge shake." Filter the mess through the coarse screen or cheesecloth to remove all visible chunks. Pour the remaining "juice and goodies" into your clean glass container. Cover the container loosely, then put it on a shelf and don't disturb it. Mark the water level with a pen and make sure that evaporation is replaced with fresh distilled or RO/DI water daily. Maintain it at tank temperature, if possible; if not, any temperature in the middle to upper 70°F range will likely work. After a few days small globs the same color as the original sponges may become visible. Some of these will move, amoeba-fashion, to find and fuse with other globs of their own color. These small masses are tiny sponges reconstituting themselves from the surviving cells of the blenderized sponges. Interestingly enough, though, cells from one sponge will be able to "recognize" other cells from their same sponge and will not fuse with cells from the other sponge.

Given the ease with which most sponges can recover from injury, it is not surprising that these animals are masters of asexual reproduction. Demosponges, particularly, grow well from fragments and in some areas clonal sponge populations contain many separate individuals resulting from widespread asexual reproduction due to fragmentation (Hartman and Reiswig, 1973; Reiswig, 1983). Additionally, a few marine sponges and many freshwater sponges produce asexual "resting bodies." These structures contain numerous archeocytes in a state of dormancy, and often they are surrounded by a resistant outer shell or coating. When the conditions change or improve, the coat ruptures and the cells within can differentiate into a small sponge.

Sponges also reproduce sexually. In general, both eggs and spermatozoa are produced in the mesoglea. During spawning, the sperm generally are released as a dense cloud of "milky" water blown out of the osculum. Ova generally are retained within the mesoglea. In many cases, fertilization is rather complicated. The sperm is caught by a choanocyte of a female sponge and ingested. The sperm's nucleus, containing its genetic material, is encased in a membrane, and the choanocyte changes into a "carrier cell" which then takes the form of an amoeba and moves though the mesoglea to find an egg and deliver the sperm's nucleus to it. Embryonic development occurs within the mesoglea until a relatively large flagellated larva is formed. Given the diversity of form within the various sponge groups, it is not surprising that several different types of sponge larvae have been described. The larva escapes or is released from the parent and swims away. Sponge larvae typically do not feed, but swim around for a while until they choose a site upon which to settle, fasten to the bottom and metamorphose.

Except for slight contractions of tubular osculae, the movement that occurs as a larva is the sum total of movement that most sponges are capable of, particularly sponges that reach relatively large sizes. Some smaller sponges, however, including several species of Tethya that are commonly found in aquaria, are quite mobile and capable of both changing their shape and moving at rates of several centimeters per day across acceptable substrates. (See movies of sponge movement.) The mechanism by which such movement occurs is not yet completely understood, but it appears to be the result of a sort of amoeboid movement by the sponge's basal cells in contact with the substrate.

One of the more interesting things that has happened over the last ten years or so has been a change in the "appreciation" of sponges. We used to think of all possible spongy animals as being both similar in structure and closely-related; such a view is reflected in their treatment in older invertebrate zoology textbooks, such as those by Kozloff (1990) and Ruppert and Barnes (1994). Genetic investigation has changed that viewpoint rather significantly, and has discovered that modern sponges are probably three distinct groups arising at different times and relatively distantly related to one another. In other words, the concept of the phylum "Porifera" as a taxonomic unit containing all the sponges, and descended from a common ancestor, is no longer supported. It now appears that the three groups previously considered the major taxonomic subdivisions or classes of the phylum Porifera should each be considered to be distinct, and a group unto itself (Halanych, 2004). As the groups are each distinct and are each descended from a common ancestor, there should be no problem assigning each to its own phylum. Presently the discipline of animal taxonomy, however, is undergoing considerable flux, and the whole concept of a phylum, or something like it, is changing. For the moment, it seems prudent to say that three distinct living types of animals may be called sponges and that at a basic level they may be easily distinguished by the type of skeleton they possess (Ruppert et al, 2003). In addition to variations in skeletal composition, numerous other characteristics also separate these groups. Those characteristics' differences often are obscure and require microscopic examination, so it seems easiest for this discussion to define the types of sponges by their skeletons. These groups are:

The hexactinellids are the most unusual sponges in a lot of ways. Except for their gametes, they lack cells. Instead of cells, their skeleton of fused spicules is covered by a thin protoplasmic mass that is elaborated into cellular-like structures, but few, if any, cell membranes separate or delineate these structures. Such an acellular multinucleate mass is termed a "syncytium." Syncytia are common among smaller invertebrates; for example, most tissues or organs in rotifers or roundworms don't have cells, either. However, in these groups, these syncytia often are considered to be adaptations for their small size, the idea being that in an animal the size of a rotifer, cell membranes literally take up too much space. Hexactinellid sponges, however, are often large; presumably, they have developed syncytia for other reasons. The mesogleal layer is reduced to a skeleton of fused spicules and free spicules imbedded in the syncytial mass. The skeleton may be composed of spicules of various shapes, but spicules with six rays (called hexacts) predominate. Typically, many such spicules are fused together to form a brittle and inflexible skeleton. Relatively few animals have the capability to metabolize and secrete silicon dioxide, but sponges do and the hexactinellids are masters of this art. The fusion of different spicules, which is done inside the syncytial mass, results in a relatively strong, solid structural mass. Many of the hexactinellid sponges are asymmetrical, but others may be cylindrical with the spicules arranged in beautiful geometric patterns.

Glass sponges are typically found in deep waters and are, in some instances, characteristic of the deep seas. Many of them tend to be relatively large animals; individual sponges are commonly more than a meter (3.3 ft) in height or diameter. They are immobile as adults; their only mobile form is their larval form. They have no sensory organs or structures; however, their entire syncytial surface probably is sensitive to various chemical and tactile stimuli.

Little is known about the natural history and ecological relationships of most hexactinellids, but some work has recently been done, mostly on temperate forms (Leys and Lauzon, 1998). They tend to be long-lived, slow growing animals, and the ages of some have been determined to be in excess of 200 years. They are relatively uncommon on coral reefs, and it is unlikely that any reef aquarist would encounter one. Many of the basic questions about them remain to be answered; for example, in many cases we do not know what their predators are, or how they are protected from predation, although the fact that their body contains myriads of spicules that are really nothing more than shards of sharp glass is presumed to have something to do with the fact that most predators seem to avoid them. It is likely that they are chemically protected against predation as well.

The majority of sponges are demosponges. As do the hexactinellids, they typically have spicules made of silica, but the spicules are never fused to form a solid lattice. Nevertheless, the spicules are often cemented together with proteins into a network that may be as complex as that found in glass sponges, if not as permanent. As befits a group containing thousands of species, there is a lot of diversity of skeletal structure. A few demosponges lack spicules altogether and have only a proteinaceous skeleton; these are the classic "bath sponges." Some others, called "sclerosponges," secrete a massive calcareous skeleton, in which the silicate spicules are imbedded. A thin tissue layer overlies this massive skeleton. Fossils that appear to be very similar to such sponges are fairly common in the fossil record from the mid-Paleozoic, when they were reef-forming animals. While predominantly a marine group, the Demonspongiae also contains the only sponges found in freshwater. Freshwater sponges are not very diverse, but they are very common, being found in most non-polluted freshwater ecosystems.

Figure 4. Large demosponges, such as this Niphates digitalis, are
common in Caribbean coral reefs.

Demosponges tend to be moderately-sized animals, but some are found growing as only thin layers over rocks. In contrast, others may exceed a meter in height. In many temperate nutrient-rich areas and deep-water coral reef areas (Suchanek, et al, 1983), demosponges are the dominant benthic animals. These sponges tend to grow more rapidly than hexactinellids, and it is not surprising that they are commonly found in virtually all coral reef habitats, and hitchhike into reef tanks in or on live rock. In general, those sponges found living in crevices, within rocks or in lagoons tend to do best in reef aquaria. Those demoponges that require a lot of currents, such as the brightly colored so-called "tree sponges" and "ball sponges," generally perish after a short period in reef tanks.

Many animals eat demosponges, but on coral reefs their primary predators are fishes, various snails, such as nudibranchs, and sea stars. Having so many predators, natural selection has casued the evolution of the sponges either to hide or to be very toxic. Generally, the large, evident sponges on coral reefs appear to contain toxic chemicals. These sponges are often long-lived and many of them are homes to other animals that live on or in them in various symbiotic relationships. Many of these animals are commensals, which benefit from living on the sponge, but whose presence doesn't benefit the sponge. Others appear to be ectoparasites, intercepting and eating foods brought to the sponge by its filtering currents. Still others may benefit the sponge in some manner.

Lacking structures for attacking other animals, sponges might seem to be at a disadvantage in the rough and tumble competitive world that constitutes a coral reef's benthic environment. This is decidedly not the case, however. Sponges are powerhouses of chemical synthesis, and many produce highly toxic chemicals. These may serve to make them unpalatable to predators, but similar (or the same) chemicals also may be liberated from the sponge to kill any nearby animals. On a natural reef, where water exchanges are continuous, these chemicals generally act over only very short distances, from a few millimeters to a centimeter or two. In an aquarium, the water movement is not sufficient to flush the chemicals from the system. In such enclosed environments, highly toxic sponges may well have the potential either to kill potential competitors, such as corals, or to stress them so severely that they become diseased and die. Unfortunately, while it is clear that many sponges may be able to do this, there is no practical way to determine if such chemical releases are occurring and what their affect is; however, it is beyond neither belief nor reality that sponges are responsible for many of the mysterious deaths that plague aquarists.

Sponges with skeletal spicules made of calcium carbonate form the third group of sponges, which contains a few hundred species. Some calcareous sponges are commonly found in many reef aquaria. These sponges tend to be small animals, seldom reaching 15 cm, about 6 inches in any dimension. They also tend to be simpler in structure than the demosponges and hexactinellids.

Figure 5. Calcareous sponges, such as these 2 cm (0.8 inch)
high individuals of a species of Leucandra, tend to be
small and drably colored.

Sponges are normal and common components of coral reef ecosystems, but may or may not be good things to have in reef aquaria. Many sponges are good competitors and can crowd out more "desirable" livestock; additionally, many of them are highly toxic. These properties, coupled with the difficulty in identification, make sponge husbandry a most interesting topic, and one I will discuss in detail in next month's column.

Figure 6. Two coral reef sponges spawning. These are males releasing a sperm suspension in the water currents leaving the sponge through the osculum. The females retain the eggs and fertilization will occur internally. Images courtesy of, and with thanks to, Eric Borneman.