Featuring fascinating biodiversity and particularly unique adaptations, sponges are sessile aquatic animals found in both freshwater and saltwater environments at a full range of depths that span everything from tidal waters to deep-water environments of 8,800 meters or more. Sponges lack many of the basic systems found in other animals, including digestive, circulatory, and nervous systems, but instead rely on a number of adaptations enabling them to utilize the constant flow of water through their bodies as a means for securing both oxygen and food as well as for the removal of waste products.
Sponges can be further differentiated from other animals by the total absence of organs and tissues along with the fact that their asymmetrical bodies are shaped in a manner yielding maximal water flow efficiency through a central cavity in which nutrients are deposited and waste products are cleared through the osculum. Although sponges share some characteristics with jellyfish -- a sponge’s body takes it shape due to the presence of mesohyl, the jelly-like mass found between two thin layers of cells and composed mostly of collagen -- the majority also feature internal skeletal structures composed of silicon dioxide or calcium carbonate.
Classification
Belonging to the Porifera phylum, it was only relatively recently that a fourth class was added due to studies delineating the phylogenetic separation between Homoscleromorpha and Demospongiae, as the former once belonged to the latter class. The four classes of sponges belonging to the phylum Porifera are as follows:
• Calcarea
• Hexactinellida
• Demospongiae
• Homoscleromorpha
The phylum to which the sponge belongs is quite aptly named, as Porifera translates to “pore bearer,” a reflection of the porous bodies of the sponge that, along with its channels, allow water to efficiently circulate through its body in order to perform necessary survival functions. The classes dividing the sponges are primarily determined by the skeletal composition of each species, with a specific focus on the characteristics associated with the nature of the spicules, spongin fibers, exoskeleton, and body form.
Anatomical Structure
Cell Types, Water Flow, and the Skeletal System
It is the presence of mesohyl that gives shape to the hollow body of the sponge, but the body itself is reinforced by a network of densely arranged fibers made of the same substance primarily found in mesohyl: collagen. Cells shaped like cylinders and cones -- which are called choanocytes -- cover the inner surface of the sponge with a single flagellum surrounded by a single cell. The choanocytes and flagella work in concert to create the motion needed to ensure water flows through the sponge in the most efficient manner possible.
The water is then moved through the ostia, which are the channels that lead to the internal portion of the sponge in which the mesohyl is located. A system of valves and a variety of other cells work to continue to move water through the sponge while taking in food particles and performing a number of other essential functions. Other cells found in the mesohyl include each of the following, some of which are responsible for signaling the sponge to contract or to act as a sponge’s unique version of an immune system, while other cells are responsible for producing and secreting collagen; secreting polysaccharides and spicules; reproduction; feeding; removing debris and waste products; and transforming into a different type of cell based on the needs of the sponge:
• Archaeocytes
• Collencytes
• Lophocytes
• Myocytes
• Oocytes
• Pinacocytes
• Porocytes
• Rhabdiferous
• Sclerocytes
• Spermatocytes
The intake of water by the sponge begins at its base and is moved through the hollow central cavity before exiting through the osculum. Sponges are exceptionally efficient in this task and even benefit from the suction effect created by the presence of currents, with the rate of water flow being dictated by the sponge according to the changing conditions that surround it. The sponge is even able to cease the flow of water entirely if there is an excessive amount of debris, sand, or silt present.
Sponges are not only able to dictate the flow of water by altering the pace of the flagella’s movement in concert with the opening or closing of the internal valves and channels, they are also able to alter their shape entirely to further ensure the greatest possible efficiency of water flow through its central cavity. With the flexibility made possible by the presence of mesohyl, sponges can adopt each of the following body structures:
• Asconoid
• Leuconoid
• Syconoid
The sponge adopts a specific shape based on a variety of factors, with the shape it takes ultimately dictating the size it will be able to achieve over the course of its life. Asconoid-shaped sponges, for example, have a limited water-pumping capacity and are therefore rarely larger than 1 millimeter in diameter. Leuconoid-shaped sponges, on the other hand, have a greater pumping capacity and often grow to a diameter of 1 meter or more.
Despite its inherent flexibility and the lack of joints or biomineralization, the mesohyl of the sponge’s interior still qualifies as an endoskeleton. Mineral spicules -- which are typically made up of calcium carbonate or silica -- and spongin fibers contribute to the relative hardening of the mesohyl. While most sponges possess this kind of endoskeleton, some species -- hard sponges, for example -- actually secrete an exoskeleton made of calcium carbonate rather than an endoskeleton.
Movement and Other Essential Functions
Even though they are rightly recognized as a sessile aquatic animal, sponges are not necessarily incapable of movement. In fact, some species are able to move in an imperceptible fashion, progressing no more than a few millimeters per day while relying on cells like pinacocytes to move them along the ocean floor.
A number of sponges are capable of contracting their bodies to varying degrees, with some even able to contract the entirety of their body through the use of its myocytes. These cells are often referred to as “muscle cells,” as it is the signal these cells deliver that causes the sponge to contract certain parts of its body when necessary.
Sponges do not have many of the essential systems possessed by other animals and must therefore rely on the flow of water through its central cavity to perform all of its most critical functions. Sponges filter particles of food from the water taken in and then utilize a variety of cells to consume the food particles through several different methods determined according to the size of each particle being consumed.
The smallest of the particles a sponge takes in usually makes up about 80 percent of its food supply and are consumed by the choanocytes after passing through the ostia. The larger particles -- which cannot make it through the ostia’s walls -- are instead consumed by pinacocytes and archaeocytes.
Carnivores, Endosymbionts, and Immunities
Carnivorous sponges are quite rare and are typically found in environments in which the quality of the food supply is especially poor. Although there are up to 10,000 species of sponge in freshwater and saltwater environments, only 137 species have been found to be carnivorous. Feeding primarily on small crustaceans typically measuring no more than a single millimeter, carnivorous sponges are usually located in deep water of up to 8,800 meters. There are always exceptions, of course, and it is indeed the case that some carnivorous sponges have been discovered in caves at a depth of approximately 20 meters.
Sponges have also developed several important endosymbiotic relationships with organisms like dinoflagellates and cyanobacteria, with the latter being the more common of the two. These organisms reside in the cells of the sponge and provide a tremendous amount of energy to the sponge through photosynthesis.
The sponges conduct light into the mesohyl in which these photosynthesizing organisms reside, providing the microorganisms vital protection and access to light while gaining much-needed energy from the products of photosynthesis -- which is especially important for sponges in nutrient-deficient environs. These endosymbiotic relationships often result in the production of more food and more oxygen than is consumed. In fact, it is believed the oxygen produced is as much as three times the amount consumed, so it is clear that these collaborative relationships have a profoundly positive impact on entire aquatic ecosystems, including the Great Barrier Reef of Australia.
Although they lack the complexity of other animals and are therefore without a traditional immune system, sponges are still able to identify and reject grafts from species that are not their own. It is believed that the gray cells of the sponge are responsible for this pseudo-immune system, as they produce a chemical that immediately signals all cells to avoid the grafted area in order to ensure that the internal transport systems so critical to the sponge’s survival cannot be corrupted.
If preventing intrusion is not enough to deter the invader, then the gray cells densely accumulate in the affected area. Once there, the gray cells begin to release a toxin intended to kill all cells within the affected area to further ensure the sponge’s internal transport systems are not in any way harmed by the attack.
Ecology
There are very few limiting ecological factors affecting the sponge, as they are known to reside in both freshwater and saltwater environments and are able to thrive in both the darkest depths of the ocean and the shallow tidal pools of the tropics. In fact, the only limiting factor appears to be associated with the abundance of debris, sand, and sediment that could have an adverse affect on feeding and breathing if the sponge’s pores are routinely blocked.
As a result, the environmental factor most commonly associated with all species of sponge is clear and quiet waters in which sand and sediment is unlikely to be stirred up on any kind of regular basis. The warm tropical waters feature a greater degree of sponge diversity than temperate waters, which is likely the reason that there is a greater abundance of sponges in temperate waters but not nearly as much diversity given the relative lack of predatory organisms.
Lifecycle and Reproduction
The life expectancy of the many different species of sponge varies considerably by region, with the longevity of sponges residing in temperate climates differing significantly from that of the sponges in tropical climates. Although there are differences within each region, a sponge in temperate waters may only live for only several years at best. Sponges in the tropics, on the other hand, can expect to live for several centuries and it has even been estimated -- based on the size in diameter of certain types of sponge -- that some species may be up to 5,000 years old if it is assumed that the rate of growth always occurs at a constant rate over the course of its lifetime.
Sponge reproduction occurs through both asexual and sexual means, and the overwhelming majority of sponges are hermaphroditic. In most cases, the sponges retain their fertilized eggs until they hatch into larvae capable of swimming through the use of its cilia and flagellae, but some sponges do release their eggs into the water once they are fertilized. The hatched larvae then swim for several days until settling in a location in which they can begin to develop the specific cells they need to survive based on the location in which they have settled.
The asexual means of reproduction vary and can occur through budding, fragmentation, and also through the production of gemmules. When a current, wave, or some other force causes a fragment of the sponge to break away, that fragment will quickly alter its shape due to the presence of the mesohyl and will eventually find a suitable surface to which it can attach and begin the process of regenerating. In just a few days it is rebuilt as a smaller but entirely functional sponge.
Although budding is not particularly commonplace among the 10,000 or more different species of sponge, gemmules can be produced by the thousands and are released as part of a regular reproduction cycle or in the event of an acute threat to the sponge’s existence. This is more common among freshwater sponges, but there are some marine sponges that release gemmules under similar circumstances.
