There are over 8600 species of sponges worldwide. They are found in all marine environments, from shallow pools to the deep ocean, and are common in reef, mangrove, and seagrass ecosystems. Though they are primarily marine, about 250 species live in freshwater habitats. Fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian, over 600 million years ago.
A sponge is, in essence, a multicellular cooperative in the form of a perforated sac. Though not typically considered a colonial organism, the cells are often so loosely associated that if you run some sponges through a filter and scramble the cells, they will re-associate to form another sponge. However, though the cells exhibit considerable independence, they have become specialized enough that we no longer think of sponges as cellular co-ops. If separated, they must reassemble; not every cell can live alone.
Sponges have no organs. Organs, such as hearts, kidneys, and digestive tracts are unnecessary simply because most of the sponge's cells are exposed to circulating water (provider of oxygen and food, remover of wastes, praise unto thee, etc.). Sponges are composed of three layers:
1) the outer layer (sac), consisting of thin, often leathery, tightly-packed polygonal cells, called pinacocytes;
2) the middle layer (mesohyl), consisting of a gelatinous protein/carbohydrate material, a range of mobile cells running around on various errands, and a skeleton of spicules (minute, sharp-pointed mineral structures) or spongin (elastic protein fibers); and
3) the inner layer, consisting of flagellated cells called choanocytes.
The sac (the outer layer) is not solid. It is perforated by a system of pores, through which the sponge draws water. Special cells, called myocytes and porocytes, surround these pores and can contract the openings, similar to muscle cells. This allows the sponge to "hold its breath" in dirty water, such as from pollution or sediment influx. They also allow some species to creep very, very ... very slowly across the substrate. The mobile cells in the mesohyl are called archaeocytes, or amoebocytes. Amoebocytes are multitaskers; they can change into all the other types of sponge cells. They are constantly working digesting food particles, carrying nutrients and oxygen to other cells, disposing of waste products, and maintaining the structure of the sponge. Also in the mesohyl are the sclerocytes and spongocytes, whose job it is to secrete the skeleton, which may be calcareous (made of calcium carbonate) or siliceous (made of silica, a white crystalline compound), depending on the class. On the interior wall, choanocytes also have their hands, or rather, flagella, full. These cells create the active pumping of water through the sponge, drawing water in through the pores and sending it out through one or more main openings, the osculae.
Sponge bodies come in a variety of forms, including, but not limited to, arborescent (tree-like), flabellate (fan-shaped), caliculate (cup-shaped), tubular (tube-shaped), globular (ball-shaped), and amorphous (shapeless). For all of the many, diverse forms, there are only three major types of body construction (which have little to do with the taxonomic classification of sponges):
1) Asconoid: a simple tube perforated by pores with a single osculum;
2) Syconoid: also a tubular body with a single osculum, but generally larger and with a thicker body wall (requiring canals in addition to simple pores) than the asconoids; and
3) Leuconoid: the largest and most complex sponges, containing not only pores and canals, but chambers, through which to move water; infolding of the body wall allows many more shapes to be achieved. The vast majority of sponges have the leuconoid organization.
Size is limited by the rate at which water can flow in and out. Sponges with the smallest surface area, the asconoids, are typically the smallest in the phylum. Leuconoid sponges, with the most surface area, claim some of the largest members.
Time for taxonomy! Sponges belong to the phylum Porifera, from Latin, meaning "pore bearing." They are subdivided by their skeletal structures into four classes: Calcarea, Hexactinellida, Homoscleromorpha, and Demospongiae. Calcarea (calcareous sponges) skeletons consist mainly of calcium carbonate; individual spicules can be straight or composed of 3 to 4 pointed rays and often protrude through the outer layer, giving the sponge a rough texture. These sponges are small, usually only a few inches high, often dull in appearance (though some species are brightly colored), and are the simplest sponges, with representatives from all three major morphologies (asconoid, syconoid, and leuconoid). Hexactinellida (glass sponges) skeletons consist of silica; individual spicules are composed of 6 or more intersecting rays and often form a latticework. These are deep-sea sponges. They are pale in color and often cup- or basket-shaped, having a body plan between syconoid and leuconoid. Their cylindrical skeletons often have the appearance of spun glass. Homoscleromorpha used to be a part of Demospongiae but was reclassified, on the basis of molecular and morphological evidence, in the early 2000s. They have a simple anatomy and are often small and delicate with few (or zero) spicules. Usually encrusting-type sponges, many resemble a dense sprinkle of candle drippings. Demospongiae skeletons consist of spongin and/or silica. This is the most diverse group; species vary in size from small, encrusting forms to very large, irregular masses. All are leuconoid; many are brightly colored. The freshwater sponges all belong to this class, as do the common bath sponges and the boring sponges (not boring as in How much longer can she possibly talk about sponge shapes?? boring as in drilling).
As you may have guessed, (most) sponges are filter feeders. They strain the water for oxygen and nutrients. Food particles that bump into the choanocytes stick to them and are digested or picked up by passing amoebocytes who get the job done. The volume of water passing through a sponge can be enormous; one cubic centimeter of a sponge can filter more than 20 liters of water a day. Some sponges are symbiotic with algae and get a little extra energy on the side with that relationship. A few deep-sea sponges are carnivorous; they don't even have all the equipment to filter water anymore.
The sheer abundance and variety of sponges make them an obvious meal target. The fact that they move so slowly (like 6 mm a day), if at all (because most are sessile), means that they can hardly avoid being eaten. A mix of sea turtles, crustaceans, fish, and echinoderms all prey on sponges. They're almost a free meal if you have the right utensils. Luckily, sponges have more than one means to replenish the population.
Sponges are hermaphroditic (having both male and female sexual characteristics) and reproduce both sexually and asexually. Asexual reproduction is achieved through budding and gemmules. In budding, the parent releases a specialized mass of cells that can develop into another, genetically identical, adult sponge. Gemmules are small buds that are encapsulated within the parent and are capable of surviving extremely unfavorable conditions, such as drying or cold, that cause the rest of the sponge to die. Sexual reproduction takes place in the mesohyl. Sperm, fashioned from choanocytes, are broadcast into the water, sometimes in masses so dense that the sponges appear to be smoking. Once the choanocyte-turned-sperm is inhaled by a sponge of the same species, it is trapped and fused with a local choanocyte. This choanocyte-sperm fusion then metamorphoses into a specialized amoeba-like cell and crawls toward an egg. The egg forms when an amoebocyte engulfs special cells to form a yolk. The choanocyte-sperm-amoeba fuses with the egg, and presto chango! A zygote. In some species, the sponge releases the eggs right after fertilization; in others, the sponge retains the eggs, and some development takes place within the parent. Some larvae settle directly after release and transform into adult sponges; some are planktonic for a while, tooling about looking for a suitable substrate to set up shop.
Sponges are competitors for space, and many secrete chemicals to ensure themselves a place in the neighborhood. Many species contain toxic substances, probably to discourage predators. Certain other marine creatures take advantage of this characteristic by placing adult sponges on their bodies, where the sponges then attach and grow. Despite being stingy with surface area, sponges provide homes for a number of small marine plants, which live in and around their pores and canals. Symbiotic relationships with bacteria and algae also exist. Humans have jumped on the spongewagon, too. Some of the chemicals that sponges produce have beneficial pharmaceutical effects in respiratory, cardiovascular, gastrointestinal, anti-inflammatory, anti-tumor, and antibiotic treatments.
In the Gulf of Mexico, sponges are one of the dominant sessile invertebrates. The Gulf has a rich sponge fauna, with over 300 documented species. However, very few sponges live in the bays and estuaries, especially in Texas and Louisiana. Low winter temperatures and reduced salinities discourage many species common in the Caribbean, southern Florida, and Yucatan. Rivers deposit sediments that smother sponges growing too near the bottom. Rapid summer algal growth smothers sponges growing too near the surface. Consequently, the northern Gulf's sponges thrive best on submerged reefs far from shore, where the water is clearer. The bays usually have mostly encrusting forms of the genera Haliclona and Microciona, and boring sponges of the genus Cliona.
Haliclona (class Demospongiae, family Chalinidae) spreads over the substrate in a mat, ranging from a little over 1 mm to nearly 20 mm (3/4 inch) in thickness, but it is easily overlooked because of its encrusting form and general lack of color. Their morphology varies considerably. Some individuals have their osculae raised on pinnacles resembling tiny volcanos. Other individuals are so smooth that they are easily mistaken underwater for a coat of paint. Color is also variable, ranging from brick red to yellowish-green to lavender and deep purple, but is most often tan, cream, or dingy gray. Obviously, sponges cannot be reliably identified solely on shape or color. Microscopic examination of the spicules is usually required.
Microciona (class Demospongiae, family Microcionidae) extends well up into Texas bays in years of low rainfall; it was seen in Copano Bay in 1949 (when the salinity was between 18 and 23 parts per thousand). This family, which includes encrusting, massive, lobate, fan-shaped, and branching growth forms, is possibly the most abundant and diverse of any family of Porifera and is widely distributed in all the world's oceans.
Boring sponges (class Demospongiae, family Clionidae) excavate the surface of corals and mollusks, sometimes causing significant degradation or death to the mollusk. They begin their boring as larvae and spend their lives in the tunnels they form, seeking protection by sinking into the hard structures they erode. Even this process has some beneficial effects, however (not that any oyster would agree), in that it is an important part of the process by which calcium is recycled. Sponge-eroded calcareous chips may compose up to 30 or 40 percent of sediments in low-energy reef environments.
Cliona celata (from Latin, meaning "concealed or hidden"), one of the sulfur sponges, is the most conspicuous of the boring sponges. The large dome-shaped or vase-like forms which occur in the open Gulf aren't found in the bays, but encrusting forms of C. celata are fairly common and bright yellow boring forms can be found in old oysters and dead shells.
Happily, there are too many species in the bays to cover in one article. So for now, no more boring sponges!
Where I learned about sponges, and you can too!
Beachcomber's Guide to Gulf Coast Marine Life: Texas, Louisiana, Mississippi, Alabama, and Florida
By Susan B. Rothschild
Houston Advanced Research Center Gulf Coast
Marine Species Identification Portal
World Porifera Database
"Notes on the Boring Sponges in Gulf Coast Estuaries and Their Relation to Salinity"
By Sewell H. Hopkins
Agricultural and Mechanical College of Texas and Texas A&M Research Foundation
The Nature Conservancy
Tree of Life Web Project
Animal Diversity Web
Department of Zoology at ANDC
University of California Museum of Paleontology
Sidwell Friends School
*Special thanks to Allen Collins (Smithsonian National Museum of Natural History) and Tracey Bates for helping with this article.