The collar is made of fine tubes surrounding a long whiplike thread called a flagellum. As flagella plural of flagellum in the collar cells move back and forth, they create a current of water that moves into the ostia and out the osculum. Several gallons of water can circulate through a fist-sized sponge in a single day, bringing in tiny food particles such as suspended bacteria, bits of plant and animal matter, and tiny drifting planktonic organisms.
As the water circulates, the fine tubes of the collar cells filter out the food particles and take them into the cells for digestion. For this reason sponges are described as filter feeders. Between the outer surface of epithelial cells and the inner surface of collar cells is a jellylike material. In this jelly are the structures that support the sponge.
There are also free-moving cells called amoebocytes Fig. During feeding, some of the particles taken in by the collar cells are passed on to amoebocytes, which carry them to other cells of the sponge. Several kinds of amoebocytes serve special functions, like producing the sponge skeleton, digesting and transferring nutrients, or reproducing themselves. The skeletal elements of the sponge are produced by the amoebocytes.
The amoebocytes produce spongin , the soft fiber that forms natural bath sponges. These sponges feel soft and springy to the touch because they have soft skeletons made of flexible fibrous spongin. Other sponges have a stiff skeleton that feels prickly because it is made of hard, sliver-like spicules , which are also built by the amoebocytes.
Some sponges have both spicules and spongin and feel both prickly and flexible. Many species of sponges can be identified by the shape and composition of their spicules Fig. Siliceous sponges have spicules made of silicon. Calcareous sponges have spicules made of calcium. Spicules also have many shapes and sizes. While some sponges have no spicules, others have so many that they look and feel like lacy skeletons of glass Fig.
All cells in a sponge are in contact with or near to seawater. Because each cell exchanges oxygen and carbon dioxide and discharges waste products into the seawater, a sponge has no respiratory, circulatory, or excretory system.
Sponges can reproduce either asexually or sexually. Asexually reproduction without eggs and sperm often occurs by budding , similar to growing a new branch on a tree. Cells on the side or base of the parent begin to bulge out and form a new organism. The buds may remain attached to the parent, or they may detach and settle down nearby to form a separate organism. Sponges also reproduce sexually when specialized gametocyte cells produce sperm and eggs. Sponges undergo synchronous spawning and eject sperm and egg cells into the water.
If gametes sex cells; either sperm or egg from the same species meet, they form a larval sponge. After a period of planktonic drifting, the larva settles to a suitable location on the bottom and grows into an adult sponge.
The drifting larval stage means that sponges can colonize new locations, even though as adults they remain attached in a sessile lifestyle. Freshwater sponges can live in areas that are subject to cyclical wet and dry periods.
Prevailing theories suggest that sponges are early animals which produced no subsequent evolutionary line. It is the thinness of the sponge body and the fact that its cells are exposed to circulating water—which supplies food and oxygen, and removes waste—that make organs unnecessary. Sponges may have been the first multicellular animals.
Multicellularity which means that cells adhere to one another, communicate, are mutually dependent for survival, and specialize to perform different tasks is the key to producing more complex organisms. So, sponges are in fact the original animal hipster; they were multicellular before it was cool. Frazer, Jennifer. Animals are spoken of as one "organism". Yet when you shed skin cells or digestive cells, or when any cells in your body voluntarily die without offspring, they are sacrificing themselves for the good of this larger, albeit genetically identical, entity.
One of the three major groups of sponges are the glass sponges, distinguished by their glassy, six-pointed silicious spicules and sometimes-delicate, sculpted appearances. Here is one preserved in the Sant Ocean Hall at the Smithsonian.
Notice its long rooting spicules, a feature many sponges share. These would be covered in mud in the ocean. The Venus's Flower Basket, a glass sponge. They look pretty much like this in the water, too. Creative Commons Ryan Somma. Click Image for link.
When sponges live deep -- really deep -- beyond the domain where filter feeding is a profitable business, they tend to become, amazingly, carnivorous. One might not expect such a feat for an organism that typically has all the mobility of a potted plant, but then again, plants have produced more than a few carnivores themselves.
One of the most bizarre-looking organisms on the planet is a carnivorous sponge. Don't believe me? Behold the ping-pong tree sponge , Chondrocladia lampoglobalis, a deep-sea dweller that resembles a 60's-retro chandelier popular among hipster home decorators.
These curiosities are typically about 50 cm 20 in high and live at to m 8, to ft deep. Small crustaceans or worms so unfortunate as to land on the ping-pong tree sponge find themselves velcroed to it by tiny grappling-hook-like spicules. This triggers digestive cells to migrate to the site and spend the next few days chowing down.
When supper is finished, they return to their original locations. Other carnivorous sponges deploy similar weaponry. In addition to hooked spicules, they may use sticky threads to entangle and digest prey. Most carnivorous sponges have lost their choanocytes and resulting hydraulic systems, as these would be pointless in the absence of filter feeding. Chondrocladia has not. It uses a highly-modified hydraulic system to inflate the ping-pong balls at the ends of its branches.
A few sponges -- perhaps 50 species -- live in freshwater. It is likely a species of Spongilla and if anyone knows the exact species, George would like to know , the same genus as this specimen of Spongilla lacustris captured in the upper reaches of the Columbia River in Washington state:. A photosynthetic, symbiotic freshwater sponge, Spongilla lacustris. Creative Commons Kurt L. You will notice that it is green. That is because it, like many freshwater sponges, hosts a green algal symbiont.
Some marine species partner with blue-green algae also called cyanobacteria or even photosynthetic dinoflagellates a kind of protist , which can provide over half of their energy demands in places where the filter feeding is not so hot.
Ahh, biology. How I love you. Finally, I cannot end this post without mentioning the plague of potato sponges that washed ashore following Hurricane Irene this year. The poor homely things seem to have been scoured from the seabed by the storm, whence they drifted, dying, ashore, only to be accused of being foul-smelling and disgusting.
If your corpse were floating for several days in Chesapeake Bay, I'll wager it would not be so sweet-smelling either.
The views expressed are those of the author s and are not necessarily those of Scientific American. She has degrees in biology, plant pathology and science writing. Already a subscriber?
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