A brief introduction before I begin. I've decided to test out a new series of posts about life on earth. Most of these posts will be about different types of animals, but plants will be included intermittently.
Life has been present on earth for over 3.5 billion years. It has since diversified into a vast assortment of different organisms. Animals all share two key features: first, they obtain their energy from food (as opposed to plants, which produce their own food by photosynthesis); second, they are multi-cellular, which distinguishes them from single celled organisms.
My inaugural post on animal life concerns the simplest of all animals...
Sponges were once thought to be plants because they spend their entire adult life fixed to the sea floor, unable to move around. On land, animals must move around in search of food. In the ocean, water currents carry an abundant supply of food in the form of microscopic plankton. Fixed animals can take advantage of this by filtering their food from the water, without having to move from place to place.
When it is time to reproduce, they shed sperm and eggs into the water. Currents distribute them to new area, where they can settle and grow.
The anatomy of a sponge has no symmetry, nor do they have distinctive body parts. A sponge consists of a cooperating community of individual cells, which surround a system of canals through which water is pumped. As water passes through, plankton and particles of organic matter are trapped inside. Rigidity is provided by a skeleton of tiny, bony splinters called spicules, scattered throughout the body.
Some sponges grow to about 6 1/2 feet tall. The largest may be hundreds of years old. There are more than 15,000 different types of sponges in a breathtaking variety of colors. Grays and browns predominate in deeper waters, brighter colors in the shallows.
Six species of sponges are considered commercially marketable, especially a type called demosponges, whose populations are now badly affected by over-collection. Mediterranean sponges are the softest and best. These are gathered by divers and then cut into the familiar blocks seen on supermarket shelves. Fortunately, most kitchen and bath sponges are now synthetic, and were never really living sponges at all.
The elegant branches of the tube sponge are easily torn, so this variety is found only in very deep water where wave turbulence is minimal. Tube sponges are most common in the tropical waters of the south Pacific, and they typically grow to about 3 feet in length. It is sometimes seen as a single tube, but more often as a series of branches joined at the base. When this sponge releases its sperm into the water, it resembles a smoking chimney.
Most sponges need a hard surface for attachment, although a few can bore through soft sediment. For this reason they often form beautiful "gardens" in shipwrecks and on coral reefs. The largest populations exist where tidal currents are strong, which brings extra food. Other animals such as crabs and worms sometimes live inside of sponges.
There aren't too many interesting books just about sponges, but there are lots of good ones about all kinds of animal life. Animal: The Definitive Visual Guide, by David Burnie and Don Wilson is a good first step for an interest in animals in general. Ocean, by Philip Eales, David Burnie, and Frances Dipper is a good one about sea life. Both are filled with interesting facts and lots of glossy pictures, perfect for yourself or as a holiday gift.