Despite the fact that sponges are large-bodied multicellular animals, they function largely like organisms at the unicellular grade of complexity (subkingdom Parazoa). Their strategies of nutrition, cellular organization, gas exchange, reproduction, and response to environmental stimuli are more like those of single-celled organisms than those of a typical animal.
The General Characteristics of Sponges
The members of phylum Porifera exhibit a number of unique morphological (structural) characteristics.
- Their multicellular body is a loose aggregate of cells without true tissues. Adult sponges are asymmetrical or radially symmetrical.
- Sponges are all aquatic, most are marine. Terrestrial (land) sponges do not and cannot exist.
- All adults are sessile (not moving about freely) suspension filter feeders. However, the larval stages of sponges are motile (mobile).
- Their body is perforated by ostia (pores), canals, and chambers that serve for the passage of water.
- Water currents move through the chambers and canals due to the action of unique flagellated cells known as choanocytes.
- Sponges lack a true skeleton. Their support network is composed of calcareous or siliceous crystalline spicules, often combined with coarse collagen fibers (spongin).
- Asexual reproduction occurs by means of regeneration, buds or gemmules while sexual reproduction involves eggs and sperm. Sponge larvae are ciliated and free-swimming.
Sponges are the Masters of Regeneration
Sponge cells tend to be totipotent. This means that they retain a high degree of mobility and are capable of changing form and function. This potential for plasticity has helped sponges survive for over 500 million years.
Sponges can respond to unfavorable environmental changes by simply crawling slowly away, rebuilding their canal systems as they go. If one is damaged by an angelfish (one of the few fishes that eat sponges), instead of building new tissue to repair the damage like a more advanced animal might, a sponge merely mobilizes existing cells to move over and cover the wound, often leaving a scar.
Sponges use this plasticity potential to change shape and spread out, filling irregular open spaces on reefs and rocks, and crowding out more structurally complex animals. The prize is the rarest commodity on the seafloor: space to settle down and live.
Sponges may technically be immortal because in undisturbed natural surroundings they never die. The only natural death threat sponges face is total dehydration and drying or being froze solid.
Macerate a sponge into tiny pieces and most of those pieces will regenerate a whole new sponge. In fact, if you force a sponge through a fine silk mesh and break it up into individual single cells, each of those individual cells has the potential to regenerate an entire sponge. (Imagine the mess you would have if you tried that with a chunk of human flesh, let alone an entire human body.) Sponges just keep on keeping on.
If one ever has the opportunity to confront a living sponge, do not dismiss it off hand as a primitive being lacking even a single organ. Rather, respect and even admire this strange entity that would laugh (if it possessed the proper body structures with which to laugh) at the fragility and short life span of the so-called complex human body.
“Sponges are pretty successful. They have existed for millions of years and the fact that they still exist in very simple forms indicates that they are able to do what they do without increased complexity. Bacteria are simple, but they rule the world. Innovation does not render the old way of doing things obsolete or unusable; the complex (new) way just lets you do different things like grow larger, become mobile, move to land, and chase prey.”
(Matthew Nelson)
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