Marine Sponges

Sponge bodies are diverse in form, ranging from encrusting sheets, to volcano-shaped mounds, to tubes as small as one millimeter or as large as one meter. In all cases, sponges have a canal system, through which they pump water. Water enters through pores called ostia, flows through canals to a spacious chamber called a spongocoel, and finally exits through large openings called oscula.
Often, sponges are distinguished by the level of complexity exhibited by their bodies. The simplest form consists of a single tube two cell layers thick. Sponges with this type of architecture are necessarily very small due to surface area to volume constraints. In order for a sponge to attain greater size, the sponge wall must fold in on itself.
Like cnidarians (jellyfish, etc.) and ctenophores (comb jellies), and unlike all other known metazoans, sponges bodies consist of a non-living jelly-like mass sandwiched between two main layers of cells. Cnidarians and ctenophores have simple nervous systems, and their cell layers are bound by internal connections and by being mounted on a basement membrane (thin fibrous mat, also known as "basal lamina"). Sponges have no nervous systems, their middle jelly-like layers have large and varied populations of cells, and some types of cell in their outer layers may move into the middle layer and change their functions.

Most sessile or slow moving animals on tropical reefs have developed the means in which to produce a wide array of chemical compounds for self defense or to prevent other organisms from growing upon themselves. The brightly colored Nudibranch are a good example of an animal group that not only produces toxic, defensive compounds, but advertises that they do by being brightly colored. While many sponges do not advertise their toxicity through colorfull displays, they can and do produce some of the most elaborate compounds found anywhere on the world's reefs. For many years now, science has been collecting and analyzing a great many sponge compounds for use in the medical field to combat human cancers and for their use as antibacterials. It is also thought that many of these compounds may be used by the sponges as a form of chemical warfare (allelopathic) against other encrusting or encroaching organisms including other sponge species. This is where sponges can become a real problem in our aquariums should they feel the need to defend themselves or die and cause the release of these compounds. Which could of course cause the rapid demise of the other organisms we are trying to keep as part of our captive reef ecosystems. A number of times I have collected specific sponges with the thought of adding them to my aquarium only to find that they had released some form of toxin while in my collection bucket on the way home from our local reef which promptly killed any and all other creatures that I had collected and were sharing the same bucket. Over time and through trial and error I have come to learn which types or species are not prone to cause such harm and will try to include photos of those that I have found to be suitable for collection and keeping within an aquarium.

Sponges filter food particles out of the water flowing through them. Particles larger than 50 micrometres can not enter the ostia and pinacocytes consume them by phagocytosis (engulfing and internal digestion). Particles from 0.5 to 50 micrometres (2.0×10−5 to 0.00197 in) are trapped in the ostia, which taper from the outer to inner ends. These particles are consumed by pinacocytes or by archaeocytes which partially extrude themselves through the walls of the ostia. Bacteria-sized particles, below 0.5 micrometres, pass through the ostia and are caught and consumed by choanocytes. Since the smallest particles are by far the most common, choanocytes typically capture 80% of a sponge's food supply. Archaeocytes transport food packaged in vesicles from cells that directly digest food to those that do not.
It was recently discovered by that the Halisarca caerulea sponges that grow in the deep cavities beneath reefs, obtained 90% of their diet by consuming dissolved organic carbon, which is inedible for most other reef inhabitants. When the amount of carbon that sponges consumed was measured it was found that they consume half of their own weight each day, yet they never grew. So what were the sponges doing with the carbon? As it turns out, the sponges were growing, but discarded so many of their own cells that it offset new growth, with the discarded cells becoming food for many other organisms. Essentially, the sponges recycle carbon that would otherwise be lost to the reef.
Most hobbyist seem to have very poor success rates with purchased sponges since any exposure to air will kill most species and those few types that are commonly sold are simply not suited for aquarium life as our providing the exact particle size and type of food, which for many species consists of phytoplankton and/or bacteria, are not usually available in any great quantity in a typical reef aquarium due to heavy filtration by skimmers. However, there are a few species that live in waters where the supply of food particles is very poor and instead, prey upon crustaceans and other small animals. Most belong to the family Cladorhizidae, but a few members of the Guitarridae and Esperiopsidae are also carnivores. In most cases little is known about how they actually capture prey, although some species are thought to use either sticky threads or hooked spicules. Most carnivorous sponges live in deep waters, up to 8,840 metres (5.49 mi) making it highly unlikely that we will ever find them within our aquariums but I thought it worthy to note simply to show how diverse this group of animals are.

I have yet to collect any tunicate species from the reefs that have done well for me over the long term, although I am attempting to keep a collected species as shown in the above photo. The few that I have been able to maintain so far, all appeared as hitch hikers on live rock and have to assume that it was only by chance that the species that happened along did well for me. I also have to note that there were more species that did not do well than those that did. In short, if you do find a tunicate species thriving in your aquarium system, count yourself lucky that your system and what it provides just happens to have met the needs of that specific species. For more information and photos concerning the Tunicates, please see the relevent section within my Hitch Hikers Guide to the Reefs.

Related Reading : The SeaSlug Forum Fact Sheet - A good place to start

Tunicates or Sea Squirts - A great article by Dr. Ron Shimek