An aquarium keeper who intensely deals with the internal structure and physiology of sponges, will presumably be surprised. With a relative and quite big probability, indeed, he will find out that living sponges, in their functionality, are surprisingly similar to those devices developed during the last few years in aquariology for water cleaning treatments. As a matter of fact, a living sponge can be defined as a perfectly formed biological filtering aggregate. In this essay, I would like to provide some information regarding what I elaborated since 1997, when I asked myself about the effective and specific employment of said “living filtering aggregates” in a reef aquarium, as a natural method of supplementary filtering, or in place of a filtering implant and a skimmer. In the USA, natural filtering methods, during these last few years, have become very popular. This is demonstrated with multiple filtering methods regarding reef aquariology, like the Jaubert Method, the Adey System with the algae based filter, the Dutch mini reef and also what has become known here in the USA as the “Berliner System”. All these systems are based on the employment of living rocks as biological components in the water treatment. Since the beginnings around the middle of the nineties, I prepared many reef tanks based on the principle that was very popular at that time: the “Berliner System”. The method combines extensive quantities of living rocks with a skimmer, and the aquariums that I prepared in this manner, have developed with remarkable success. Photographs of these tanks soon appeared in all kinds of aquariology books, and many aquarium keepers soon after had the impression that said method was “bomb proof” and the best path to ride on. Starting from 1996, though, reef aquarium keepers in the USA had to suffer a recoil: RTN (rapid tissue necrosis), that is the sudden degeneration of tissues. The majority of us supposed that the cause of such a catastrophe was to be looked upon in some sort of importation of pathogen bacteria with the corals, from a specific export area in the Pacific, that someway arrived into our aquariums. Soon after a possible offender was found: a particularly virulent Vibrio species, which spread itself from a particular exportation site. All across the country, every aquarium keeper who owned hard coral populated tanks had to confront themselves with the massive propagation of this problem, losing a great quantity of corals. In this article I do not want to discuss the RTN problem in detail, but I would like to remember that it protracted itself almost up until the end of 1997. The experience with this import disease, however, has since then induced me into intensively reflecting about the aquarium system, used by all of us at that time. I had the impression that even one skimmer had its functional limits. It is not capable, for example, to keep the populating density of pelagic bacteria (the ones who swim freely) under control. It is barely able to indirectly influence it, in part removing several organic substances from water (solid, not dissolved), therefore subtracting nutritional bases to certain bacteria. Between 1995 and 1996, in many spots inside my aquarium, I observed for the first time a phenomenal growth in sponges. In a non-illuminated, living rock filled filtering tank, for example, many big sponges had developed, growing out of rocky material. Some exotic sponges developed in the aquarium too, for example in the obscured spots under a big protuberance. Given that in the years between 1997 and 1999 many aquarium keepers were still overcoming the financial losses caused by RTN, my simple and economically effective method aroused a lot of interest back then. Therefore, for the monetary aspect too, I tried to push this aspect forward. But, pressed by the spreading of RTN, I also strived to find a convenient way to control the populating density of pelagic bacteria. In this sense, UV sterilization apparatuses, ozonators, activated carbon filters and a strong skimming proved to be effective, but their purchase and use required a notable disbursement. The more I dived into the reading of sponge related scientific literature, the stronger got in me the belief that these organisms constituted the best solution for my system.
The filtering ability of living sponges
Science has studied living sponges, discovering much about their structure and physiology, about their filtering ability and their nutritional necessities. Sponges are able to let water flow through their external body openings (hosts), towards the inside. It is estimated that a sponge with a one liter body volume, can be able to filter, depending on its species, up to 570 liters per hour. Input channels convolute towards the inside of the sponge and become tighter, then finally water arrives to the choanocytes camera. This is the location of small handling mechanisms able to circulate water, not only towards the inside of the sponge but also all through it. In these chambers micro sized bacteria and dissolved organic substances are kept. Every chonanocyte cell is fitted with a scourge which is then shaken, putting water into motion and pushing it outside the chamber. From these chambers water flows towards effluent channels, usually collected in the exit opening (Tyre, 1998). It is known that sponges will filter pelagic bacteria out of the water in a very effective manner. Analysis of a tropical Demospongiae sponge has evidenced that 96.1% of bacteria that flow towards the insides of the sponge with water, are extracted via filtration and kept in the micro sized choanocytes cells. Phytoplankton is also filtered very effectively by sponges. In specimens located in Jamaica’s Discovery Bay, it was evidenced that the keeping ratio of phytoplankton was 86.5%. In the case of dinoflagellates, diatoms and fungal spores, this ratio was always fixed around 48.7% after all. Even the micro sized debris, present in the water, was kept in the order of 41.9%. Organic dissolved substances contain, in the water column belonging to the tropical reef areas, around 86-88% of the total organic carbon found in it. In the case of three tropical Demospongiae, for what organic substances are concerned, a keeping ratio of 35.1% has been established (Reiswig, 1971). Tropical sponges combine then a strong pumping yield with the ability of keeping micro sized particles. Of all marine living organisms, they have the most efficient structures for filtering of micro sized bacteria and dissolved organic substances. In this manner they have become, in this vital environment, the dominant filtering group for particles of said size. The majority of other filtering organisms in the reef, those who are not part of the sponge species, have specialized in a particle size superior to 50 µm, and those who anyway capture smaller particles, will do so with different techniques from those used by sponges, for example with the use of viscous nets or other passive methods, which may depend however on the presence of water currents (Reiswig, 1971,1975). This means that sponges have the ability of absorbing and utilizing dissolved organic substances from reef water. This ability makes sponges a natural substitute for the skimmer in the aquarium. They also offer to the aquariophile the possibility of controlling the population of pelagic bacteria in a natural way. My examinations of water samples belonging to many reef aquariums have proven that tanks with an elevated concentration of organic compounds also had the tendency to feature a major population of said bacteria (Tyree, 2000). All the reef aquariology methods employed today feature the use of the living rocks belonging to the coral reef. These rocks are normally transported towards the arrival country dry, which means that said material has to be “cured” before being introduced in a reef aquarium. In practical terms, this means that they have to be kept in sea water in order to allow the decomposition of dead organisms, disappearing from the rock. Those who already have a degree of experience with the “nurturing” of recently transported lively rocks, will know that many of these organisms are sponges. These sponges constitute a crucial part of living rocks, therefore most reef aquariums are prepared with an unnaturally reduced population.