Everyone knows what a sponge is; they go in the bath or shower and are great for washing. They can appear or be placed in the marine aquarium as well, though they’ll look different!
Living sponges vary in size from very small, 1″ (circa 2.5cm) or so, to huge, 18″ (circa 45.75cm) or so in diameter. There are many different ones, the total numbering 9000 plus*. The sponge is not a single celled organism but multi. They are always stationary and meet success or failure in that one position. In a similar way to soft corals, the sponge holds its shape by means of calcium spicules, though some are silica which divides sponges into calcareous or siliceous types. Different types can be found from very shallow to very deep seawater.
Sponges feed by drawing seawater in and filtering out nutrients. Once the filtration is complete the seawater is ejected with any waste. This is a continuous process.
Sponges are fairly common in retailers and come in various colours and shapes. The shape usually indicates the area where the sponge originated, flat encrusting types originally living in high water flow and taller wider ones offering more resistance to flow coming from quieter areas. Sponges are sometimes found on coral rocks when the aquarist acquires a new coral. They could also be growing on live rock, perhaps unseen as they are hidden from view.
Some sponges seem to be hardy, these being the encrusting types which can be found on living rock and coral rock. They often survive without specific feeding and are welcome.
The larger, more upright and brightly coloured sponges are more difficult. When on the captive reef they without doubt add bright colour and shape, and can be a real temptation to an aquarist. The aquarist should resist the impulse to buy, unless he/she has experience enough to maintain corals successfully, that is maintaining them long term in a healthy and growing condition. This at least gives an indication of husbandry abilities. Sponges are sensitive to declining seawater quality.
If a sponge is bought, it must not be exposed to the air at any time. Transfer into the home aquarium should take time allowing the seawater to equalize. The aquarist should not touch the sponge but maneuver it by the attached stone. This makes the point that there should be an attached stone; a purchase shouldn’t be made without one. On no account place a sponge on or into sand, it will die.
Position in the aquarium is very important. Reef aquariums use very bright lighting for the sake of the corals and this could lead to some encrusting and other algae. Algae are one of the enemies of sponges. Therefore they should be positioned in a dimly lit area, under an overhang perhaps, or low down where light is reduced. The position should also have gentle seawater flow.
The major problem with sponges is feeding them. Nutrient levels in reef systems are low, with efficient protein skimming and routine seawater changing. It is very unlikely that there will be sufficient food matter to sustain a sponge. I don’t know if a sponge is able to make use of dissolved organic matter (DOM) but placing the protein skimmer on a timer and running it for half the usual period may be useful? A close watch would need to be kept on seawater quality in case corals objected. Perhaps the way to feed a sponge with the best hope of success is to use a pipette with very fine foodstuffs, such as the foods commercially available targeted at filter feeding corals and fan worms. The corals and fan worms will also benefit and hopefully the sponge will obtain nutrients. This feeding will place an additional strain on the bio-filter as it is likely not all the food will be consumed. Again, a watch will be needed to ensure there isn’t excessive deterioration of seawater quality.
Sponges are food for some livestock. There are fish that eat them and a good example is the rock beauty angelfish (Holocanthus tricolor). Other angelfish and butterflyfish are a danger to them. There are more, including some snails and urchins. So it is essential, as always, to check compatibility with existing livestock before purchase.
The sponge I remember was purchased because of its lovely bright orange colour. It was properly attached to a rock and was shaped like a lollipop, about 5″ (circa 12.75cm) tall. The sponge was introduced properly and carefully positioned.
A demand was certainly placed on that virtue of marine aquarists, patience. This was required when feeding took place – a commercial filter feeding food was used. Great care was taken to ensure a small cloud of food surrounded the sponge once, sometimes twice a day. Seawater quality didn’t deteriorate, I assume because the aquarium had other filter feeders. I didn’t reduce the period the protein skimmer ran; it was left on full time. Perhaps I should have experimented with that.
The sponge didn’t grow or deteriorate. It maintained full colour and shape for about 12 months. Then a little grey was noticed, and this was seen to be very slowly spreading. The sponge was removed from the aquarium, as I was afraid of sudden failure and pollution.
If an attempt is to be made in keeping a sponge purchased from a retailer, then it is suggested that notes are kept on the techniques used from the very beginning, including introduction to the aquarium, positioning, seawater quality and particularly feeding. If reasonably successful then let your local club or internet forum know about the procedures used.
The link gives pages of sponge pictures.
(* Reference: Marine Aquarist Manual Comprehensive Edition. Dr. P. V. Loiselle & Hans A. Baensch)