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Sponges do not enjoy a solid reputation in the trade/hobby as hardy captive organisms. Many people consider them simply short-lived (or not living at all) ornaments. This is inaccurate and fortunately there are some varieties that, given proper collection, transport and intermittent care, do do well in captivity.
Has the specimen been exposed to the air? It should not be! Air is easily tapped inside the many channels inside the animal, ostensibly killing off those parts. Sponges should never be lifted out of the water! "Bag" and move them underneath the surface.
The excurrent (outflowing) "mouth" (oscula) of the sponge should be open, indicating internal activity/water flow. Impugned sponges "close up" their exits.
Purchase sponges with little or no "dead" areas, grey or whitish masses. If possible ask the dealer to cut out bad spots and "hold" the prospective purchase for you if/while it is regenerating.
Living sponges are relatively firm, well (consistently) colored, and do not "smell bad". Stinky, limp and blotchy-colored ones are probably done for.
Buy from a reputable dealer, who knows what sponges are and is willing to "cure" them for a week or more for you in order to assure their vitality and acclimate them.
If you purchase "live-rock" for a reef system, you may well be purchasing a considerable amount of live sponge material "free". If some of your sponges start growing, encourage them by placement in a sump, refugium (out of the light if they're non-photosynthates), away from predators. Sponge growth is generally a sign of a viable, healthy system and in turn, Sponges help to keep the biological balance.
Most sponges offered in the trade have broad environmental tolerance, nevertheless a few broad conditions apply: Your chances of keeping live Sponges are much enhanced by placing them only in well-established reef systems; among other reasons to assure the organic material and bacterial populations to nourish them. Tied in with this idea is not to be over-zealous regarding absolutely "pure" system water quality. Use of silicate removing chemical filtrants and "super-skimming" and/or ultraviolet sterilizing is counterproductive to Sponge culture.
Placement is very important. Sponges engage in "chemical warfare" as celebrated as the Soft Corals. They should be put out of harms way of all stony corals and provided with growing room around them. Encrusting forms must be provided with a "break" in their substrate to stop them from spreading.
Maintenance of sponges is pretty simple. They do need to be "vacuumed" or "blown" clean of detritus, gravel and "gunk" periodically. Adequate light and feeding are the most important factors in success, once a healthy specimen has been secured.
About lighting: Some reef-building species (e.g. the bright purple and blue genera Halichondria, Callyspongia and Dysidea) prefer bright direct red-end-shifted lighting. The majority of species offered exist in low light habitats and should be accommodated likewise. If algae are growing on your sponges, something is wrong: Placement, photo-strength, duration, quality and/or nutrient levels are out of whack. Greenish specimens may be harboring symbiotic algae (zooxanthellae) requiring more light. Non-hermatypic species should be placed in low light, and especially out of the light should filamentous or encrusting algae appear on them.
A strong current is appreciated, from power head(s), filters, air-lifts... Pay particular attention that any matter not stay on your sponges surfaces. If circulation doesn't wash off sand, food, what-have-you, gently siphon this material off or blast it with a handy powerhead.
Predator/prey relations: Many opportunistic omnivores will chomp on your sponges. The Dwarf Angel genus Centropyge generally are not too bad, but the larger pomacanthids make up a considerable part of their diet in sponges in the wild. Some species of Filefishes and Triggers are also known sponge munchers. Nudibranchs, Cowries and several other mollusks are not to be trusted carte blanche. As usual, you will have to experiment and be observant!
Toxic Sponge Effect: Some Fire and Red Ball sponges are known to produce a "wipe out syndrome" if injured or sufficiently disturbed. On introduction, mixing water, disposing of most mixed water and careful placement are warranted. By the by, some "fire" sponges are toxic to humans as well; wear rubber gloves where/when in doubt.
Symbioses: Partners and Parasites. All manner of other life lives in and on (Shown, Istrochota and zoanthids) Sponges, (or their remains). Crabs (shown, Decorator Sponge Crabs that attach Sponge for camouflage and its bad taste), lobsters (Shown, gobies and lobster in Sponge), starfishes (show Basketstar, Astrophyton muricatum in and on Sponge/feeding at night), real fishes, worms, crinoids, Sea Cucumbers, and they in turn grow on and through other organisms. These relationships are typically commensal, but they run the gamut from predatory, parasitic to mutualistic.
Sponges are known to reproduce in several ways, sexual and asexual. They have several variations on "budding" and "fragmenting" themes as well as produce eggs and sperm released into the water column with meteorological and chemical cues. Some develop planktonic young, others are fertilized and develop internally.
Though sponges will spread and reproduce asexually on their own, they can be easily "fragged" by cutting off a piece of an established, stable colony and tying it to a piece of rock with a piece of fishing line. This should only be attempted with large-enough colonies and never in conditions that favor micro-algae growth.
Sponges are largely filter feeding organisms, constantly whipping their (choanocyte) flagella, sweeping in food (bacteria, organic matter and phyto- and zooplankton) and oxygen and creating an excurrent stream flushing out carbon dioxide, other wastes and occasionally gametes. Poor nutrition is a close runner-up to inappropriate collection as the number one source of sponge mortality. Some writers suggest not feeding tropical reef sponges, but unless your system is established well enough (from Live Rock and Sand) to support its other filter feeding organisms without supplemental feeding, I would; but sparingly.
Feeding is accomplished by periodically temporarily turning off power filters, if any, and introducing a food-broth in the area around or in the case of larger specimens, into the sponge(s). The actual particles ingested are tiny (microscopic). Mancini suggests liquid foods (Liquifry (tm), fresh juice from mollusks (for human consumption), foods for newly hatched brine shrimp, dried or live Chlorella, and finely divided dry prepared foods. These foods are made into a suspension (I suggest using a blender) with tank water and applied at the animal's surface with a baster or syringe. More frequent (several times daily ideally) feedings are preferable. Omitting a too-clean particulate filter mechanism in systems with considerable sponge bio-mass is endorsed. Cleanliness is not sterility!
Again, there are some hermatypic sponge species that derive up to 100% of their nutrition from "making food with light". These organisms' zooxanthellae require intense full-spectrum lighting as other photosynthetic reef life.
Sponges are no longer considered delicate, short-lived, dangerous life forms by reef aquarists. You should recognize that they are an important part of all natural tropical shallow marine environments and encourage them even if only as bioassay organisms, incidental food, and accidental filter adjuncts.
Should you want to keep purposeful sponge material remember to select the tougher aquarium species with some substrate attached, not expose them to air, predatory tankmates.... keep them free from debris and in areas of good circulation.
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