Coral reef aquaria, and the science of maintaining them, have advanced by leaps and bounds in the last decade. Much of this owes to progress in the area of foods and feeding. Many marine creatures (especially filter-feeding invertebrates like sponges) that were once deemed difficult or even impossible to keep are now cultured with regularity. These developments will surely result in a much greater selection and availability of “oddball” aquarium livestock in coming years.
Sponges, at least as a group, are not rare in any way. Indeed, they are found in great abundance and diversity on most coral reefs. If anything, they are rather poorly represented in the aquarium hobby. Despite their sometimes stunning beauty, they are unusual to aquarists for one reason: They have a really, really bad record of survivability in captivity. This low success rate is attributable to several things. Sometimes, it’s as simple as bad pairing. For example, a single pomacanthid angelfish can consume a large sponge in a matter of days. Other reasons are not always so obvious; too often, improper harvesting/handling practices (e.g. air exposure) dooms an otherwise healthy specimen before it even reaches a retailer. But the biggest longstanding concern for captive sponges is nutrition.
To address this concern, it may be helpful to understand the basic anatomy of these “animals” and how they feed.
Many many mouths
Of all animals, sponges (Phylum Porifera) are most closely related to the colonial choanoflagellates that are the progenitors of the animal kingdom. These very primitive creatures are so simple in structure that they really are a lot more like colonies than distinct organisms. They lack nerves and muscles. In fact, overall, their cells are so unspecialized that they lack (at least in the opinion of some biologists) true tissues. In a lot of ways, sponges are really just a big pile of loosely associated cells.
That being said, individual cells can not only sense and react to environmental stimuli but communicate with other cells. The simplest of sponge bodies are composed of a hollow sack or sheet that is perforated with numerous tiny pores. The body is more or less composed of just two kinds of cells, the flagellated choanocytes and the wandering amoebocytes. The sponge draws water into itself through these pores whereupon it enters a central cavity called a spongocoel. From there, water exits through a duct called an osculum.
Alright, it can get a little bit more complicated than that. More advanced sponges exhibit modest structural complexity in the form of distinct body shapes (candelabras, vases, etc.), folded body walls, highly branched water canals, multiple oscula, etc. But the body plan of all sponges is very much centered on their feeding activity.
Sponges are (with few exceptions) filter-feeders. Aside from the frenetic activity of their many flagella, sponges are almost completely incapable of any kind of deliberate movement. Thus, to eat, they must push tiny morsels to and through their bodies via miniscule, self-generated water currents. It is the choanocyte that pushes water and traps food particles; it is the amoebocyte that engulfs and digests them. A sponge must, on average, filter approximately 275 gallons of water to capture enough food to gain three ounces of biomass.
Feeding your captive sponges
The vast majority of sponges are nanoplanktivores, feeding upon the tiniest of plankton. This group includes bacteria and the smallest species of phytoplankton. In some cases, minute bits of particulate organic matter may be consumed. In just a few cases, sponges can harbor zooxanthellate algae much like corals and tridacnid clams.
Aside from good water quality and strong water movement, captive sponges need lots and lots of planktonic foods of the appropriate size. Some of this food certainly is produced in situ, as most systems have at least minimal amounts of bacteria and phytoplankton in the water column. Blooms of heterotrophic bacterioplankton, for example, take place for short periods of time following carbon dosing. Of all the planktonic organisms in aquarium water, however, nanoplankton are the first to be removed/killed by UV sterilization and protein skimming. And then, of course, there is competition from other filter-feeders. Corals, clams, tube worms and other filter-feeders can very selectively (and very quickly!) clear the water of choice planktonic food items.
For these reasons and more, successfully maintaining sponges long-term in aquaria requires that foods be added frequently. This usually cannot be the willy-nilly addition of artificial, liquid “invert” diets. While each sponge species has its own dietary requirements, sponges generally demand a live nanoplankter. Such can be supplied with the nutritious bacteria (Rhodopseudomonas palustris) in PNS Probio or PNS YelloSno by Hydrospace LLC, or with the very small algae (Nannochloropsis gaditana) in OceanMagik. Used as directed, these products likely supply sufficient nourishment for a single specimen in the average system. However, in a system that is packed with sponges and/or other fine filter-feeders, more frequent additions or even continuous dosing may be necessary!
The future for aquarium sponges
There are many species of sponge that show up (albeit sporadically) in the trade. The red finger sponge (Haliclona rubens), for example, appears from time to time. Their curious and famous ability to regenerate from small fragments might cause one to imagine a day when they will be sold on small plugs. Attractive, fascinating and rewarding, these simple animals could become far more popular as quality live aquarium foods continue to become available.