Ancient shape-shifters

Giovanna Fasanelli | VOLUME 30, ISSUE 5

Don’t be fooled by their simple appearance – sponges are intricate organisms perfectly adapted to their habitat.

The humble sponge. There doesn’t seem to be a lot going on when one gazes at their strange, motionless form, apart from a pleasing array of fashionable colours and textures. But upon closer inspection, at the microscopic level, the true wonder of these ancient life forms is revealed.

A sponge is a colonial, filter-feeding organism comprising countless individual cells assigned to different functions: some are used as protective support, some are involved in the mechanics of feeding or in nutrient and waste transport, and others in reproduction. Some cells are totipotent, meaning they’re capable of morphing into any kind of cell that is needed to keep the colony healthy and happy.

Holding all this together in the middle is a gelatinous, collagen-filled matrix containing tiny, species-specific needles, known as spicules, as well as stiffening spongin fibres, all of which help to provide necessary structural integrity to the whole.

Local environmental conditions, especially those dictating current flow, will alter the shape into which a sponge grows. The same species that normally forms small, tube-like fingers in calm conditions may respond to a habitat of strong currents by growing into an encrusting layer over the reef’s surface. Either way, the aim is to create a form that allows for maximum exposure to water flow, without tearing or breaking apart.

Sponges can grow to be massive, hulking frameworks seven feet high, sheltering a myriad of other creatures from predatory snails and colourful crabs and shrimps, to brittle stars, sea cucumbers and fish.

Other than the contributions by photosynthetic algae embedded within the tissues of some sponges, the food used to fuel these highly intricate colonies are morsels of planktonic particles drawn into the sponge via innumerable channels known as ostia, which pepper the sponge’s exterior. Lining the internal surfaces of the sponge are armies of specialised feeding cells, known as choanocytes, which use their beating, whip-like cilia to funnel the oxygenated and nutrient-rich seawater around the internal chambers of the sponge. And for those cells that are hard at work elsewhere in the colony, the totipotent archaeocytes deliver food packages from the choanocytes, ensuring these remote specialists continue performing their specific duties. It really is the most miraculous biological machine.

Perhaps as remarkable is the fact that, were you to force a sponge through a very fine sieve, such that only a milky substance oozes out of the bottom, you could watch under a microscope as the individual cells reorganise themselves, until a recognisable sponge is eventually restored. A real live Terminator!

The fascinating story of these creatures doesn’t end there. Their talent for chemical warfare cannot be overstated and is currently a source of much excitement in the pharmaceutical industry. Being glued to the seafloor for the better part of 600 million years, sponges have evolved highly effective strategies of fighting for the best spaces, and for keeping at bay most predators and fouling organisms. Some sponges are even capable of chemically boring into shells, rocks, corals and reef.

All of this activity has yielded over 5000 compounds of interest to scientists, who have used them to fashion everything from antimalarial, antifungal and antiviral agents, to anticoagulants and antitumor drugs. It’s amazing to consider that our fight against cancer is growing in confidence with the help of the humble sponge’s magical powers.

So next time you scrub down the dirt of the day with your trusty ol’ bath sponge, spend a few seconds contemplating the true value of this seemingly simple creature.

Creature Features