An image of parallels and asymmetry.
In normal, human, terrestrial, domestic urban or rural life, the concept of a chimney is nothing special to people. To some, the romanticised idea of a warm fireplace, to some bringing about thoughts of Christmas, and others simply curiously wondering at such ancient forms of energy acquisition. On a larger scale, like in industry, chimneys are seen as a symbol of pollution and chemical toxicity, with factories pumping clouds of black smoke into the atmosphere to produce whatever they produce. Or even larger icons of destruction, are the monstrous chimneys of Nuclear Powerplants, massive funnels billowing steam reminding us of scale of energy contained and being produced within the facility.
This design, to eject masses of by-products into surrounding environment, long pre-dates human thinking. Coral reefs around the world are filled with its own chimneys, often well over the height of the tallest ape and sometimes several meters in circumference, Barrel Sponges are the towering columns of the marine world. These structures are some of the simplest, and are descendants of some of the oldest forms of life on our planet, and they serve an often overlooked role in saving coral reefs. Above the waves, chimneys direct waste gasses into the air above to dissipate over a wider area and the same mechanism is utilised by the barrel sponge on coral reefs. The reefs of Koh Tao have a diverse range of sponges but none are more impressive and dominating the reefscape than the Barrel Sponge (in this case, Xestospongia testudinaria).
Barrel sponges as the name suggests are a type of sponge, classed in the Phylum Porifera which amongst many things, are recognised by their lack of symmetry in body shape. These simple unicellular organisms form dense colonies, with each individual having a particular role in an overall system, the colonies can grow to be surprisingly complex and large. Sponges are primarily filter feeders, in that they draw in water, feeding on the organic matter within it, before expelling the now nutrient deficient water back into the ecosystem. The largest and most productive of these, are the barrel sponges. Estimates suggest that the largest barrel sponges can filter many thousands of litres of water in a single day. It draws in water through pores on its thick wall, separating nutrients, before expelling nutrient deficient water back into the ecosystem, allowing this low nutrient water to dissipate and spread bringing down the nutrient levels for the whole reef just a little.
Every coral reef in the world faces a never ending battle to survive and grow, one of many, which is the battle against overgrowth. Corals are in continuous competition with algae, tunicates and many other forms of life that respond very positively to an increase in nutrients, including sponges. An increase in nutrients in the ecosystem allows more resources to be available to these organisms which undergo an explosion in growth, whereas coral polyps and their zooxanthellae can make little use of this increase. Macroalgae being a key player in this competition responds by out-competing and often overgrowing coral colonies and surrounding substrate they may be able to grow or recover on. Though uncommon, rapid coral overgrowth by sponges has been witnessed at a few locations before, and is thought to be driven by the same nutrient dependant mechanism that fuel’s the growth of other mentioned fouling organisms.
With an increase in development and marine pollution, the amount of nutrients entering the marine ecosystem are increasing at a rate often unchecked, leaving few mechanisms to cycle these nutrients before they fuel an overgrowth. Historically rapid increases in nutrients would have been to much smaller scales, much more localised and far less frequent, however with rapid anthropogenic growth, a wide ranging continuous supply of nutrients is found at most coral reef locations in the world. Additionally, in the past, powerful filter feeding organisms were significantly greater in number, such as Giant Clams of the genus Tridacna, most of which are now threatened due to the culinary and souvenir trade. Today relatively few barrel sponges and large clams remain throughout many coral reefs, particularly in South East Asia. You can read more about Giant Clams, sponges and many others at our key invertebrates page here.
Beyond the exceptional expulsion powers of the colonies, the sub-marine chimneys also act as a sort of faunal pitcher plant. The colonies attract a number of organisms due to their structure and size, contributing immensely to the structural complexity of the reef, in turn acting as attractive habitat for vertebrate and invertebrate species alike. The sponges may even be said to support their own miniature micro-ecosystem, complete with food web. As fish on the reef hide inside the sponge for various lengths of time, they give into ‘natures call’ and make indiscriminate depositions of fecal matter into the base of the colony. This in turn attracts scavangers and deteritivores, such as invertebrates like small shrimp and hermit crabs. This then attracts larger predators like larger crustaceans and smaller fish, which may in time attract predators of those fish.
The image at the top shows two large barrel sponges, one that has been half destroyed. However sponges are resilient animals, capable of surviving physical damage remarkably well and retaining function during and after severe physical damage (cue story of the sponge colony in a blender….or just google it). They are after all, colonies, so as long as there is structure to their form, which can be rebuilt after destruction, the colony can continue its industrious existence. All sponges in the image are still functional, and actively pulling away the nutrients that we are passively pumping into the ecosystem. To prevent risk of anthropomorphising them, filter feeders do this through no act of altruism and that should not be forgotten, but the value they provide to coral reefs is often forgotten and for that, we are at continuous risk of taking for granting the remaining few clams and sponges that benefit the coral reefs.