The Beauty of the Less Complex: An Introduction to Sponges

by Alyssa Allchurch

Everyone loves sharks, turtles and manta rays but often the less charismatic species are ignored. One of these easily glossed over group of organisms that I want to delve into today, are sponges. With over 5,000 species stretching from the deep oceans to coral reefs, these abundant animals are a common site in our oceans. The phylum Porifera comprises the oldest known animal life present on earth.  With some sponge-like species dating back 650 million years (Maloof et al., 2010). That makes them older than trees! Sponges are the simplest of any multi-cellular organism. This lack of complexity stems from not requiring any organs or central nervous system. Instead of organized tissues, sponges depend on groups of specialized cells to function. These cells are only loosely organized, allowing sponges to have the miraculous ability to reform themselves after being poured out of a blender.

One of the important cells, Choanocytes, line the inner portion of the organism through which water flows. These cells aid the movement of water through the beating of their tails, which create currents. This water movement allows for the trapping and ingesting of food particles. Several gallons of water can be circulated a day, with certain species being able to filter up to 50,000 times their own volume (Reiswig 1971). The skeleton of the sponge is maintained by spongin, a soft flexible fiber, and spicules, made of a harder mineral that supports the sponge’s structure. Not all sponge species have both spicules and spongin, allowing for the variety between hard and softer sponge species. These differences between the presence or absence of spicules and spongin differentiates the four classes of sponges; calcareous sponges (Calcarea), glass sponges (Hexactinellida), demosponges (Demospongiae), and encrusting sponges (Homoscleromorpha).

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Sponges can be found in a wide range of habitats, mostly in saltwater, with some specialized species in freshwater. They are also extremely long-lived animals, with the oldest individual thought to be over 2300 years old. Within their given environments, sponges are important nutrient cyclers. Their primary food choice is dissolved nutrients and detritus, which allows them to be great at improving water clarity. Most ocean environments are low in nutrients, meaning slow growth for sponges. However, in recent years, human activities have been offloading huge amounts of nutrients into marine environments. These large nutrient influxes can cause massive sponge growth and even lead to their domination over other sessile organisms, such as corals. This phenomenon is becoming more common due to industrial farming, which continues to use huge amounts of fertilizer that often, through run-off, end up in our oceans.

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The delicate balance of an ecosystem, once affected by nutrient influxes, may sometimes be shifted back through sponge predation. However, sponges are preyed upon by only a limited number of organisms. This is mainly due to defensive toxins that make digestion difficult. This defense mechanism does not dissuade organisms such as hawksbill turtles, nudibranchs and certain fish species. These predators can withstand the toxins, and in the case of nudibranchs, can actually incorporate them into physical defensive mechanisms. Some of these toxic substances are currently being researched by scientists for their potential in pharmaceutical and biotechnology industries.

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We have only begun to understand the role sponges play in the ecosystem, especially in intricate ecosystems such as coral reefs. In areas where sponge overgrowth causes mass coral mortality, understanding the physiology and ecology of sponges can help inform decisions by conservationists. We at ATMEC are excited to be continuing sponge research, because before you can help an ecosystem you must understand it.

References

• Maloof, A., Rose, C., Beach, R., Samuels, B.M., Calmet, C. C., Erwin, D. H., Poirier, G. R., Yao, N., Simons, F. J. 2010. Possible animal-body fossils in pre-Marinoan limestones from South Australia. Nature Geosci 3: 653–659.
• Reiswig, H. M. 1971. In situ pumping activities of tropical Demospongiae. Marine Biology 9: 38-50.