It is ironic that biologically diverse habitats are usually rather poor in nutrients. Coral reefs are one of them. Crystal clear waters of the tropical seas is a “clear” indication of nutrient poor environment. Because nutrients in the water column are scarce microscopic plants and animals (planktons) that form the basis of food webs cannot maintain high numbers and the water column remains clear. Dissolved organic carbon is a nutrient that is inedible for most organisms living in a reef.
Evolutionarily, sponges are considered to be the oldest and most ancestral surviving species of the multicellular animal lineage going back to more than 750 million years ago. They are notorious filter feeders. They have an extraordinary capacity to filter dissolved nutrients through a specialized group of cells called choanocyte. Encrusting sponge tissues are made of choanocytes forming canals that converge at the openings called oscula where filtered water is discharged like a chimney.
A species of Star Encrusting Sponge Halisarca caerulea grows in the deep dark cavities beneath reefs, and 90% of their diet is composed of dissolved organic carbon. The sponge must filter vast amounts of organic carbon in order to survive in the nutrient poor environment it lives. Every day these sponges consume half of their own weight in organic compounds, yet they hardly ever grow. Jasper De Goeij and his study group started studying these sponges in Dutch Antilles. Sponges collected from reefs were placed in small chambers and were exposed to 5-bromo-2′-deoxyuridine (BrdU). The BrdU is incorporated into the DNA of dividing cells if they take up carbon and grow. However the chemical label was nowhere to be found. Cells were very quick to divide (in less than 4.5 h) yet no carbon was stored.
It turned out that choanocytes were shedding at a very high rate. Tiny piles of brown material could be seen next to the sponges growing in enclosures every morning. The sponges were shedding the newly divided cells providing reef residents food to consume. In this way, a significant resource found diluted in water column is concentrated for utilization of higher trophic levels such as reef fish.
Major groups of reef sessile animals, such as polychaetes (tube worms), bivalves and gastropods, barnacles, sponges, tunicates and corals can filter and accumulate particles from the surrounding water column. Sponges have a much greater role in reef ecosystems than previously known. They are especially efficient in converting much finer dissolved organic carbon into edible particles, providing an important food source for other organisms.
In terrestrial environments our understanding of biomass production is rather dominated by primary producers like plants. We do not have an analogous organism that function by filter-feeding like the sponge. Instead we have super-organisms like ant colonies collecting and concentrating large amounts of nutrients into point sources. For instance, take the fungus gardens grown by the leaf cutter ants. Leaves widely available but largely inedible are collected throughout the forest column and fed to the subterranean fungus. Fungus metabolizes the leaf material and produce fruiting bodies which ants feed on. The sponge and the fungus consume vast amounts of carbon but don’t grow. Instead feed others.
Knowledge of sponge biology and ecology has direct effect on management of aquatic ecosystems. When designing artificial reefs special effort should be made to create habitats suitable for their proliferation. Vertical hard structures can create upwelling water movement carrying dissolved nutrients. Algae have hardtime growing on vertical surfaces simply because of the diminished angle of light driving photosythesis. Plant life therefore are limited to flat sunny surfaces on reefs. Sponges turn non-productive surfaces into an extra source expanding the base of food chain.
This documentary was recorded by Mert Gökalp in Curacao last September 2012. It was aired on Netherlands’ TV broadcaster NTR-HET KLOKHUIS in 31st of October same year. Gökalp is also the author of an underwater field guide book covering species found along Turkish coast.