The question of how much deeper the ocean would be without sponges, members of the phylum Porifera, is complex. Sponges have existed for approximately 600 million years, and their sheer numbers mean they are not passive inhabitants of the seafloor. Over deep time, these organisms have significantly altered the ocean’s topography and crustal composition through the materials they incorporate and the processes they facilitate. While the total physical difference in ocean depth is difficult to calculate precisely, their contribution is measurable on both a localized and geological scale.
The Physical Contribution of Sponges to Ocean Structure
Sponges impact the physical structure of the ocean floor primarily through their skeletal elements, known as spicules. These minute structures are composed of either calcium carbonate in calcareous sponges or opaline silica in glass sponges and demosponges. When a sponge dies, its soft tissues decay, but the mineralized spicules are highly resistant to dissolution and become part of the marine sediment.
Siliceous spicules represent a major sink for silicon in the global ocean. These remnants accumulate in enormous masses, forming structures like spicule mats or geological formations called spiculites. In certain areas, the skeletons of glass sponges can fuse together to create massive, reef-like structures known as bioherms, which significantly elevate the local seabed.
Conversely, some sponges actively contribute to the breakdown of existing structures through bioerosion. Excavating sponges, such as those in the genus Cliona, bore into calcareous substrates like coral skeletons and mollusk shells. They achieve this by chemically etching and mechanically chipping away tiny fragments of calcium carbonate, releasing fine silt-sized sediment. This process can be highly destructive, with some species capable of eroding over 20 kilograms of substrate per square meter annually.
Addressing the Hypothetical Depth Change
Quantifying the global depth change caused by the absence of sponges is challenging because the contribution must be viewed across vast geological timescales. On a localized level, the impact is profound; for example, massive glass sponge bioherms have been found to rise as much as 25 meters above the surrounding seafloor. Removing these localized structures would immediately deepen the seabed in those specific areas by dozens of feet.
Averaged across the entire planet, however, the direct global increase in ocean depth would be negligible, likely measured in millimeters or a few inches at most. The true scale of their impact is evident in the geological record, which spans the entire Phanerozoic Eon. Over hundreds of millions of years, the accumulation of siliceous sponge spicules has been a primary source material for geological formations, including chert and vast spiculite beds.
Sponges facilitate a global burial flux of silicon, representing a significant output from the marine silicon cycle. Their skeletons are far more resistant to dissolution than those of planktonic organisms like diatoms. This means a higher percentage of their silica is permanently locked away in the crust, and the immense volume of this sedimentary rock, built over eons, has permanently altered the density and thickness of the Earth’s oceanic crust and continental shelf deposits.
Ecological Consequences Beyond Physical Depth
The absence of sponges would create immediate and profound changes in the marine environment that extend far beyond a measurable increase in ocean depth.
Filter Feeding and Water Quality
Sponges are highly efficient filter feeders, pumping large volumes of water through their bodies to extract bacteria and organic particles. This process plays a substantial role in regulating water quality and clarity across diverse marine habitats.
Nutrient Cycling
Their feeding habits also make them important agents in nutrient cycling. Sponges process carbon, nitrogen, and phosphorus, then release waste products in a form readily available to other organisms, often referred to as “sponge loops.” This process fuels productivity in nutrient-poor environments, particularly on coral reefs, where they can be the dominant biomass.
Habitat Provision
Sponges also serve as essential habitat providers and ecosystem engineers. Their complex, three-dimensional bodies offer shelter, refuge, and nursery grounds for countless small invertebrates, fish, and other organisms. A sponge-less ocean would suffer a dramatic loss of biodiversity and a fundamental disruption of the food web.