Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 140

ecosystems some 580 million years ago, but recent finds of fossils
from southern Australia, and of chemical traces of sponges from
rocks of the Arabian Peninsula, 83 may push this date back further
to at least 650 million years ago. This is important, because it sug-
gests that the development of multicellular life in the oceans
occurred during the period of Earth's history that is characterized by
the intense, world-embracing Late Proterozoic snowball glaciations,
when ocean productivity and the nutrient cycle may have been
severely curtailed.
Sponges play a wider role. Before they appeared in Proterozoic
oceans there were no suspension-feeding organisms, and so the seas
would have been commonly turbid from clouds of suspended living
and dead cells and microscopic organic debris. By themselves, such
tiny, low-density particles settle to the sea floor only extremely slowly,
and so can accumulate within the water. Sponges filter-feed particles
from water down to cyanobacterial size, and as a result they remove
turbidity from the water. How does that help other organisms to
thrive? The Proterozoic ocean biosphere largely consisted of cyano-
bacteria that lived in the surface waters. Because of their small size,
and the absence of mechanisms to concentrate them into clumps,
much of the organic material in Proterozoic oceans would have
remained buoyant and near the surface. This also reduced the pene-
tration of light into the oceans and therefore limited the development
of larger eukaryotic and multicellular photosynthesizing organisms.
Sponges not only clean the water, but they accumulate a large
quantity of organic matter that other organisms can use for food; 84 in
some settings sponges accumulate perhaps as much as half their body
mass each day. They can use the dissolved organic matter in seawater
to build organic tissues, and they then expel these tissues as filter cells
that can be consumed by other reef organisms. Sponges may have
been integral to increasing the flow of energy from one trophic level