The silica skeleton of the Venus’ flower basket sea sponge is tough and stable because multiple levels of organization each help to manage forces.

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Venus’ flower basket (Euplectella aspergillum) is a marine animal that lives anchored to the deep ocean floor near the Philippines. Looking more like delicate sculptures than animals, these tube-shaped sea sponges typically stand 10-30 cm tall and filter tiny food particles from the seawater as it flows through their bodies. Also known as glass sponges, their cylindrical skeletons are made out of silica, the main component of glass. While glass is normally a brittle and fragile material, the Venus’ flower basket’s skeleton is tough and stable owing to its composition and how it’s organized. There are at least six levels of organization in the skeleton that span from nanometers to centimeters in size.

The sponge’s glass skeleton is made up of spicules, tubule structures of concentric layers of amorphous hydrated silica separated by thin organic layers, like a Parisian pastry with just a tease of sweet cream between flaky crusts. But these thin organic layers go a long way to impart the spicules with considerable toughness. Even the pair of symbiotic shrimp that live their lives trapped within each Venus’s woven glass basket can’t break out. Unlike biomineralization in other organisms such as the abalone, the mineral portion does not appear to have a regular crystalline pattern. Experiments suggests that the silica layers are made up of colloidal spheres of silica about 50 to 200 nm in diameter, which are in turn made up of smaller spheres about 2.8 nanometers in diameter. By comparison, the smallest sand grains on a beach (also usually silica) are about 60 nm in diameter.

Each spicule consists of alternating layers of inorganic silica and organic compounds, all around a central protein filament. The inorganic layers are made from hydrated silica nanoparticles and are relatively stiff. The organic layers, however, appear to be weaker and able to absorb energy. This laminated organization of alternating stiff and weak layers can prevent cracks at the surface of a spicule from spreading deep into the core.

At a higher level of organization, spicules are arranged into a square lattice rolled up into a tube. This is the main shape of the glass sponge. Two separate but overlapping lattices make up the main frame, and because these lattices can still move relative to one another, the skeleton can be flexible while it’s growing. The squares of the lattice are reinforced by struts that run vertically, horizontally, and diagonally. These struts are made of bundles of spicules and further support the lattices against bending, sliding, and twisting forces. Helical ridges made of spicules form on the surface of the tube-shaped structure and spiral around in opposite directions. These ridges also help the skeleton resist crushing or twisting forces.

A cap at the top of the cylinder keeps it from collapsing, while a flexible bundle of anchor spicules keep the whole skeleton attached to the ocean floor and able to withstand forces coming from the side. Finally, a silica matrix with small spicules embedded throughout cements the whole structure together and further increases strength.

Each hierarchical level of organization in the Venus’ flower basket’s skeleton contributes to its overall mechanical performance. The result is a complex structure that’s tough and stable even though its main ingredient is a naturally fragile material.

To learn more about hierarchical structures in various living systems, check out the case study, “Little Things Multiply Up: Hierarchical Structures” in Zygote Quarterly 9:

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References

“Despite its inherent mechanical fragility, silica is widely used as a skeletal material in a great diversity of organisms ranging from diatoms and radiolaria to sponges and higher plants. In addition to their micro- and nanoscale structural regularity, many of these hard tissues form complex hierarchically ordered composites. One such example is found in the siliceous skeletal system of the Western Pacific hexactinellid sponge, Euplectella aspergillum. In this species, the skeleton comprises an elaborate cylindrical lattice-like structure with at least six hierarchical levels spanning the length scale from nanometers to centimeters. The basic building blocks are laminated skeletal elements (spicules) that consist of a central proteinaceous axial filament surrounded by alternating concentric domains of consolidated silica nanoparticles and organic interlayers. Two intersecting grids of non-planar cruciform spicules define a locally quadrate, globally cylindrical skeletal lattice that provides the framework onto which other skeletal constituents are deposited. The grids are supported by bundles of spicules that form vertical, horizontal and diagonally ordered struts. The overall cylindrical lattice is capped at its upper end by a terminal sieve plate and rooted into the sea floor at its base by a flexible cluster of barbed fibrillar anchor spicules. External diagonally oriented spiral ridges that extend perpendicular to the surface further strengthen the lattice. A secondarily deposited laminated silica matrix that cements the structure together additionally reinforces the resulting skeletal mass.” (Weaver et al. 2007:93)

Journal article
Hierarchical assembly of the siliceous skeletal lattice of the hexactinellid sponge Euplectella aspergillumJournal of Structural Biology, 158: 93–106April 1, 2007
Weaver JC; Aizenberg J; Fantner GE; Kisailus D; Woesz A; Allen P; Fields K; Porter MJ; Zok FW; Hansma PK; Fratzl P; Morse DE

Journal article
Skeleton of Euplectella sp.: Structural Hierarchy from the Nanoscale to the MacroscaleScience, 309(5732): 275-278July 8, 2005
Aizenberg J; Weaver JC; Thanawala MS; Sundar VC; Morse DE; Fratzl P

Journal article
Micromechanical properties of biological silica in skeletons of deep-sea spongesJournal of Materials Research, 21(8): 2068-2078August 1, 2006
Woesz A; Weaver J; Kazanci M; Dauphin Y; Aizenberg J; Morse D; Fratzl P

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Venus's Flower BasketEuplectella aspergillumSpecies


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