Marine sponges are some of the oldest living animals on earth, with a fossil record that dates back about 600 million years to the Precambrian period, the earliest period of life on Earth. There are approximately 8,550 living species of sponge that have been identified and they are classified in the phylum Porifera. This phylum comprises four distinct classes: Demospongiae (containing about 90% of all sponges), Hexactinellida (glass sponges), Calcerea (calcareous sponges), and Homoscleromorpha.

Although sponges and corals are often discussed in the same conversation due to them both being immobile and sometimes occurring in the same habitats, they are vastly different animals, with a distinct anatomy, feeding methods and reproductive processes. Sponges live in diverse habitats, such as coral reefs, the tropics, the deep sea and even fresh water.

Sponges have an intricate skeletal type that adapts well to its particular habitat, allowing it to live on both hard substrates and soft sediments. Water flows through a sponge’s porous exterior, and inside the sponge cavity, small hair-like structures called flagella trap food and also filter bacteria out of the sponge’s cells. Their unique skeletal structures help sponges withstand the high volume of water that flows through them daily.

Glass Sponges

Glass sponges are unique among sponges, as they lack cell membranes, and their skeletons are based on a hexagonal design. Glass sponges form a massive skeleton, made of silica, for structural support. The silica is formed into needle-like structures called “spicules”. The ends of these spicules are fused together, much like scaffolding used in construction projects, to create a sturdy lattice-like skeleton. Glass sponges are able to interact with neighboring glass sponges and gain support from them, creating a large three-dimensional reef. Even after the sponge dies, these skeletons remain intact, contributing to gigantic reefs that create a habitat for many other species.

Recently, the skeletal system of the Venus’ flower basket (Euplectella aspergillum) has received a lot of attention from engineering and material science communities due to its hierarchical architecture and mechanical robustness. This sponge species is found attached to rocky areas of the seafloor, in the western Pacific Ocean. They are deep-sea sponges, found from 100 to 1,000 meters below the surface of the ocean, and are most common at depths greater than 500 meters. The spicules of this species consists of a core surrounded by concentric layers of silica nanoparticles and thin organic interlayers. Similar grid-like open-cell lattices are commonly used in an engineering context due to their reduced weight, high energy absorption and ability to control the propagation of acoustic and thermal waves. Many lightweight bridges and skyscrapers have been inspired by the skeletal structure of these sponges.

Using Skeletal Structures Of Glass Sponges In Engineering

A recent study published in Nature Materials investigated the skeletal anatomy of E. aspergillum as inspiration for the design of mechanically robust lattice architectures. By thoroughly analyzing the skeletal organization of this species, the researchers discovered that its double-diagonal, checkerboard-like square lattice design provides enhanced mechanical performance compared to similar existing architectural structures. The sponge lattice was compared to other common diagonally reinforced square lattices, as well as a non-diagonally reinforced lattice, all with the same total mass, and it was found that the sponge design provided a superior mechanism for withstanding loads for a wide range of loading conditions.

Although it did not form a primary focus of the study, the results also shed light on functional aspects of the skeletal organization of E. aspergillum. When maturing, the skeletal design goes through two distinct phases. When it is young, it has a flexible phase, however after maturation it becomes rigid. In the early, flexible stage, the vertical, horizontal, and diagonal skeletal struts are not fused together, and can thus accommodate radial expansion of the skeletal cylinder. During this phase, the mechanical movement of the sponge skeleton is due to the properties of the individual spicules. Once the maximum size of the cylindrical lattice is achieved, the skeleton goes through several rigidification steps, which results in a progressive stiffening of the skeleton, and fusing of the ends of the spicules through the deposition of laminated silica cement.

Fluid Flows and Sponges

In another study, published in Nature, state-of-the-art fluid dynamic simulations were used to look in detail at fluid flows around and through these sponges. The results of the study showed that the sponge’s structural elements reduce the impact of continuous hydrodynamic forces on the organism and generate internal circulation patterns that are required for feeding and reproduction. In water flows of 1-10 centimeters per second, solid cylinders that are roughly the size of E. aspergillum undergo vortex shedding – a process where swirling vortexes form downstream of an object. This causes velocity fluctuations and hydrodynamic drag. The researchers discovered that the skeletal structure of E. aspergillum suppresses such velocity fluctuations and decreases drag. Considering that hydrodynamic forces can dislodge a sea sponge, reduced drag means the sponge can withstand strong flows.

Although the study greatly advanced the knowledge of fluid flow through E. aspergillum, there are still a lot of unknowns. The study was able to prove that the external ridges of the sponge increased residence times of particles in the central cavity, but the extent with which this helps with filter feeding and gas exchange is not yet certain. It is also not yet understood how the flagella, that drive fluid flow through the sponge via their movements, interact with environmentally driven flows, particularly in unsteady currents.

Conclusion

The studies published in Nature Materials and Nature can be applied to many structures in nature, and the fluid flow results can be applied to other systems of food filtering, gas exchange, drag reduction, as well as pollen capture and heat loss. The studies also reveal how complex geometries found in nature can be used by organisms to enhance their survival, such as mechanical support and drag reduction. The lessons learned from studying the skeletal structure of E. aspergillum could inspire improved engineering structures that are lightweight and structurally sound.