Concentric Layers Strengthen Cables

white floral sea sponge against black background

Biological Strategy

Venus's flower-basket

Eleanor Banwell

Image: Arctium Lappa / Shutterstock / Some rights reserved

Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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Sponges

Phylum Porifera (“pore-bearing”): Sponges

Sponges may seem simple, but there’s more to them than meets the eye. True, they don’t have nerves, muscles, or organs. But they do have unique, specialized cells that help them survive. Collar cells have long tails or flagella that line the inner sponge cavity and funnel water through the body. Sponges are filter feeders, taking in large amounts of water and trapping tiny particles to eat. They also have small spiky spicules made of silica or calcium carbonate that help them keep their structure. There are at least 5,000 known species of sponges living in aquatic systems, with 98% of them in the ocean.

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Glass cables that anchor sponges strengthened by concentric layering

Sponges are some of the simplest animals. They have no internal organs and collect all their food and oxygen by filtering it from water. Like most sponges Euplectella aspergillum, or Venus’ flower basket, works like a chimney, taking water in at the bottom and then ejecting it through the opening at the top and through the pores in its body. All sponges have skeletons and in the case of Venus’ flower basket and the appropriately named “glass sponges,” that skeleton is made from needle-like shards of glass.

Glass sponges live in very deep water where they must be capable of remaining anchored to the soft sediment of the deep ocean floor. Euplectella aspergillum anchors itself with numerous hair-like glass cables called spicules, each up to 10 cm long. The spicules are bundled together where they attach to the sponge’s body, forming a thick cable, but further from the sponge’s body they spread out singly into multiple tiny threads that run through the sediment.

Anchoring spicules must resist tension and be able to cope with uneven loads as the sponge moves in the ocean current. Glass is a brittle and inflexible material that is not traditionally associated with applications like cabling. Glass sponges cope with this issue by creating a glass composite material. Spicules have a solid glass core surrounded by up to 50 concentric layers of glass with a biological layer sandwiched in between each one. The concentric glass layers give strength under tension to the cables, while the biological layers act as a glue, prevent cracks propagating through the entire cable thickness, and give the spicules flexibility.

Concentric layers of glass in spicules are of different thicknesses. Near the core, layers are relatively thick, while closer to the surface of the cables, glass layers become thinner, with an increasing percentage of the cable being made of the flexible biological layer. This arrangement helps to accommodate the greater movement that occurs at the surface of the cables.