Barbed Filaments Anchor

close up photograph of a white sea sponge against a black background

Biological Strategy

Venus's flower-basket

Eleanor Banwell

Image: Arctium Lappa / Shutterstock / Some rights reserved

Attach Permanently

A living system can conserve energy by attaching permanently to a particular site because it can take advantage of resources that come its way, rather than expending energy to move to resources. A permanent attachment, intended to last the lifetime of the living system, creates special challenges. For example, physical mechanisms, such as the anchor that holds a marine algae to the ocean’s bottom, must be able to withstand forces that can pull it off its substrate. Chemical mechanisms, such as a barnacle’s glue, must avoid both physical and chemical breakdown, such as being dissolved by water.

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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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Filaments anchor sponges in soft sediments using barbed tips

Sponges are some of the most simple of animals. They lack body symmetry and have no organs, getting their food and oxygen from the water. Most sponges work like chimneys, taking water in at the bottom and then ejecting it through the opening at the top and through the pores in their bodies. All sponges have skeletons and in some, the appropriately named “glass sponges”, the skeleton is made up of 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, Venus’s flower basket, anchors itself with numerous hair-like glass skeletal elements 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 they spread out singly into multiple tiny threads that run through the sediment. Each spicule has a series of barbs along its length and at the terminal end is a crown of barbs that works much like a sea anchor, holding the sponge steady in the soft shifting sand and silt.

Like many interactions in nature, Venus’s flower basket makes use of very large numbers of relatively weak attachments that when combined form a strong bond. In this instance, where the substrate itself is very weak and a single strong bond would not help, this method is particularly useful.