Apple #573: Venus' Flower Basket

I just got back from a good but tiring vacation. I must get some sleep, but before I do, I want to find out about something I encountered on my vacation -- a Venus' Flower Basket.

I was in a natural history museum when I saw the skeleton of this thing. The note next to it said that the skeleton is made of silica -- glass. My jaw dropped. A living creature's skeleton is made of glass? I must know more.



The bleached skeleton of a Venus' flower basket, or a glass sponge. The fibers are made of silica, a.k.a. glass.
(Photo from the Natural History Museum, London)

• This animal -- yes, it's an animal -- is a type of sponge.

• Venus' flower baskets (Euplectella or Eupectella aspergillum) live in the ocean where the water has lots of silica and is very cold, which often also means that it's very deep, anywhere from 10 to 5,000 meters below the surface.

• They live mainly in the South Pacific off the coast of Japan and the Philippines, but they've also recently been found off the coast of Australia.

• When they're alive, the sponges can range in color from white to creamy yellow. The "skeletons" that are often displayed in museums are typically the tissue of the sponge which has been dried and bleached to give it an even whiter color.

Three Euplectella living underwater. They don't live in colonies the way coral do; it's just that these three happened to set down roots near each other.
(Photo from telecomBlog)

• Their shape is what allows these sponges to survive in the huge pressures that exist at such ocean depths. How they make their shape has been the subject of a lot of research.
  

• Like all sponges, Venus' flower baskets are filter feeders. One of the things they filter from the water is silica.

• The sponge has special cells called spicules. The spicules extract silica from the seawater, and then they set the silica bits on top of a wire of collagen protein. So the structure gets built bit by bit, but very delicately. This process is known as biomineralization.

Looking down into the internal cavity of a Venus' flower basket
(Photo from Tecnologia e Desenvolvimento Sustentavel)

• Lots of sponges do this biomineralization thing. What's unique to the Venus' flower basket is the way in which it builds its layers of silica.
  

• The spicules themselves have three perpendicular rays, which gives them 6 points. As the sponge biomineralizes, it forms more spicules. It lays them down, layer by layer, each a few millimeters thick. Between each layer is a thin layer of "organic matrix" that functions like glue. There are seven different types of layers, each arranged concentrically and hierarchically.
  

• Outside the mesh of spicules is another set of cells. Some people call these filaments, some call it a trabecular net, others call it a synctium, still others call it a cobweb. Whatever you call it, these form an outer layer and they get wrapped around the mesh in various orientations: horizontally, vertically, and diagonally.
  

• This outer layer helps to make the structure a little more sturdy, which is very important since glass is very brittle, and this animal's entirely-glass skeleton has to survive at very pressure-heavy depths.

This image gives you a good sense of the underlying mesh and the syncytium that winds around the top of it.
(Photo from Richard L. Howey's page on the Euplectella aspergillum)

• "As yet," says one researcher, "it is not clear how a primitive organism can produce such a complex and optimized structure at all."

• But wait, there's more.

• Inside the mesh is an open cavity where the ocean waters wash through and get filtered. One of the things that floats into the cavity are tiny shrimp larvae.
  

• The shrimp larvae stay in there, eating happily away until one fateful day when they discover they've gotten too big to swim back out again. Sometimes a few get trapped in there, but often it will be a pair, male and female.
  

• So people have made up a quaint little story that the shrimp have found each other and live out their wedded bliss together until death do they part within the chambers of the sponge.

• This is why the sponge is named after Venus, the goddess of love.

• This is also why the Japanese often give each other these sponges (after they've been collected and dried) as tokens of love or as wedding gifts.

• What I find even more fascinating than that sentimental gush is the fact that a sponge which is entirely made of glass can not only survive super-high pressures of 1,000 meter ocean depths, but it can also survive little pincer bites from the pair of shrimp who take up residence inside its body. Tough little cusses, these underwater glass nets.

• And in real life, the shrimp and sponge live symbiotically. The shrimp clean the interior walls of their sponge-cage with their little pincers, and in return, the shrimp get to eat some of the food the sponge takes in.

Shrimp inside a Venus' love basket. Is it eternal love between shrimp, or is it really intraspecies cooperation -- which may itself be a form of love?
(Photo from Fresh Photons)

• Oh, did you think that was all these things can do? No no, my friends. There's still more.

• OK, so the sponge's eggs mature inside the females and get fertilized there, and when they become larvae, they get spewed out of the sponge. Released into the wide ocean, the larvae beat their little ciliae to propel themselves through the water, looking for their new home.
  

• These little freddies can swim for several days, but most tend to find their favorite spot on the ocean floor after about 12 hours. Once they've landed, a tuft of fibers at the end of the animal attaches to the ocean floor, and then they begin developing into adults.

• Here's the jaw-dropping part. That tuft of fibers is not just a mass of weird-looking hairs. The fibers are very fine and very long, sometimes up to 175 meters. Because they're so fine and they're made of silica, their composition happens to be a lot like fiber-optic cables. Exactly the same, in fact. Which means that these funny little hairs sticking out from the end of this animal can trap and transmit light.

Most of the bleached skeletons have lost the little fibers at the end, but here they're intact and very visible. This sponge has tons of those little hairs. Which can act like a fiber optic cable.
(Photo by Ryan Somma, sourced from oneworldoneocean.org)

• Wait, did I say they're exactly the same as our fiber optics? I was wrong. They actually work better than the fiber optic cables we make. And the animals are more efficient at making their fiber optics than we are.
  

• No one is sure if the sponges use their fibers' ability to transmit light for a particular purpose and if so, what that purpose might be. Some researchers speculate that those fibers are how the animal attracts one of its favorite foods, which is algae.
  

• Or perhaps the light is what attracts those shrimp larvae, which are bioluminescent. This theory is that the shrimp see the light from the little hairs and, thinking it's more shrimp, swim inside. Thus the sponge gains its symbiotic partners.
  

• Oh, and by the way, what with those bioluminescent shrimp in there and the fiber optic hairs floating off the bottom, these creatures glow in the super-dark that is the very deep depths where they live.

Like all sponges, these animals have no brain, no nervous system at all, no organs like a heart or stomach or kidney. They don't even have that many types of cells. Yet they build their own bodies in these complex, mathematical shapes, they make their own fiber optic cables, they glow in the dark, and they house a pair of shrimp for life to boot. Pretty incredible stuff.


Sources
Natural History Museum of London, Eupectella aspergillum (Venus' flower basket)
Encyclopedia Britannica, Venus's flower basket
Max Planck Research, Secrets of the Venus' Flower Basket, April 2005
Queensland Museum, Animals of Queensland, Unique features of sponges, Organic & inorganic skeletons
University of Michigan Museum of Zoology, Euplectella aspergillum
Una esponja submarina genera fibras ópticas mejores que las industriales, telecomBLOG
Peter Weiss, Channeling light in the deep sea, Science News Online