So why sponges?

I suppose now is as good a time as any to introduce you to the marvellous, magical things we call sponges. For one thing, it gets me away from the packing, plus I've not written any fantasy for ages. [Actually, I'll only make up one thing - see if you can spot it.*] First off, there's a lot more to sponges than meets the bathroom. They include crawling species, shape-changers, predators, and quite possibly the oldest living individuals on the planet. They also go back in history a very long way, which is rather handy from my point of view. The general view among biologists and palaeontologists now is that sponges either are our earliest animal ancestors, or they evolved directly from them.

Lucy meets a distant relative

The textbooks will tell you (as will Sir David Attenborough in his latest series) that sponges are pretty dull things, little more than collections of single-celled choanoflagellates stuck together. The thing is, though, they've been evolving for just as long as we have, and that view really does them a disservice. They may not have brains and nervous systems, but they've evolved ways around it - many glass sponges have electrical ion channels instead, and other groups have complex internal chemical signalling processes. They also have very interesting immune systems, which can respond to invasion in a more subtle range of ways than can, say, a tarantula. In other words, if they're bundles of cells stuck together, then they've stuck in a very intricate way. 

A typical sponge? Note the chimneys (exhalent canals) and small pores (inhalent canals).

Fundamentally, though, a sponge is a living sieve. It's a body with pores all over it, leading to canals that join together, eventually leading to one or more large holes at the top. The canals are lined with cells that have little whip-like flagellae, which all beat in the same direction to suck water in through the pores, and spit it out at the top. Sponges may not have organs, it's true. But they do have a surprising array of different cell types. Some of them create currents and catch food, others are for oxygen exchange, others for secreting the skeleton, others for reproduction and so on. Some of them, the archaeocytes, can become any other type - a very neat trick similar to the stem cells so beloved of modern medicine.

The organic (collagenous/chitinous) skeleton of a modern "bath sponge"

I mentioned a skeleton, and here's the one of  a typical bath sponge. It's made of collagen fibres forming a network, and this supports the sponge's soft tissues. However... it's not normal. Most sponges have a skeleton made of interwoven or fused spicules of either opal-A (amorphous silica, similar to quartz, similar to glass, but not quite either) or calcium carbonate (calcite). This is a bit of the skeleton of Euplectella ("Venus' Flower Basket") - it's an amazingly exquisite construction of rods and cross-shaped spicules woven together:

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To finish off for the moment, if you want to really follow my sponge posts in future, remember these three major groups. A big chunk of early sponge fossil research is about how they're related.

Hexactinellida ("glass sponges" with siliceous, often cross-shaped spicules)

Demospongea ("common sponges", with siliceous calthrops- or rod-shaped spicules, or an organic skeleton, or no skeleton at all)

Calcarea (calcareous sponges, with a skeleton of calcite spicules).

Next time I get bored, I'll get onto the fossil record of these wonderful beasties. Toodle-pip!

[*Actually none of it was made up. There really are predatory sponges like Asbestopluma that eat crustaceans, many sponges crawl (slowly) by reorbing some bits of themselves and growing in others, and some of the really big Antarctic sponges are thought to be over 10,000 years old - although proving it is complicated, which is probably why I've only ever seen it at a conference as yet...]