Why sponges are animals

October 14, 2009 • 1:31 am

by Matthew Cobb

Aristotle thought they were plants; for centuries, we have used them to scrub ourselves clean, while millions of kids round the planet think they wear square pants and play with stupid starfish. Nevertheless, sponges are in fact the oldest living animals, one of the most amazing success stories on the planet.

Argument rages over exactly how old the earliest sponge is. Some fossils that are claimed to be sponges have been dated back to around 710 MY ago, but not everyone accepts these. Everyone agrees, however, that they were around shortly before the Cambrian Explosion, around 560MY.

They now cover every part of the ocean, and can account for up to 75% of the biomass on the floor of the Antarctic ocean. 15,000 species have been identified, most of them marine, but there are around 150 freshwater species. There are three main extant groups, and – confusingly – three main body forms, which don’t overlap (i.e. the body forms don’t form distinct evolutionary lineages).

One group of sponges – the Archaeocyaths – flowered briefly (metaphorically speaking) during the Cambrian, but disappeared after around 15 MY. Most sponges are filter feeders, but a few deep sea sponges are carnivorous, spiking tiny crustaceans on their spicules and then absorbing their body contents.

Sponges have no neurons, no muscles, no cell-cell junctions, no gut, no front/back, no reproductive organs.

So what makes them animals?

Studies of DNA show that all the animals – “metazoa” – form what is called a monophyletic group. In other words, all us animals share a common ancestor which is not shared by any other life form. Animals evolved only once, and the earliest branch is the sponges, says the data.

But strictly speaking, that doesn’t make them an animal. They could be some multicellular non-animal, which branched off before the rest of us started on our animally way.

However, recent studies have revealed some amazing things about sponges that clearly place them with the animals – they share fundamental characters with us that no other life-form shares. We can see in the sponge the beginnings of our own way of organizing our bodies and behaving.

They have a simple immune system – a sponge will accept a graft of its own flesh, but will reject that of another sponge. This rejection can be stopped by using an immunosuppressant widely used in medicine. In other words, we share a biochemical pathway that the body uses to distinguish self and non-self.

They show coordinated movement – Ephydatia muelleri, a small sponge, “sneezes” (rather slowly!) when it is agitated or has inedible ink poured over it. This movement can be induced by pouring known animal neurotransmitters over the sponge, showing the sponge cells are sensitive to stuff that we use to activate our neurons.

They may not have neurons, but they have the tiny ion channels that are required to make a neuron work. Express these genes in another organism, and they work.

They have a simple form of the genes required to make neurons. Express these genes in a toad or fly and they start to make neurons there.

This is exactly what we would expect would have happened at the earliest stages of animal multicellularity. Animals didn’t suddenly appear as running, flying, swimming things. Our earliest, multicellular ancestors would have been perfectly adapted to their environment, but would also have used simple forms of cell-cell communication to build their bodies, and to respond to that environment.

Over the vast depths of evolutionary time, those methods were honed and shaped by natural selection, eventually leading to the stupendous array of animal life that the planet has carried, including me, typing these words to you.

But that doesn’t make sponges “primitive”, or in any way lesser beings. They have been on our planet for longer than any other animal – around 10,000 times longer than our species. They have survived repeated mass extinction events, and that suggests they will probably be around for at least another 600 MY. Not bad for an animal that Aristotle took for a plant.

[Added later: Current Biology has this “Quick Guide” to sponges [should be free], while the excellent Tree of Life website has this page on sponges.]

27 thoughts on “Why sponges are animals

  1. “…Sponges have no neurons, no muscles, no cell-cell junctions, no gut, no front/back, no reproductive organs.
    So what makes them animals?”

    The same question may well be asked of politicians.

  2. I did not known sponges shared all these other body-organization traits with the rest of animals. However, I thought the classic cut-up point to circumscribe metazoa was the presence of a blastula stage, for which sponges comply.

    Oh, and regarding politicians, that’s just secondary loss. So I’m afraid phylogenetically they will always belong with the rest of us.

    1. Only some sponges show a blastula-like embryo. Many of them don’t. (Another example of that unlawful lawfulness of biology!)

  3. “Primitive” is one of those words that just doesn’t make sense in the light of evolution, other than in historical discussions. Every living thing on Earth today has exactly as “modern” as any other because they all share a common ancestor back at the beginnings of life. Sponges may not seem particularly “advanced”, but with 15,000 species they clearly have done plenty of evolving.

    1. I think “primitive” just means “more like a common ancestor” in some aspect.

      Obviously all species have had the same time to evolve from their MRCA’s.

  4. I realize that sponges are animals not plants, but which of the three processes you listed: self versus non-self recognition, signal transduction/ion channels, or coordinated movement (many plants move in response to light) could not apply to plants?

    1. Plants don’t have the kind of immune system we do (think of all those grafted fruit trees – you can carry out grafts between genera!), they don’t have ion channels (tho they do have signal transduction), and although some species do show coordinated movement (sensitive plant, venus flytrap), most “movement” in response to light is in fact differential growth (the plant cells grow more on the dark side of the shoot, because of the inhibitory effect of light on plant hormones).

      However, even if all these characters were also seen in plants, the question would then be what mechanisms are involved. What’s striking in the case of sponges is that we share the neurogenic genes, the neurotransmitters and the immune system, whereas plants have very different ways of doing (some of) these things.

      MC

      1. But given that, as you say: “Sponges have no neurons, no muscles, no cell-cell junctions, no gut, no front/back, no reproductive organs”, why class sponges as an animal, rather than neither a plant nor an animal, but some other third kind of thing?

      2. If we categorized sponges as something non-animal, and non-plant, they would get lumped in with all the other problematic wierdness filling out the many branches of the eukaryotic parts of the tree of life.
        Sponges are similar enough to animals that they’re more similar to [rest of animals] than they are to any other group. So removing them from Animalia would lead to more exceptions in descriptions of Animalia than it would solve.

        As one example, sponges have collagen, just like all other animals. So dropping sponges from Animalia would change the definition of Animalia from including “multicellular eukaryotes with collagen” to “multicellular eukaryotes with collagen, except Porifera”. That “except Porifera” would have to get stuck into lots of parts of the description of the group “animals”.

        All of this is rather semantic, of course – sponges are going to keep on being sponges regardless of how we think about them.

  5. Has anyone studied whether they produce neurotransmitters and use the ion channels and neurotransmitters to pass signals? Or are the ion channels just there to absorb nutrients or maintain osmotic pressure?

  6. Hey! You say nice things about Patrick! He’s cool!

    “It’s not only stupid – it’s also dumb!”

    – Patrick Star on Sponge Bob Square Pants

    And think about this: A TV series about an animated sponge who lives in a pineapple and has the occupation of fry cook has been one of the most successful television programs for over a decade!

    These creatures are indeed amazing!

  7. I have read there are some people who think that sponges evolved from more complex ancestors and secondarily lost that complexity. What is the current thinking on that?

  8. I can’t see head or tail in the question if sponges (or their larvae) have a body plan with symmetry axes, but I guess not. (Adults “adjust their shapes” and larvae are “balls”, according to Wikipedia.)

    Then I find it interesting that closer to animals with symmetry axes like bilaterians we have placozoans which I gather don’t have a symmetry axes either, but what I would describe as a broken symmetry: “it is possible to distinguish a back or dorsal side from a belly or ventral side in Trichoplax adhaerens [Wikipedia]”.

    I’m not sure that has any deeper meaning at all, but I find the “no symmetry axes – broken symmetry along an axes – true symmetry axes” path intriguing. Conceivably broken symmetry could easily evolve into later axes.

    1. [I should probably exemplify instead of leaving it vague.

      Say by a placozoan relative evolving a “mouth orifice” instead using digestive cavities on the belly side, which could eventually set the broken symmetry on a path to a longitudinal axis which a symmetry can organize around.]

  9. Good article. Also:

    1. Some sponges are more closely related to us than to other sponges, so they aren’t even a sister group.

    2. You say no front or back end, but some sponge larvae does have an anteroposterior axis. This is very interested, as is the mounting evidence that they once had Hox and ParaHox clusters. Sponges might be secondarily simplified.

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