Sponges are thought to be the first type – the top systematic group – of animals that differentiated from the stump of the Tree of Life. The earliest known (alleged) sponges were found in Australia and are dated to 666 million years. Sponges are only found underwater – but they spread to every water you can find. From arctic, cold and salty waters to sweet lakes and rivers – they colonized it all.
Sponges don’t have any organs nor organ systems. They don’t move a lot much – that’s why they are mistakenly took for being plants. But they ARE animals. Their bodies are in a shape of a tube-shaped orifice, and the walls of the tube are perforated, so water filled with bacteria/detrytus and oxygen can flow through. They don’t have specialized tissues either – instead they have a few different types of cells. Sponged tend to be mostly tube- or vase-shaped, in a way that makes the water flow the most efficient. Some have radial symmetry, but most are asymmetrical. Their bodies are hardened by collagen structures, others have mineral spicules made of silica or calcium carbonate.
These hard structures might suggest that sponges could fossilize very well. It should be easy to find Very Early Sponges in Very Old Rocks, but actually the fossil record is quite problematic. One scientist might discuss, that definitive evidence of sponges are to be found in rocks dating after the Cambrian explosion; there are others that suggest that they found primitive sponges in layers dated to 600-640 million years ago.
Sponges really could be one of the oldest forms of life on Earth. To me they look perfectly simple. A tube, orifices, and taking advantage of the flowing water to feed and breathe. The earliest known sponges that I mentioned were found at the Edicara site in Australia. You can learn more about it with PBS Eons1 .
Paleontologists found out, that even before the famous Cambrian explosion, multi-cellular life found its ways in the primordial oceans. They suggest, that after the gigantic ice ace (a.k.a. Snowball Earth Hypothesis), melting ice fed the oceans an enormous amount of new, fresh and tasty ingredients, allowing single-celled organisms to multiply and thrive. The influx of nutrients possibly encouraged algae blooms, and that resulted in more oxygen available underwater. They called that time the Avalon Explosion – from the name of the site of course.
The fauna (?) from this time is highly problematic and very strange. We see imprints of multicellular life, that we don’t see anymore today. It is hard to decide if those creatures are animals at all. The Edicarian period is so ancient and fossils so sparse, that it is very hard to understand the relations inbetween those petrified critters, and it’s entirely another quest to find out how we and modern life relate to them. We are expecting – due to molecular data form DNA – that sponges were supposed to be alive and well in Edicarian period. This article in Nautre2 argues about one of the fossils to actually be one of early sponge.
I also found that there are resources online that are far more in-depth that what I can find in books on my shelf: one of those is this sweet presentation by Calla Carbone, from 20173. I really recommend listening to it and paying attention to the fossil photographs: it is a peek into life so ancient, it might not have relatives living today.
So, what makes a Sponge an Animal?
To call a living Thing an animal, scientist came up with different qualities The Thing must posses to be one, and as it turns out, sponges do have (some) of those qualities. I’ll by trying to translate from my own copy of Biology, Seventh Edition, Solomon, Berg, Martin et al4. It’s been translated and published in polish in 2012, so, behold: my re-translation! This book is unfortinately not available on Archive.org to borrow, I couldn’t find it anywhere online. I think it’s a very popular one to find in academic libraries, so I recommend checking it out in your local library!
Tab. 1: Animal characteristics by Solomon, Berg, Martin, 2012.
| 1. | Animals are multicellular eukaryotes (their cells have cell nuclei). |
| 2. | Animals are heterotrophic. Most of them digest nutrients inside their digestive system. |
| 3. | Cells are differentiated and they serve different purposes. Most of animals have cells that make up tissues, that make up organs etc. Only the most primitive animals don’t follow this pattern. |
| 4. | Most of animals have muscular and nervous systems. These systems make make it easy to quickly observe rapid changes in the environment and react to them fast. |
| 5. | Animals are able to move in some or all of their life stages. Some are able to move in their larval (young) stages. |
| 6. | Most of them are diploid – they have two types of reproductive cells, one is huge and immobile (eggg), the other is small and mobile. Haploid sperm merges with haploid egg, creating a diploid zygote. Then it gets through a series of mitosis, that creates a blastula. Then the blastula gets “layered”, the cells start diverging into different lineages, thus creating a gastrula, that, as mentioned above, is sweetly layered. |
A Sponge does have cells with nuclei – that’s a check. They are also heterotrophic – they eat other living things and can’t make food out of sunlight, as plants do. The differentated cells look a bit problematic, but they do have at least three types of them: two types of reproductive cells (sperm and egg) and choanocytes. I’ll be talking about them a bit more later. The next cryterium looks like we loose: the organ systems. Sponges do not have those, but still are able to rapidly react to what’s happening around them, by contracting in an organized manner (Dawkins, Wang 2016). Most sponges do live their lives in one place, but many of them can actually move at a slow pace, and most sponges in larval stages can swim freely. An the last criterium is also a bit complicated…
Dawkins and Wong, in their “Pilgrimage to the Beginnings of Life”5, spend a whole chapter of their book to talk about sponges. They mention that even though a sponge has differentiated cells, they don’t group up into specific tissues: all of cells can change their job anytime, as needed. “Sponges’ cells are totipotent (…)”, the authors stated. There are only several types of sponge cells: one of those have flagellae that help in moving water though a sponge (choanocytes). Other types are the reproducing cells: sperm and eggs. What is strange, sponges, unlike all other animals, don’t have separate lineages of cells for somatic (building your organism) and reproductive purposes. Animals like mammals, for egggsample a woman, have her amount of eggs (reproductive lineage), and it is finite. Those cells don’t take part in everyday living of the organism. They just wait for the Ovulation Time to get released, wait for the conception (by the sperm cell of another human), and then either Die Like a Diva (menstruation) or start its own life (zygote –> blastula –>gastrula etc). In the beginnings of its life, a part of cells gets put aside and create the reproductive lineage that is able to make those crazy amounts of divisions to grow into a mammal. The somatic lineage cells can go through a restricted amount of divisions and then die (apart fro cancer cells, ofc.). That is not what a sponge does though.
The early life of a sponge is not very animal-like. Well, it is a bit “animalish”, but as Dawkins and Wong mention, the cells of a freshly fertilized egg move around and behave almost like a group of very social protozoans that decide to party together, instead of all the zygote-blastula-gastrula phases. Despite that, many of contemporary specialists and zoologists insist, that sponges are indeed animals, not a colony of protozoans. They call them “the most primitive of the -zoans”. Some molecular taxonomists (or as I call them, DNA-zoologists) found that some taxa of sponges are more closely related to us than other sponge taxa, and that would mean that we all share a common ancestor, a proto-sponge-animal that diverged into sponges and us.
Dawkins and Wong suggest that protozoans did really merged together to create first multicellular organisms, and metazoans – sponges – and that marked the milestone of evolution of life.
In 1907, Henry Van Peters Wilson took a sponge, grind it through a gause and deposited the sponge-mousse in water filled petri dish. He observed the cells in the water: they behaved like amoebas, but when they met another “amoeba”, they connected, thus creating bigger and bigger agglomerations of cells: finally, all of those agglomerations created a bunch of new, functional sponges. He also tried mixing up sponge-mousses made of two different sponge species, and to his disappointment the cells connected and merged only with their own species cells. No double-specied sponges for you, Henry!
Choanocytes of sponges really look like some flaggelate protozoans. Dawkins and Wong write about Choanoflagellatea that are still alive today: they suggest, that those animals diverged and appeared 800 million years ago; we, humans, would share a common ancestor with them that long ago. Writing about choanoflagellateas, they are looking for a good tale of how multi-cellular organisms came to be. The same idea is proposed in Solomon, Berg and Martin. They also inform politely, that Choanoflagellates, Fungi and Animals are merged together into a group called Opisthokonts – characterized by flagellate cells ofc.
So, WTF is a sponge?
I guess we are asking a silly question, trying to fit one phylum of life into our neat boxes. In the end, those categories are really helpful, but they are just a way of describing the world around us, and that doesn’t mean it is the truth about the world. It can be argued if those categories do really exist outside of human perception – and I personally would argue that they are not indeed “real” as sponges are. We, as a type of macro-fauna, and the only species that we talk with about our world, have a very simplified and antropocentric way of looking at the world. It is obvious to us, that a zebra is a completely different animal than a shart; or a sponge is something eons away from a Homo sapiens sapiens. But we can’t really say what is the difference between closely related bird species (look up Corvus corone and Corvus corone corone), or try to look for differences in between weeds like Hyracelium. The whole type of protozoans and their relations are an ongoing, never ending debate. The closer we look, the more blurred the categories become.
To sum up, I’d agree with world leading scientists and say that a sponge is an animal. But philosophically I’d argue a bit and ponder more about categories and how we try to build them around the observable phenomena. In my opinion it is crucial to remember, that categories are helpful with understanding the world, but they are not a full description of it.
Sources and further reading:
- PBS Eons, The other Explosion You Should Know About: https://youtu.be/Jpi2VJj5PhY?si=4PJp-40bemQmZ5HP ↩︎
- Wang, X., Liu, A.G., Chen, Z. et al. A late-Ediacaran crown-group sponge animal. Nature 630, 905–911 (2024). https://doi.org/10.1038/s41586-024-07520-y ↩︎
- Royal Tyrell Museum of Paleontology, The Edicaran Period: The Ediacaran Period: Glimpses of the Earth’s Earliest Animals ↩︎
- Solomon, Berg, Martin, Biology. Cengage Learning 2004 https://books.google.pl/books/about/Biology.html?id=qBOPoEc-zu4C&redir_esc=y my version’s been published in 2012 by MULTICO Oficyna Wydawnicza ↩︎
- Dawkins, Wong, The Ancestor’s Tale. A Pilgrimage to the Dawn of Life. 2016 ↩︎