Sponges look more like plants or rocks than animals. They never wander, and they have no head, mouth, brain or organs. Biologists still classify them as animals because of how they are built, how they feed and how they grow. Sponges are also now thought to be the oldest surviving branch of the animal family tree, so their simplicity is a clue to what the first animals were like.1PubMed. Integrative phylogenomics positions sponges at the root of the animal tree
What Sorts a Living Thing Into the Animal Kingdom
An animal is a many-celled living thing whose cells lack the stiff walls that plant cells have. Animals also can’t make food from sunlight, so they must eat other living things or their remains. A sponge strains tiny food out of seawater and has no green, light-capturing parts, so it feeds like an animal. Most animals also reproduce sexually, with a fertilized egg dividing into an organized embryo.
None of this requires moving or having a brain. Adult barnacles, corals and oysters stay put, yet nobody calls them plants. Movement is common in animals but was never the defining rule. Feeding alone isn’t enough either, since fungi also eat other living things without being animals. It’s the full set of traits that counts. Sponge cells can crawl and change roles, and in a sponge pulled apart you can watch the cells move and regroup, which plant tissue cannot do.
Sponges also hold animal-only building materials. Animal bodies are held together by collagen, a protein meshwork that also forms human skin, tendons and bone. Sponges make the same main kinds of collagen found in humans, including the sheet-forming type that lines tissues.2PubMed Central. The Evolution of Extracellular Matrix Real collagen fibers have been seen in their bodies.3PubMed Central. The Evolution of Extracellular Matrix The single-celled relatives of animals lack this sheet-forming collagen, which marks it as an invention of the first animals.4eLife. Collagen IV and basement membrane at the evolutionary dawn of metazoan tissues
How a Sponge Eats Without a Mouth
A sponge’s body is riddled with small pores that let water in and canals that carry it through. Inside, chambers are lined with collar cells. Each has a whip-like tail ringed by a collar of tiny finger-like projections.5PubMed. Feeding in a calcareous sponge: particle uptake by pseudopodia The tail beats to draw water through the collar, which works like a sieve.6PubMed. Feeding in a calcareous sponge: particle uptake by pseudopodia The flow is arranged so water is steered into the collars instead of slipping past them.7PubMed Central. Hydrodynamics of sponge pumps and evolution of the sponge body plan The cells then engulf caught particles, such as bacteria, by wrapping parts of themselves around them.8PubMed. Feeding in a calcareous sponge: particle uptake by pseudopodia A sponge relies on a huge number of collar cells working together rather than any one big bite.
In a living sponge, water enters through countless small pores all over the surface and leaves through one or a few larger openings, often at the top. A sponge can filter many times its own volume of water each day.
Eating by swallowing whole particles is an animal way to feed. Human white blood cells take in particles by engulfing them too. Collar cells also closely resemble single-celled organisms called choanoflagellates, the nearest non-animal relatives of animals.9PubMed. Feeding in a calcareous sponge: particle uptake by pseudopodia Some differences exist between the two, which is expected after hundreds of millions of years of separate evolution, much as wings and fins differ yet share an origin.10PubMed Central. The architecture of cell differentiation in choanoflagellates and sponge choanocytes Digestion happens inside individual cells rather than in a stomach, and food reaches every part of the body through the moving water and wandering cells.
Sponges Sense and React More Than They Seem To
Sponges pump, contract and repair themselves constantly, but their slowness hid this for a long time. Sponges can slowly squeeze their canals closed to stop water coming in.11PubMed Central. Bodily Complexity: Integrated Multicellular Organizations for Contraction-Based Motility This protects the feeding chambers from damage and pushes out material they can’t eat.12PubMed Central. Bodily Complexity: Integrated Multicellular Organizations for Contraction-Based Motility Some sponges expel waste with a set series of whole-body contractions.13PubMed Central. Evolutionary origins of sensation in metazoans: functional evidence for a new sensory organ in sponges Afterward the sponge relaxes, its channels reopen and water flow resumes. This is whole-body behavior without muscles, driven by cells that can change shape and tighten. Early naturalists noticed that sponges change the width of their central cavity, one of the first hints they were animals.14UCL Researchers in Museums. Question of the Week: Why are sea sponges considered animals?
Without nerves, signals spread slowly from cell to cell instead of as fast nerve pulses. A contraction can take many minutes to travel across the sponge, more like a sneeze in slow motion than a reflex.
The sponge even has something like a sensory organ. The sponge’s main outflow opening is lined with short hair-like structures that don’t move.15PubMed Central. Evolutionary origins of sensation in metazoans: functional evidence for a new sensory organ in sponges Since they don’t beat, they are unlikely to push water, and in other animals such still hairs act as sensors.16PubMed Central. Evolutionary origins of sensation in metazoans: functional evidence for a new sensory organ in sponges Removing these hairs, or blocking the channels they use in other animals, stops the contractions until they regrow or the block wears off.17PubMed Central. Evolutionary origins of sensation in metazoans: functional evidence for a new sensory organ in sponges Sponges likely sense water flow and use it to coordinate the whole body.
Sponges Grow From Embryos and Defend Themselves
Sponges reproduce sexually, and their embryos develop through an organized series of cell divisions and rearrangements as in other animals.18Canadian Journal of Zoology. Embryogenesis and larval differentiation in sponges Most embryos become larvae with layers of cells, a front and back end, and the ability to sense their surroundings.19Canadian Journal of Zoology. Embryogenesis and larval differentiation in sponges In some sponges, the larva’s outer cells later move inward and turn into the collar cells of the young sponge.20Invertebrate Biology. Embryogenesis and metamorphosis in a haplosclerid demosponge: gastrulation and transdifferentiation of larval ciliated cells to choanocytes
The larva swims using tiny beating hairs and looks for a suitable surface on the seabed. After a period of swimming it attaches, ending its mobile phase. Its cells then reorganize, with layers rearranging and canals and chambers forming, until the body can pump water and filter food. The animal then stays in that spot for life.
Sponges also repair and defend themselves. They carry wandering cells thought to be able to become many other cell types, much like stem cells.21PubMed. Purification and in vitro cultivation of archaeocytes (stem cells) of the marine sponge Hymeniacidon perleve (Demospongiae) They can tell their own tissue from another sponge’s, and some reject grafts from others.22Proceedings of the Royal Society of London. Series B, Biological Sciences. Allogeneic cell interactions during graft rejection in Callyspongia diffusa (Porifera, Demospongia); a study with monoclonal antibodies When foreign tissue is attached, they switch on defense genes related to those in mammals.23PubMed. Histocompatibility reaction in tissue and cells of the marine sponge Suberites domuncula in vitro and in vivo: central role of the allograft inflammatory factor 1 Many living sponges also make defensive chemicals against predators, which researchers study as possible medicines.24PubMed Central. Marine Sponges as a Drug Treasure
The Same Genes That Build Human Bodies
A sponge genome contains many genes shared by all animals, controlling cell division, growth, planned cell death and how cells stick together.25Berkeley News. Genome of ancient sponge reveals origins of first animals, cancer It also holds genes for signaling during development and for forming different cell types.26Berkeley News. Genome of ancient sponge reveals origins of first animals, cancer The building blocks for a many-celled body were in place in this ancient lineage.27Berkeley News. Genome of ancient sponge reveals origins of first animals, cancer
The surprises go further. One sponge has genes matching about 125 human genes used at the junctions between nerve cells.28PubMed Central. Co-expression of synaptic genes in the sponge Amphimedon queenslandica uncovers ancient neural submodules One of these genes has the exact same key contact points as its human version.29PubMed Central. A Post-Synaptic Scaffold at the Origin of the Animal Kingdom Sponges lack some receptors that nerves rely on, so they hold the parts but not a working nervous system.30PubMed Central. A Post-Synaptic Scaffold at the Origin of the Animal Kingdom Sponges also have real tissues, not a loose pile of cells. Freshwater sponges have sealed skin-like cell layers that control what passes in and out.31PubMed Central. Freshwater Sponges Have Functional, Sealing Epithelia with High Transepithelial Resistance and Negative Transepithelial Potential That undermines the old idea that the sea simply flows through them.32PubMed Central. Freshwater Sponges Have Functional, Sealing Epithelia with High Transepithelial Resistance and Negative Transepithelial Potential
Are Sponges the Oldest Animals
For years scientists debated whether sponges or comb jellies branched off first.33PubMed Central. Evidence for sponges as sister to all other animals from partitioned phylogenomics with mixture models and recoding A large analysis then found that all clear-cut results favored sponges and none favored comb jellies.34PubMed. Integrative phylogenomics positions sponges at the root of the animal tree Sponges are best seen as the sister group to every other animal, with comb jellies next.35PubMed. A Large and Consistent Phylogenomic Dataset Supports Sponges as the Sister Group to All Other Animals The first animal likely had no nerves, muscles, mouth or gut.36PubMed Central. Evidence for sponges as sister to all other animals from partitioned phylogenomics with mixture models and recoding
“Oldest branch” doesn’t mean modern sponges are our ancestors. It means sponges split off before other animal groups, and living sponges have kept evolving for hundreds of millions of years too.
Rocks offer another line of evidence. Chemical leftovers of sponge fats found in rocks have been read as signs of sponges over 635 million years ago.37PubMed. Fossil steroids record the appearance of Demospongiae during the Cryogenian period But one of those chemicals can also form from algae, so it may not prove sponges were there.38PubMed. Algal origin of sponge sterane biomarkers negates the oldest evidence for animals in the rock record Another rarer chemical found only in sponges has turned up in similar rocks, supporting the early dates.39PubMed Central. Demosponge steroid biomarker 26-methylstigmastane provides evidence for Neoproterozoic animals Sponges were probably among the earliest animals, but the oldest dates remain unsettled.
What Makes a Sponge Skeleton and Bath Sponges
Many sponges build tiny glass needles from silica using an enzyme.40PubMed. Silicateins, silicatein interactors and cellular interplay in sponge skeletogenesis: formation of glass fiber-like spicules The glass dries out and hardens after it forms.41PubMed. Hardening of bio-silica in sponge spicules involves an aging process after its enzymatic polycondensation: evidence for an aquaporin-mediated water absorption Other sponges are supported instead by spongin, a relative of collagen.42eLife. Collagen IV and basement membrane at the evolutionary dawn of metazoan tissues
Natural bath sponges are the soft spongin skeletons left after the living cells are removed. They are harvested from the seafloor while alive, then left to die and rot so the living tissue can be washed or squeezed out. What remains is the springy skeleton, which is cleaned and sold, so the product is the framework of a once-living animal. Most household sponges today are made from plastic foam or plant fiber, not animals.
Why Sponges Matter in the Ocean
Sponges absorb dissolved organic matter from seawater and quickly shed old filter cells as debris.43PubMed. Surviving in a marine desert: the sponge loop retains resources within coral reefs Reef animals eat this debris, passing the food along.44PubMed. Surviving in a marine desert: the sponge loop retains resources within coral reefs This helps explain how rich coral reefs survive in nutrient-poor seas.45PubMed. Surviving in a marine desert: the sponge loop retains resources within coral reefs Deep-sea sponges pass food to animals like brittle stars the same way.46Frontiers in Marine Science. A Deep-Sea Sponge Loop? Sponges Transfer Dissolved and Particulate Organic Carbon and Nitrogen to Associated Fauna
Microbes living in sponges help defend them against harmful bacteria and supply essential nutrients.47bioRxiv. Taxonomically different symbiotic communities of sympatric Arctic sponge species show functional similarity with specialization at species level Sponges and their microbes trade nutrients, recycling nitrogen within the sponge.48PubMed Central. Integrated metabolism in sponge-microbe symbiosis revealed by genome-centered metatranscriptomics A single sponge is home to huge numbers of microbes, and small animals like shrimp and brittle stars shelter in and on it. Scientists often treat sponge and microbes as one working unit.