It’s a common misconception that sponges are some form of a plant.Sponge creatures lack organs. They are sessile plants, meaning they are stuck to one place without movement.No wonder these deep ocean creatures are mistaken as plants. The truth is they are animals and they might even have the same ancestor as us! Their appearance can be deceiving but there are several scientific reasons why these organisms are considered animals. Sponges are the simplest multicellular animals that have porous bodies. As such, they are members of the phylum Porifera, which means ‘pore bearer’. These pores allow the circulation of water for the absorption of nutrients and oxygen. Sponges attach themselves to solid surfaces, which serve as a good source of food to grow. There are over 6,000 species of sponges across the world’s seas and oceans and some sponge species can grow up to 6 ft (1.83 m) wide! They may not look like it but they are important components of the coral ecosystem and life cycle. The scientific community is forever discovering new and exciting facts about sponges. In this article, we aim to answer your questions about these freaks of nature. Hopefully, by the end of it, sponges will earn a cool reputation in your eyes!If you like what you read, check out these barrel sponge facts and find out the answer to are wolf spiders poisonous? Here at Kidadl!Why are sponges classified as animals?You are not alone in thinking of sponges as plants. Even the great Aristotle touted them as much. Even though they look ridiculously like inhabitants of the plant kingdom, the multicellular Porifera or sponges exhibit similar properties as many complex animals, for example, humans. It is safe to say that they are closely related even! That is the main reason why sponges are animals.It was only in the 18th century that scientists first discovered animal-like characteristics in sponges. Before that, scientists made the same mistakes as us and classified sponges as plants. Their lack of organs, immobility, and the fact that some of them have branches made this conclusion seems obvious. However, in 1755, scientists first discovered zoological characteristics including changes in the size of the central cavity and the creation of distinct water currents. We can only imagine their bewilderment! Since then, molecular testing showed that sea sponges have the same ancestor as some of the larger species in the animal kingdom. For example, their skeletal composition is made up of collagen, a protein found in our bodies as well. Sponges are also found to have elements of the immune system of mammals. These are qualities often considered as clear demarcations between plants and animals. Research now suggests that the sea sponge was probably the earliest multicellular animal to appear on Earth at least 543 million years ago!A fun fact here is that the phylum Porifera contains several carnivorous species and organisms. For example, the ping-pong tree sponge preys on assuming crustaceans and other small ocean animals that perch on its branch. The branch consists of tiny hook-like spicules. These spicules are used to immediately latch on to the prey. Then, digestive cells reach the spicules at the site of the meal to do the rest of the work. So not really an inanimate tree (pun intended) is it?Animal bones are used to make the natural sponges we use in our baths. Bath sponges are comprised of a highly porous network of fibers produced from spongin, a collagen protein. Cutting the developing sponges and immersing the cut sections in water until the flesh rots away yields the skeletons.Why are sponges considered basal animals?The umbrella term ‘basal animal’ consists of four phyla of animals, Porifera (sponges), Cnidaria (sea anemones, corals, and jellyfish), Ctenophora (comb jellies), and the Placozoa. Basal animals have simple body plans with radial symmetry and diploblastic characteristics, which means that they are made up of only two germ cell layers.Sponges lack body symmetry. They are diploblastic, which means that they have only two embryonic germ layers. They have a jelly-like substance called mesohyl floating between these two layers of cells. The mesohyl is made of collagen and holds the sponge body in shape. The inner cell layer is covered which choanocytes, a cylinder-collared cell. Sponges do not have a nervous or digestive system as they lack a well-defined tissue system. The mesohyl is a bank of multiple types of sponge cells (motile, fixed) and fibers that perform a range of functions.Sponge cells are a study in themselves in the variety of cellular functions and activities they undertake. The sponge cells are easily modified during their larval and adult life. They often form clusters, layers, or membranes. Sponge cells can also migrate along the whole body of the sponge with the capability to transform from one cell type to another. Talk about being an all-around performer!How are sponges different from other animals?Sea sponges possess specialized cells which differentiate them from other multicellular non-animal, non-plant organisms, which do not form tissues.The sponge differs from other animals in the structure of the body in that they lack organs. Sponges define minimalism at its best! The sponge has a thin membrane covering its body. Its tiny pores, called Ostia, connect to an internal canal system lined with sticky cells called collar cells. These collar cells have two functions, one, to force water to circulate through the sponge to allow absorption of oxygen and removal of waste, and two, to absorb tiny food particles from the circulating water. This functioning makes organs unnecessary.Sponges also differ in their reproductive mechanisms. All sponge species are hermaphroditic organisms, which means that they possess male and female reproductive cells in the same body. Reproduction can happen sexually, by the releasing of spermatozoan into the water, which is carried by the ocean currents to other sponges, or asexually.Most sponge species possess the extraordinary capacity to regenerate. They can not only restore a damaged or lost part of their body but also have the tremendous capacity to regrow their entire body from a group of cells or even a single cell! The separated cells attach themselves to a favorable surface and use the nutrients to completely restore themselves. So they are practically indestructible unless they come to the end of their life on their own!Why are sponges considered unique in the evolution of animals?Zoologists regard the sea sponge as an isolated creature in the animal kingdom that did not produce an evolutionary line. They are different from other metazoans.Sponges are true freaks of nature among animals because of their feeding system. Sponges are filter feeders, which means that they eat tiny floating organic particles and planktons that they filter out from the ocean water. They don’t have mouths. In fact, they are incapable of forming tissues as mentioned above. They primarily eat through their pores. The flagella in the choanocytes pump water in a unidirectional manner, which means that water enters from one pore and is released from another, larger pore. Sponge cells present in the walls of their body filter the circulating water. The food of a sponge usually includes detritus, plankton, viruses, and bacteria. They also absorb microscopic food from the water itself through their pinacocyte cells. An exception is the Cladorhiza corona, a predatory sponge that grows in stagnant water. Because of the lack of particles to filter, these sponges have developed tentacles that grow around the victim, engulf it, and digest it! Yikes!A marine sponge feeding system can filter water up to a thousand times the volume of its body in one day. In fact, off the coasts of Canada, they filter up to 500 ft (152.4 m) of water! They filter the ocean water of dirt and toxins. Moreover, they are key players in the global carbon recycling process. In some nutrient-deficient reef systems, sea sponges restore carbon levels by excreting a certain kind of ‘sponge poop’ that acts as food for the rest of the ocean organisms who keep the carbon production going! Sponges thus perform an essential function in the marine ecosystem. (Fun fact: sponges share this trait with whale sharks, the largest fish in the ocean!)Well, that’s that. We hope this takes care of the ‘sponges considered animals or plants’ debate once and for all. The scientific community is sometimes as baffled as us when it comes to sponges and rightly so! They are truly unique creatures and we hope we were able to pique your interest in them!Here at Kidadl, we have carefully created lots of interesting family-friendly facts for everyone to enjoy! If you liked our suggestions for, what’s the truth: are sponges animals, or are sponges plants? Then why not take a look at, know the difference: are truffles mushrooms? What’s so unique about them or a writing wonder: how is ink made? Amazing facts for kids!
It’s a common misconception that sponges are some form of a plant.
