By Rachel Downey (Australia National University & British Antarctic Survey) and Claire Christian (ASOC)
Every so often, conservationists make a concerted effort to get the public to care about some humble or overlooked species. Cephalopod Awareness Day, anyone? Photos of unusual species lacking the fur or feathers typically required for cuteness, might even go viral, if they show some charisma, for example, the smiley-faced axolotl salamander. The task gets tougher if the species doesn’t have a face (unless we consider the well-loved children’s cartoon character SpongeBob). But in a series of blog posts, we – an invertebrate scientist and an Antarctic conservationist – are going to try to convince you that sea sponges are the next unusual creature you should learn to love.
First, a few basics. Although sponges are sessile (meaning they don’t move, but they can manoeuvre a small amount to get themselves in a better position) they are animals, not plants. Sponges are minimalists – they have no organs that serve as digestive, nervous, circulatory or excretory systems – they are instead composed of masses of cells in a matrix stiffened by a collagen, silica or calcium carbonate skeleton. Sponges are by-and-large filter feeders, with their bodies composed of pores and canals, pumping the surrounding water for tiny particles of food and oxygen. However, one group of sponges has abandoned filter-feeding altogether and turned carnivorous! This special group of sponges has evolved in very food-poor environments, so has modified its internal skeleton in order to ‘hook’ (a bit like Velcro strips) passing swimming animals, often tiny crustaceans, that land upon the sponge, which they ingest over several days.
This primitive-seeming member of the animal kingdom is also far more diverse than most people realize. There are over 8500 sponge species currently known (with hundreds more being described by scientists every year), and they are found throughout the world’s oceans, from the ice-covered poles to the sun-baked tropics, from the deepest ocean trenches to the rocky inter-tidal zone. Sponges come in every colour, and vary in size enormously, from just a few millimetres to over 2 metres in size, with the largest known sponge so far (the size of a small truck) found in the deep waters near Hawaii.
Many sponges have cell and canal structures which enable them to grow into innumerable shapes including irregular, massive blobs, flatter, encrusting forms, perfect spheres, enormous vases, elongated tubes and elegant funnels. This ability to grow into different shapes also helps sponges adapt to different types of environments, partly explaining their presence in every marine environment. Sponges also have the amazing ability of being able to regenerate and reconstruct their entire bodies, even if broken into tiny pieces (video here) . Combined with this, is the fact that sponge cells are totipotent, each cell is like a stem cell, so any cell in a sponge body can become another cell type if required. Sponges can regenerate and change the function of every cell in their body if required – a set of talents that humans would no doubt like to have!
Another impressive sponge ability is that it can change its metabolism. Antarctic sponges have been found to have some of the largest changes in physiology of any group of animals, not surprising when they live in a part of the world with some of the most extreme environmental changes between seasons. Polar sponges must be able to cope with going from a food-poor winter with 24 hours of darkness, to a food-rich summer with 24 hours of daylight. Sponges living in the deep sea, where food is generally scarce, might also have to cope with long intervals without food.
Finally, as fellow animals, sponges are family. All animals alive today have descended from a common ancestor. Sponges are now thought to have branched off of that ancestor first, with all other animal species descending from a different branch. Therefore they are the “sister” to all animals on Earth, no matter how unlike humans or frogs or toucans they seem.
In fact, sponges and humans share a lot more in common than we first thought, as a new finding indicates that we both share the same type of gene regulation. So, despite the sponge being a simple animal, we both share a toolkit within our bodies that regulates how and when genes are activated, meaning that this mechanism has not changed since the dawn of sponge existence. In our next post, we’ll explain how our ancient siblings exert an enormous influence on ocean environments, and how they protect themselves from a world of mobile predators.