Unexpected Assassins

About 20 years ago, scientists discovered a unique animal in the previously inaccessible hostile depths of the ocean: one with hooks on it’s body, weapons it used to capture wandering crustaceans and suck them hollow, their lifeless carapaces left behind on these barbs. 

Sounds like the beginning of an unusual horror story, doesn’t it? It gets far less scary when you realize the animal in question is actually a carnivorous sponge. Sponges are not the most charismatic animals - even the name of the phylum they belong to, ‘Porifera’, Latin for ‘pore bearer’, conjures up a far from fascinating image. And yet, they are the oldest animals on this planet. Sponges have been around for 650 million years, evolving in shallow ocean basins because the deeper oceans lacked oxygen. As the environment changed, more complex life evolved from these unassuming animals, but they lingered, stalwarts of evolution, sticking around till the present day. With over 9,000 documented species today, sponges radiated out from those primordial waters to virtually every habitable aquatic environment - they are found in tropical waters, Arctic oceans and freshwater lakes. After all this time, they are still sessile animals, with no nervous system or respiratory system.

Structure of a sponge. Image source.

Structure of a sponge. Image source.

Common sponges are two layers of cells sandwiched together by a jelly-like substance called the mesophyll, made of collagen. Sponges are hollow, attached at the base, and open to the environment on top, through an aperture called an osculum. The large cavity in their center is called the spongocoel. Their tubular shape is reinforced by glass-like structural elements called spicules, which are present throughout and can be calcareous, siliceous or composed of spongin (a specific type of collagen). They come in a variety of shapes and sizes, and different species can be identified by the unique shape of their spicules.

Types of spicules. Photo source.

Types of spicules. Photo source.

Close up of a choanocyte. Image source.

Close up of a choanocyte. Image source.

The name Porifera comes from the pores, or the ostia, that dot their body, surrounded by cells called porocytes, tube-like structures that help control the incurrent flow of water from the outer layer through the mesophyll to the inner layer. The rest of the outer surface is covered by plate-like cells called pinacocytes, which anchor the sponge to its substrate and cover the rest of it with a layer of skin.

The inner layer is made up of choanocytes, collar cells with one flagellum per cell.  These whip-like flagella work in tandem to create a wave of water that rushes from the outside of the sponge up to it’s osculum, like air through a chimney. This filtering motion brings oxygen and food particles to the sponge, with the latter being caught and digesting by the choanocytes. Any larger food particles that get stuck on the sponge are digested by the pinacocytes. Apart from the energy needed by the choanocytes, sponges are passive eaters: they do not actively catch their prey, they just wait for their food to come to them. 

Common sponges live in nutrient-rich environments, such as coral reefs. In an environment as barren as the deep ocean, however, food is hard to come by - instead of filter feeding, carnivorous sponges have undergone unique changes to hunt and capture more nutrient-dense organisms, such as crustaceans and amphipods.

A.monticola, one of the newly described species. Image source.

A.monticola, one of the newly described species. Image source.

These sponges do not look like their relatives - instead of hollow interiors, they look like bare twigs or shrubs covered in tiny hairs. Under a microscope, these hairs are actually bundles of intricate and beautiful hooks, made of modified spicules. Choanocytes are completed absent, as their flagella are too energetically expensive, and as a result there is no water flow system. These sponges are still evolving, despite being some of the oldest animals on the planet. Genetically, sponges are the most isolated phylum from the entire animal kingdom, but these genes are clearly not static!  

Close up of the hairs on Asbestoplum. Image source.

Close up of the hairs on Asbestoplum. Image source.

Individual barbs. Image source.

Individual barbs. Image source.

Carnivorous sponges were first described in 1995: Abestopluma hypoa was discovered by French researchers. Two species found earlier this year, A. monticola and A. ricketts, were added to this genus. They were described in a paper authored by MBARI marine biologist Lonny Lundsten, alongside two more species belonging to the genus Cladorhiza, C. caillieti and C. evae. 

Even their name is cute, the ping pong sea sponge! Image source.

Even their name is cute, the ping pong sea sponge! Image source.

Other carnivorous sponges, in the genus Chondrocladia, have different adaptations for their diets. Discovered in the Antarctic between 2002 and 2005, they retained the spicules and choanocytes of their relatives. Instead of getting rid of them, they repurposed this system - an inflated balloon-like structure, which they pump full of water, helps them catch their prey: unlike their sinister but beautiful cousins, these comical sponges look like something out of a Dr. Seuss book, with adorable bubbles at the ends of their stalks.

Scientists acknowledge that many more sponges await discovery, and who knows if some of those will have evolved differently? Two years ago in an undergraduate lab, I saw my first sponge under a microscope, and had no idea how severely I was underestimating it. It was an amorphous orange mass, dotted with transparent and somewhat interesting spicules, but I wanted to see the cool inverts: jellyfish, or starfish, or an octopus. I didn't realize then that this humble animal was possibly the root from which all complex animal life had sprung. Discoveries like this make me realize the unique importance of every single species in our world: sponges are easily overlooked, but secretly magnificent, a trait that is not unique to them. This is just one story related to why the conservation of entire ecosystems is so important - the most unassuming creatures could hold the key to far more complex questions, which we could carelessly lose because we just didn't pay enough attention.

Annam Raza is a recent graduate of University of California, San Diego with a BSc in Environmental Systems- Ecology, Behavior and Evolution, with a focus on marine science.