Imagine you are on a cliff with a breathtaking view. The sea stretches out underneath you like a crystal blue abyss, inviting you to jump in. Next thing you know, you are diving. The underwater world opens up in front of your eyes. You witness many colored organisms swimming and floating around you. It is the most beautiful thing you have ever seen. As you swim, you notice a beautiful reef with all sorts of colors and so much life. The experienced eye knows what you are seeing is plentiful fish, crabs, algae, sponges, and corals; all of the wonderful beings that inhabit a thriving ecosystem that is built upon a coral reef. But to someone experiencing this for the first time, it is a breathtaking palette of unknown origin. With these next words, I am taking you on a journey through life forms that have evolved through millions of years. I’m talking about corals and sponges. These life forms hold far greater significance than we often realize. Corals have rightfully taken center stage in the narrative of our oceans, and thankfully so. Unfortunately, they are increasingly vulnerable to challenges like coral bleaching, which occurs as ocean temperatures rise. This process leads to the loss of algae within coral cells, disrupting their symbiotic relationship and causing the corals to expel the algae, which leaves them vulnerable to starvation, disease, and death. But to really understand this issue, you have to know the fascinating biology of corals.

The Coral Animal

Tropical reefs are one of the most diverse ecosystems on earth. They consist of a colony of marine invertebrates called corals. Corals like anemones get their nutrition and energy by filter feeding on microorganisms that pass by on ocean currents. Hard corals build the reefs and are structured of many polyps living in colonies. These polyps are identical and feed on plankton and even small fish. Polyps have a vertical digestion system which they use to feed and execrate waste. Fun fact about these hard corals: inside these polyps, on a cellular level, algae are photosynthesizing. The process works like this: all plants photosynthesize, which is how they absorb the warmth of the sun, light energy, water, and carbon dioxide to create oxygen and sugar that provides energy, nurture, and growth. To protect their soft bodies, corals use calcium carbonate from the seawater to create a hard exoskeleton so when a coral dies, the exoskeleton remains to which other corals attach themselves and develop further. Over many hundreds of years, this process creates a coral reef. A place that is a habitat, feeding, and breeding place of many marine species. Another fun fact: coral reefs cover only about 0.1% of the ocean’s surface, yet they are home to over 25% of all marine species.

The Sponge Animal

Sponges are ancient organisms, with some species even found in fossil records. Their role in the survival and development of coral reef ecosystems is vital. Serving as natural water filters, sponges contribute significantly to maintaining the health of their environment. Unlike corals, which only thrive in saltwater, sponges can inhabit marine and freshwater ecosystems, highlighting one of their many unique differences. Their porous exterior filters water, collects bacteria, and processes carbon, nitrogen, and phosphorus which is essential for coral reefs to survive the ongoing battle with the climate crisis. Even though they are simple, they are helpers of the reef. I call them helpers because sponges excrete carbon for other organisms on the reef to feed on in nutrient-depleted areas. With this function, sponges serve as a protector of the reef ecosystem. They shelter the reef from temperature, density, and light challenges, which we now know support bleaching.

Creeping Death – Opportunity over Friendship

Sponges may appear to be the noble guardians of the coral reef ecosystem, but as in every fairy tale, the hero is not always what they seem. In this story of our extraordinary marine ecosystem, the sponges wield the gift of rapid growth, outpacing the corals and, at times, covering and overpowering the very reef they were thought to protect. More and more species of coral-killing sponges are being discovered, adding yet another item on the list of coral’s problems.It works like this: sponges take over corals by producing coral-killing molecules which than destroy the reef, section by section until it’s taken over by sponges and algae. They stretch, wrap, and spread, creeping over coral structures like a slow-moving tide. They suffocate the coral beneath them, blocking sunlight, stealing nutrients, and breaking apart the delicate skeletons that took decades or even centuries to form. Why does this happen? The answer lies in imbalance. When an ecosystem is healthy, corals and sponges coexist, each carving out its own niche. But when the water warms, when pollution rises, when overfishing removes the creatures that keep sponge populations in check, the balance shifts. Sponges take advantage of the coral’s weakness. They do not destroy out of malice but out of necessity. They are opportunists, capitalizing on decay, thriving where life falters. Some even believe that this is the future of coral reefs; sponges and algae due to their ability to survive rising temperatures and climate change. But for how long?

The Great Mix-Up on the Reef

Many of us often mistake soft corals and sponges underwater. But why? What do they have in common? When distinguishing soft corals from sponges, there are a few key characteristics to observe. Soft corals have a flexible, often leathery texture and are generally more pliable. They tend to sway gently with the water movement. In contrast, sponges have a more rigid, porous texture, their surface typically has visible holes called pores where water flows through, making them less flexible than soft corals. The shapes and structures also differ. Soft corals may form tree-like or bushy shapes, and their structure is often more defined with branching or feathery patterns. Sponges, on the other hand, can have irregular or blob-like forms and tend to look more uneven or lumped together. Some sponges may be tubular or encrusting, but they usually lack the branching or feathery structure of soft corals.Color can also offer clues. Soft corals often come in vibrant shades, such as yellow, purple, pink, red, and blue, and can appear glossy or shiny. Sponges can also be colorful, with hues like yellow, orange, red, or blue, but their texture is more matte, and their appearance is generally more uneven. Finally, symmetry plays a role. Soft corals typically show radial symmetry, with the structure radiating from a central point, especially with their polyps. Sponges, on the other hand, may appear asymmetrical or irregular, lacking the defined symmetry seen in soft corals. By observing these traits, texture, shape, movement, color, and symmetry you can typically identify whether you are looking at a soft coral or a sponge. At your next snorkel or dive experience, you are equipped with a lot of knowledge to distinguish them from one another. Don’t forget that they are animals, so no touching, and be careful with your fins!

Article by Anamarija Kolimbatovic

Photos by Sabine M. Probst