The Difference Between Corals, Anemones, Sponges, and Macroalgae

You can witness an explosion of color, texture, and movement as you pass any healthy reef aquarium. Encrusting formations, flowing fronds, branching structures, and swaying tentacles all coexist peacefully. However, many novice reef keepers are surprised to learn that not all of the life in that stunning saltwater aquarium is coral. 

Organisms from entirely different biological kingdoms, each with distinct traits, maintenance needs, and functions in the marine ecosystem, can be found in your reef tank. Knowing these distinctions makes you go from being someone who merely maintains a tank to someone who genuinely comprehends what's going on in their tiny ocean.

Corals: The Builders of Reefs

First, let's talk about the stars of the show: corals. Corals resemble plants, but they are actually animals. More precisely, they belong to the phylum Cnidaria and are colonial invertebrates, which means they are related to jellyfish and, as we will see in a moment, anemones.

What distinguishes corals in your reef tank is as follows: Hundreds or thousands of individual polyps coexist as a superorganism within each coral colony. These tiny, anemone-like polyps have a central mouth surrounded by a ring of tentacles. Coral polyps, on the other hand, share resources and nutrients because of their colonial structure, in contrast to solitary anemones.

The sugars meet up to 90% of the coral's energy requirements, which these algae produce through photosynthesis. In return, the algae receive sunlight and shelter from the coral. Because of this collaboration, adequate lighting is essential for the majority of reef aquariums because it powers a photosynthetic factory rather than just illuminating an animal.

Based on their skeletal structure, corals in saltwater aquariums can be divided into two major groups. Acropora, Montipora, and brain corals are examples of stony corals (Scleractinia) that literally construct the reef structure over time by depositing calcium carbonate skeletons. Instead of having stiff skeletons, soft corals like leather corals, Xenia, and gorgonians rely on internal structures called sclerites for support. 

We can successfully frag corals for propagation because corals reproduce both sexually through spawning events and asexually through fragmentation. By snapping an Acropora branch and fastening it to a plug, you are utilizing the coral's inherent capacity to regrow and establish new colonies.

The stinging cells known as nematocysts are one characteristic that makes corals unique. Toxins are injected into rivals and prey by these tiny harpoons. This explains why certain species of corals sting more violently than others, why corals can wage "chemical warfare" with their neighbors, and why handling corals should always be done carefully.

Anemones

Anemones and corals (Cnidaria) belong to the same phylum and share traits like symbiotic zooxanthellae, central mouths, and stinging tentacles. However, anemones' biology and way of life are essentially different.

Anemones are solitary creatures, in contrast to colonial corals. Instead of thousands of tiny polyps cooperating, each anemone is a single, large polyp. Anemones can move because they are solitary, and they occasionally meander around your aquarium in search of their favorite spot. They will use their adhesive pedal disc to slide over sand, rock, and glass until they find conditions that suit them, which may or may not be where you wanted them.

The most well-known characteristic of some anemones is their symbiotic association with clownfish. In this mutualistic relationship, the anemone receives food scraps and protection from predators, while the clownfish are shielded by the anemone's stinging tentacles, to which they are immune. Clownfish have successfully paired with some corals and even pumps in captivity, so not all anemones host clownfish, and not all clownfish need anemones.

Anemones and corals have very different feeding needs. Anemones are more aggressive carnivores that actively catch and eat fish, shrimp, and large food pieces. In your saltwater aquarium, they need to be directly fed meaty foods like krill and mysis shrimp on a regular basis, usually once or twice a week. Although they also benefit from additional feeding, corals can survive mostly on photosynthesis and dissolved nutrients.

Compared to most corals, anemones have greater requirements for water stability and quality. They are infamous for getting sucked into powerheads or walking into overflow boxes, and they are sensitive to parameter swings, especially in more recent reef tanks. While many corals can be added much sooner, the majority of seasoned reef keepers advise establishing your marine aquarium for at least six months before adding anemones.

Sponges

Sponges are completely different from corals and anemones; in fact, they belong to a different phylum called Porifera. With no true tissues, organs, or nervous systems, sponges are among the most primitive animals on Earth. 

The structure and life of sponges are what make them unique. A sponge is a living filter, with water continuously passing through its many tiny pores. They pump incredible amounts of water; for example, a single tiny sponge can filter hundreds of liters of water every day, removing dissolved nutrients, organic particles, and microscopic bacteria. Because they consume waste products that other organisms cannot use, they are an excellent natural filter.

Sponges rely solely on obtaining food from the water column, making them obligate filter feeders. Because sponges need a steady supply of microscopic food, this poses a critical care challenge in reef aquariums. Despite their intriguing biology, sponges are less frequently kept than corals because they frequently slowly starve in saltwater tanks with crystal-clear water and aggressive filtration.

Target feeding phytoplankton, liquid coral foods, or specialized sponge foods multiple times a week is necessary for successful sponge keeping in marine aquariums. They flourish in reef tanks with natural plankton populations and higher nutrient levels; copepod-cultured refugiums are especially helpful for long-term sponge maintenance.

Another peculiar vulnerability of sponges is their inability to withstand exposure to air. Bubbles can be trapped in their pore system by even a brief exposure to air, leading to tissue death. Sponges must be placed completely underwater in your aquarium, with no part ever breaking the surface. Because of this same sensitivity, sponges should not be placed in tanks where protein skimmers or other equipment could produce microbubbles.

Macroalgae

Lastly, we have macroalgae, which are essentially distinct from all the other topics we have covered because they are true plants, or more precisely, plant-like organisms from different algal groups, rather than animals.

Similar to terrestrial plants, macroalgae use chlorophyll for photosynthesis, which produces energy from light and nutrients. In contrast to the symbiotic algae found in corals, macroalgae are photosynthetic organisms. They don't have stinging cells, don't eat meat, and don't have nervous systems or conventional mobility.

Red macroalgae like Gracilaria, Caulerpa species (flowing, grape-like or feathery forms), and Chaetomorpha (a fast-growing, hair-like green algae) are common in reef keeping.

In saltwater aquariums, these organisms play crucial roles, especially in the export of nutrients. Regularly harvesting macroalgae physically removes nitrates, phosphates, and other nutrients from your reef system, a natural, efficient method of filtration, because they eat these substances during growth.

As a result, macroalgae are the foundation of marine aquarium refugium systems. In addition to producing oxygen and maintaining pH through 24-hour photosynthesis, a special refugium chamber with macroalgae under full-spectrum lighting offers natural nutrient control. Additionally, the algae serve as a home for beneficial microfauna such as amphipods and copepods, which migrate into your display tank to feed corals and fish.

Macroalgae are remarkably tolerant, in contrast to corals, which need particular trace elements and careful parameter management. As long as they have enough light, nutrients, and basic water quality, they can grow in a variety of environments. Certain species, such as Caulerpa, can become bothersome in reef aquariums due to their aggressive growth, necessitating regular trimming to keep them from taking over your tank.

Conclusion

You can build a truly balanced reef ecosystem by being aware of these differences. Corals support symbiotic algae while offering structure and aesthetic appeal. Anemones give clownfish a place to live while also adding movement and personality. Sponges process dissolved organics while discreetly filtering your water. In addition to producing oxygen and exporting nutrients, macroalgae also provide habitat for microfauna. 

Knowledge, in our opinion at Reefco Aquariums, produces better reef keepers. Knowing that your tank is more than just ‘corals and stuff’, rather, it's a complex community of animals from various phyla and true photosynthetic organisms, allows you to give each resident the special attention they need to thrive. Your aquarium gives you a close-up view of one of nature's most amazing ecosystems, and the reef is not only stunning but also a biology masterclass.