The Mysteries of Sponges and Their Role on the Coral Reef

Written by Sydney Grzep

Edited by Bobbie Renfro

You may have heard the fact that coral reefs are a diverse and important ecosystem. Coral reefs support roughly 25% of all marine life, at least at some point in these organism’s life cycles if not for their entire lives. Corals themselves are a key factor in this. They build the reefs, supplying food and shelter for their inhabitants, acting as a literal backbone structurally supporting the ecosystem. However, they are not the only organism that reef fish depend on. Sponges have their own special roles to play and have been doing it for at least 600 million years!

Figure 1. Niphates digitalis a species of Caribbean vase sponge

So, what exactly do sponges do? Just like coral, they provide shelter and food for the many creatures on the reef like shrimp, polychaete worms and brittle stars. They also have an additional integral job of keeping the reef together, acting as the squishy glue that fastens the corals and rocks down. Sponges are also the reef’s water cleaner, filtering out excess nutrients and plankton. In fact, they are so specialized for filter feeding that they are the only group of animals capable of filtering out the smallest of the plankton, the picoplankton!

Sponges are just as important to the fish on the reef as they are to the corals. They are a significant source of food for several groups of fish and even some turtles, notably for some of the flashier reef inhabitants like beautiful, huge angelfish and hawksbill sea turtles!

Figure 2. Pomachanthus paru, the French angelfish, is one of many reef spongivores aka animals that eat sponges

The 2022 publication, “Significance of fish-sponge interactions in coral reef ecosystems” (Coppack, et al. 2022) attempts to evaluate reef fish-sponge relationships based on literature from the past 30 years and finds that most of our understanding has a strong geographical bias towards the tropical Atlantic. So far, we have evidence to suggest that at least 16 fish families associate with 56 different sponge genera for shelter or food. With the decline of coral that many reefs are facing, the role of sponges may become more prominent, as they are one of the few groups of animals still remaining in high abundance on some reefs.

Another publication examining the relationship between sponges and sponge-eating fish, “Targeted predator defenses of sponges’ shape community organization and tropical marine ecosystem function” (Wulff 2021), examines how sponges in certain ecosystems defend themselves against predation. This study assesses different predators from reef, seagrass, and mangrove environments, and their ability to feed on a variety of sponge species from these same ecosystems. The results of this seem to find that sponges haves adapted defensive methods that target specific predators and are more vulnerable to predation from predators of different systems. One such example of this was a seagrass-dwelling starfish would eat less than 10% of seagrass sponges, but more than 70% of coral reef or mangrove sponge species, showing that predation may keep sponges from spreading between the reef, seagrass and mangroves but not control their abundance within each habitat. 

Figure 3. A selection of sponges used in feeding arrays. Wulff 2020.

We still have a lot to learn about sponges and their specific role in reef ecology and their relationships with fish. Most of what we do know so far is based on tropical Atlantic and Caribbean coral reefs, and it is unclear how our conclusions may apply to Indo-Pacific reefs or areas like the Mediterranean and the Red Sea. Sponges and their relationships to fish are generally understudied, but that leaves a lot more to discover and understand!

Wulff, Janie. 2020. Targeted predator defenses of sponges shape community organization and tropical marine ecosystem function. Ecological Monographs.

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