Corals Struggle to fend off increasing sponge populations

By: Ashley Gustafson

It is without question that coral reefs are synonymous with exceptional biodiversity and great beauty. Coral species themselves are a collection of unique and diverse organisms. Modern times have brought great peril to these ethereal ecosystems whether it be raising ocean temperatures or over fishing. As if corals didn’t have enough threats today, a recent survey across the Caribbean has shown results of depleting coral populations due to smothering from blooming sponge species.

Corals and sponges dsrod_spgo not play nice. Sponges are primitive, aggressive creatures that smother adjacent coral that get in their way. In this process the coral is normally killed and the sponges take over the corals space. Sponges do this by using toxins, mucus, and shading to grow on the coral’s skeleton but the question is why now? Sponges and corals have been coexisting for centuries but now in the Caribbean, sponges are increasing and corals are depleting. According to this new survey, overfishing is to blame. Overfishing results in the removal of predators that would normally maintain the sponge population and thus the coral population. While coral populations are already declining, fast-growing sponges are only adding fuel to the fire.
Dr. Joseph Pawlik of UNC Wilmington took a research team to 12 countries in the Caribbean to survey coral reefs. Pawlik and his team surveyed 25 different sites of areas with depleted fish populations due to decades of intensive fish trapping. Then, compared this data with 44 sites where fishes are plentiful due to laws protecting them. The team found that over 25% of the coral colonies at the sites with over fishing showed signs of smothering from sponges which is more than double that of the protected sites. On the protected sites, angelfish and parrotfish species were observed eating fast-growing coral species and only leaving the slow-growing species that protect themselves with chemical defenses.
As previously stated, it is no secret that corals are threatened. They are slow-growing and vulnerable to storms, diseases, and rising sea temperatures. Reef building corals protect shorelines and provide homes to thousands of sea creatures. Their importance to the world’s ocean has them protected under the Endangered Species Act and the World Conservation Union has placed 10 species on the Red List of Threatened Species.
From these new surveys it is evident that in order to save coral species we must also protect angelfish and parrotfish species that prey on sponges. This validation of the ecosystem theory can be applied to many other endangered organisms and is critical piece of information to give the general public. Indirect effects like overfishing can often be overlooked when examining population declines but it is often one of the simplest problems to fix. Global warming is a much vaster, less concentrated issue in comparison to overfishing which can be alleviated with proper legislature and enforcement. It is also evident that we cannot control the weather or the storms it creates.
While it is easy to point the finger at sponges and call it a day, other organisms are fighting for the same space as corals too. One of these contenders is seaweeds which classically have been considered the biggest threat to reef building corals. During the survey it was confirmed that seaweeds cover nearly twice the reef surface as sponges. The common conception is that reef fishes eat seaweed and keep the population in balance. These results and the results of other recent studies show that there might be more to this relationship than once considered. While this might be true there is one thing for sure, overfishing is causing a bloom in sponges that is furthering the endangerment for reef building corals.