Overfishing Aids Sponges, Hurts Corals on Caribbean Reefs

With brightly colored branches, tubes, mounds, fans and barrels, sponges are dominant animals on most reefs. Despite their quiet stillness, they are involved in an ancient and ongoing struggle with their predators and competitors — and on modern-day Caribbean reefs, sponges appear to be winning.

Sponges feed by consuming tiny particles and dissolved compounds in the seawater they pump through their bodies. Millions of flagellated cells inside sponges power this pumping, generating an outward flow from the sponges' larger openings. A population of Caribbean giant barrel sponges can process the entire volume of seawater above a reef in as few as three to 18 days, depending on sponge abundance and depth. This unusual feeding strategy sets sponges apart from the other two primary reef occupiers, corals and seaweeds, which rely primarily or exclusively on sunlight and photosynthesis for their nutrition. Using this difference to their advantage in the struggle for available space, sponges can grow to shade and smother corals and seaweed and then steal the real estate.

When my research group from the University of North Carolina Wilmington began working on sponges more than 25 years ago, the conventional wisdom was that sponges on Caribbean reefs were largely unaffected by predation. Angelfishes and hawksbill turtles had been identified as sponge predators, but it was thought that their feeding activity was evenly spread out among sponges, not causing particular harm to any one species. We discovered that several species of parrotfishes are sponge predators. Furthermore, we determined that predators avoid some sponge species because of distasteful chemical compounds in the sponges' tissues. Over the years we have worked with organic chemists to isolate and identify the chemical compounds that serve as defenses for many sponge species. Some of these compounds show interesting properties that may be useful in the development of new drugs to treat human diseases.

Our research found that not all sponge species on Caribbean reefs have chemical defenses. Sponge predators targeted some very common species, such as gray tube and green branching sponges, in a series of experiments we conducted on reefs in the Florida Keys. We observed that these undefended sponges grow faster than chemically defended sponge species. This was important evidence for what ecologists call a resource trade-off: Sponge species invest their energy in either making chemical defenses or growing faster, but they can't do both.

Based on that observation, we predicted that undefended yet faster-growing sponge species would populate new reef habitats more rapidly than defended species. We were able to test this hypothesis with the sinking of the USS Spiegel Grove as an artificial reef off the coast of Key Largo, Fla., in 2002. After only four years the wreck was covered with undefended sponge species, and very few chemically defended species were present. Since then the wreck has attracted the angelfishes that eat these sponges, and on subsequent visits we have seen the relative proportion of undefended sponges decrease as the predators graze them and the chemically defended species steadily recruit and grow on the wreck.

When sponge-eating fish are overharvested, sponges can quickly overgrow and kill reef-building corals.
To test our developing model of sponge ecology, a team of five of us conducted dive surveys of 69 reefs in 12 Caribbean countries over three years. We chose locations such as Jamaica and Martinique that have been overfished using fish trapping and netting as well as locations such as the Cayman Islands, Bonaire and the southeastern Bahamas with less-fished reefs that have been mostly protected from fishing. We predicted that the removal of sponge predators on overfished reefs would allow the undefended sponge species to flourish, while these species would be held in check by sponge predators on less-fished reefs — and that's exactly what we found. In particular, angelfishes had a dramatic effect on the sponge species they like to eat. Our results showed that overfishing of sponge predators altered the community of sponges in favor of faster-growing, undefended sponge species.

Overfishing of Caribbean reefs changed more than just the sponge community. Our Caribbean-wide study documented three times more sponge overgrowth of reef-building corals on overfished reefs, a direct consequence of undefended sponges growing unchecked by sponge predators. With the fast-growing sponges free from predation on overfished reefs, they were able to smother adjacent reef-building corals.

Divers know that corals, not sponges, build the reefs we cherish. Caribbean coral reefs provide shoreline protection for reef-adjacent countries as well as millions of dollars in annual tourist revenue. For the conservation of coral, the results of our research justify the protection of sponge predators on Caribbean reefs. The first course of action should be to ban fish-trapping and netting practices that indiscriminately remove fishes of all species. Sponge predators, particularly angelfishes, warrant special protection, especially from spearfishing.

Divers can help by encouraging Caribbean governments to enact protective legislation and to fund management programs and by supporting marine parks and protected areas that enforce fishing restrictions. These actions can help restore reef ecosystems that are severely out of balance.

© Alert Diver — Q4 Fall 2015