In my last blog post, “Oceans Before Humans: An Investigation into the Decline of Shark Populations,” I walked you through Smithsonian Tropical Research Institute (STRI) Fellow Erin Dillon’s project examining what shark populations might have looked like before humans began to harvest them. Like my previous ones, this post will look at the dynamic Caribbean ocean, but with a focus on coral reefs and how they have changed over the last 7,000 years.
A graduate of the University of California at Santa Cruz with a degree in Marine Biology, I have been working with STRI as a paleobiology (a field that combines field research of current environments and organisms with that of fossils) with research intern for Aaron O’Dea and Andrew Alteri, comparing fossil and modern coral reefs. I have been focusing specifically on the interactions between the dusky damselfish and a type of reef-building coral known as staghorn coral, wherein the damselfish causes damage to the coral’s tissue. In response to this damage, the coral produces growths known as “chimneys,” which are calcareous (made up of calcium carbonate) and as such are well preserved in the fossil record, which makes it possible to compare the frequency of chimneys on coral branches between 7000-year-old fossil reefs and modern Caribbean reefs.
The Courageous Coral and Dastardly Damselfish
Commonly thought of as plants or rocks, corals are actually animals, and are members of the phylum Cnidaria, which makes them a close relative of organisms like jellyfish. Corals are composed of numerous colonies of individual but genetically identical polyps that sit in a cup-shaped depression in the coral, and produce an internal skeleton made of calcium carbonate. Each polyp has several nematocysts, or tentacles, used in feeding which first sting and subdue prey before ushering it into the waiting mouth.
Most corals get additional energy and nutrients in the form of oxygen and carbohydrates from a unicellular organism called zooxanthellae, which live in the polyps, which in turn receive metabolic waste from the coral in the form of phosphorous, nitrogen, and carbon dioxide. Because these symbiotic organisms are photosynthetic (meaning that as plants they convert energy from sunlight into chemical energy), the corals must inhabit clear shallow tropical and subtropical waters that allow adequate light penetration.
As anyone who has Googled “coral reefs” can attest, corals are beautiful, yet most don’t know that they are also:
- Crucial to biodiversity – corals create microhabitats for countless marine species, including up to 25% of the world’s fish biodiversity!
- An indicator species – because they are so sensitive to light and temperature, corals can only tolerate a narrow range of water conditions, and thus are environmental indicators of water quality.
- Providers of ecosystem services – coral reefs aid in breaking up wave action.
- Economically important – corals contribute a significant amount of money to economic areas such as tourism (valued by the National Oceanographic and Atmospheric Administration to contribute around $9.6 billion) and fisheries.
- A carbon sink.
Now, add damselfish, our second species of interest. The dusky damselfish has a weak stomach, which means it has a difficult time digesting anything but the most delicate of algae. Because of this, it engages in a mutualistic behavior of plant-herbivore gardening with filamentous algae [a relationship akin to that of bees consuming the pollen of flowers and then dispersing it on land]. The fish deliberately damage patches of coral, effectively killing portions of the coral, permitting space for the colonization of the algae, which the fish then “farm.” In response to the bites of these damselfish, the coral produces distinct protuberances, or “chimneys.” My research seeks to quantify and compare the abundance of these chimneys on 7,000-year-old fossil reefs with those found on modern Caribbean reefs.
Overfishing has led to a marked removal of top predators from oceanic ecosystems, allowing for mesopredator release (rapid increase in medium-sized predators such as damselfish as a result of the lack of large predators). This mesopredator release may have allowed damselfish populations to increase, thus allowing them to kill more coral, and create more chimneys. (For more information on overfishing, LoaTree blogger Juliet Taylor has written an awesome piece that summarizes the key points in her post “Planet Ocean: The Status of Our Fisheries and How You Can Help.”)
A Drop in the Ocean
Fossil and recent coral branches were collected off the southwest coast of Isla Colón in the Bocas del Toro Archipelago, and were then brought back to Panama City where I examined them for the presence or absence of chimneys-like growths. Chimneys are distinguished from other growths by the following characteristics: 1). Small bumps or cylinders 0.25 – 2 cm long 2). Absent from underside of segment 3). Hollow (sometimes with algae inside) 4.)Occurring in clusters.
Preliminary results are striking and clearly demonstrate that chimneys are not only vastly more common in the modern reef, but larger as well. This suggests that a significant change has taken place in the interaction between damselfish and staghorn coral, which could be the result of the following:
- A change in the relationship between fish, coral, and algae
- An increased abundance of damselfish
- Decreased growth rate of the coral (this would result in an apparent increase in chimney density)
Pursuing this research also leads to an exhaustive list of questions. But since science is rarely straight forward it is highly unlikely that there is a simple answer, especially since we are dealing with the ocean which feels the global effects of events like nutrient pollution, sedimentation, ocean acidification, and increasing ocean temperatures. These issues and others need to be taken into consideration when asking the seemingly simple question of what is going on in these reef systems today that has caused such a change from the past?
While the loss of coral from damselfish gardening may seem negligible and insignificant when compared to larger threats such as disease, the additive effects exacerbate the issue of already poor coral health, and we now are seeing corals getting pushed towards a point where they are unable to recover properly from any kind of damage. Results from this survey have the potential to help determine if the widespread decline in corals is a shift that communities have experienced before, or if this radical decline is an unprecedented trend brought about by human actions interfering in the marine environment.
If you’re interested in learning more about coral reefs and their conservation, or even how you can get involved, I encourage you to visit: Ocean Health Index: Coral Reefs, NOAA Coral Reef Conservation Program, Coral Reef Alliance. Unfortunately while threats to these dynamic coral habitats are vast there’s still hope!
Tune in next time for something completely different!