Ecological Effects of Tube-dwelling Coral Symbionts
While working in Moorea, French Polynesia, I discovered an interesting interaction between corals in the genus Montipora, and two symbionts, previously unknown species of tube-dwelling amphipods and worms. Montipora usually grow by encrusting over the sea-floor or forming flat plates, but when the amphipods and worms are present, the corals form long finger-like branches (which I call fingers). It appears that fingers form when the coral encrusts the symbionts' tubes, and the tube dwellers must continually extend their tubes to prevent being overgrown by the coral. The end result is dramatic changes in the coral's morphology, and the addition of a lot of 3-dimensional structure to the reef.
Corals are the foundation of the coral reef ecosystem, and provide food and habitat for innumerable other creatures. Any significant changes to the amount of living coral or to the amount of coral structure present on the reef is likely to impact anything that lives in or feeds on coral. Montipora is the second most common genus of coral in Moorea, and 2/3 of the Montipora in the lagoons show significant morphological changes due to these symbionts. It seems likely, therefore, that the presence of these symbionts may have appreciable effects across the reef.
My dissertation focused on the ecological effects of this interaction, and attempted to follow both how the symbionts directly affected the coral, as well as some of the indirect effects the symbionts may have on other organisms through the morphological changes they induce in corals. The figure below illustrates some of the direct and indirect effects that occur because of the presence of these symbionts, and brief descriptions of my experiments and findings follow. The embedded slideshow above allows you to see pictures from some of the experiments I conducted. Just click on the arrow buttons to progress through the slides. You can also access a flyer that highlights some of my findings:
- Amphipod Flyer (PDF)
Direct Effects of Symbionts on Coral Growth
I am interested in how the presence of these symbionts directly affects coral, and conducted a series of experiments to determine if their presence enhances coral growth rates. My first experiment compared rates of linear extension (growth into the water column) of fingers with amphipods to those where the amphipods had been removed. The fingers with amphipods exhibited significantly greater growth over the course of one year.
I followed this with another experiment where I compared fingers with amphipods and fingers in which I replaced the amphipod tubes with artificial substrate (extruded marine epoxy). I found no difference in the relative growth rates of the two groups, meaning that the increased growth observed in the previous experiment likely stems from the presence of the amphipod tubes providing extra growth substrate, and not from some other mechanism by which the amphipods are promoting coral growth.
Direct Effects of Symbionts on Coral Reproduction
The finger structures induced by the symbionts are relatively fragile, and this influences the ability of the coral to spread through fragmentation. I have observed fingers detached from the colony where they formed reattach and continue to grow into a new colony. Surveys show this occurring throughout the reef, and indicate that this is potentially an important mechanism for dispersal. This could also act to increase reef recovery following a large wave event, such as a typhoon, as these fragments would be broken and spread throughout the reef.
Indirect Effects of Symbionts on Coral Competition
The fingers also allow Montipora to out compete neighboring corals. Montipora is a particularly fast growing coral, and its growth rate, combined with the ability to grow upward when symbionts are present, allow it to dominate many other types of corals in the race to claim reef space. I have observed fingers extend towards neighbor corals, and then upon contact, form flat plates that shade or smother their neighbor. Once the neighbor coral is dead, the Montipora is then able to attach to the substrate the other coral had previously inhabited and continue growing.