Doctoral Dissertation Research

Ecological Effects of Tube-dwelling Coral Symbionts

During my first field season 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, or look at some pictures from a few of my field seasons on Moorea:

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 may influence the ability of the coral to spread.  I have observed fingers detached from the colony where they formed reattach and continue to grow into a new colony, a form of asexual reproduction.  Surveys I have conducted 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 may 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.

Indirect Effects of Symbionts on Coral Mortality

The finger structure also appears to protect corals from predation.  Crown-of-thorns Seastars and Pin-cushion Stars are common coral predators that typically digest away their prey, leaving only dead, white coral skeleton.  However, the physical structure of the fingers seems to reduce the efficiency with which these predators are able to consume corals.  I conducted experiments where I placed coral colonies with and without fingers into tanks with crown-of-thorns and pin-cushion stars, and found that colonies with fingers were less likely to be attacked, and more likely to have living tissue remaining following an attack.  Any surviving tissue is able to continue growing and rebuild the colony, so fingers appear to provide colonies insurance against seastar attack.

Indirect Effects of Structure on Corallivore Prey Choice

Beyond simply deterring crown-of-thorns from eating Montipora with fingers, it appears to change the overall preference crown-of-thorns have for different coral species.  In preference tests I conducted with several coral genera, I discovered that crown-of-thorns typically prefer eating flat Montipora to Acropora.  However, when given a choice between fingered Montipora and Acropora, the seastars preferred the Acropora.  This indicates that the symbionts not only affect Montipora, but may also act to indirectly affect other corals by altering predator behavior. 

Indirect Effects of Symbionts on Fish Abundance and Recruitment

Corals provide food and habitat to a diverse array of fish and invertebrates, so it stands to reason that changes in coral structure will alter fish and invertebrate diversity and abundance.  I conducted an experiment where I placed small artificial reefs, some with fingers and some without, in a large sand flat, and monitored them through time to see which ones would attract more fish.  I found that reefs with fingers attracted a greater diversity and greater abundance of reef fish than those without fingers.  Furthermore, I found that juvenile fish were more likely to recruit to reefs with fingers than to those without, showing that the presence of symbionts may influence the number of fish across the reef.