Role of coralliths in coral reef recovery and expansion

Abstract

In this thesis, I examined the ecological importance of a unique group of corals called coralliths and their role in reef recovery following disturbance events. Coral reefs are one of the most important habitats on Earth, supporting many ecosystem services essential to coastal communities in the tropics. However, they are threatened by human-induced and environmental disturbances, leading to increased degradation. These disturbances affect some of the most common species of coral present on reefs, but some more resilient species and morphologies can withstand these stressors. One such group of resilient corals are called coralliths. They are unattached, mobile corals moved passively by wave action and fish grazing. This movement means they encounter more environmental variation than sessile corals. This mobile lifestyle may precondition coralliths to be more tolerant of future climatic conditions. In this thesis, I investigated the key factors enabling corallith formation. Is it 1) a coral’s ability to adapt to changes in the light environment or 2) the ability to recover quickly from physical damage? I also set out to understand the role coralliths may play in coral reef recovery post-disturbance and whether their resilience may mean they play a more significant role on coral reefs in the future.

Using PAM fluorometry, microscopy and CT scanning techniques in the lab, I found that a coral species’ ability to recover from physical damage plays a more critical role in corallith formation than their ability to adapt to low light conditions. Using benthic surveys, I looked at changes in the coral community in the Caribbean over the last 40 years to discover that corallith-forming species (CFS) represent a larger-than-expected proportion of current Caribbean reefs. Looking more closely at the reefs of Utila and Tela, Honduras, I found that the proportion of CFS increased after local bleaching events. This suggests that CFS were benefitting from these otherwise harmful disturbance events. To learn whether this increase was restricted to the Caribbean, I used data collected in the Indian Ocean to look at the change in the CFS Porites rus after successive bleaching events. This species is found throughout the Indo-Pacific and is known to form huge encrusting colonies. However, despite their large size, they do not provide as much 3D structure as other coral species and, therefore, have fewer inhabitable niches. I showed that P. rus not only survived these events but increased its cover. The mechanism for this increase I show is through its propensity to form coralliths and corallith formation is adaptive and not merely a morphological anomaly. Like P. rus, many CFS are encrusting and massive species of corals. A future reef with a higher proportion of CFS could provide less habitat for biodiversity, which relies on there being a variety of different ecological niches on reefs. This would directly impact the ecosystem services that coral reefs provide. By drawing conclusions on future reefs' community structure, we can better prepare communities to respond.