Are Endosymbionts better Taxonomists than Scientists?

Two morphologies of Millepora, currently classified as separate species, exist in the Bahamas. Millepora complanata, is found primarily in shallow waters, possessing wide, smooth branches whereas Millepora alcicornis, is found primarily in deeper waters, possessing thinner, knobby branches. Upon discovery of a range of intermediate morphologies, questions arose concerning whether these different morphologies represent different species or whether the differences result from phenotypic plasticity of the two recognized species. Results from reef abundance surveys and ultrastructure morphometric analyses support the two species hypothesis. However, a genetic analysis of rDNA revealed the presence of two distinct cryptic clades that are independent of morphology. This suggests that the current taxonomy of the Millepores may not be accurate.

In order to resolve this taxonomic dilemma, we are taking advantage of the mutualistic association that Millepores form with the dinoflagellate, Symbiodinium. These obligate photosynthetic symbionts allow corals to thrive in shallow, nutrient-poor tropical seas and provide energy required to deposit calcium carbonate skeletons. Recent evidence has suggested that these symbionts are extremely diverse and fall into two categories: generalists and specialists. Specialists have been shown to form species-specific associations with their host corals. We are attempting to determine whether Symbiodinium-host specificity can be used as a diagnostic tool to better understand the phylogenetic relationship between the Millepores.

We have isolated Symbiodinium DNA from coral tissue and have begun a genetic analysis of the symbiont. Our hope is to uncover host-specific specialists that will lead us to a comprehensive understanding of speciation in the Millepore complex.