May Update: Growth Form, Polyp Budding, Colony Formation, and Polymorphism
Since the conclusion of our fundraising campaign, our team has been investigating the light-scattering properties and skeletal architecture of 88 coral species with the goal of identifying which skeletal characters are linked with bleaching susceptibility. We have been examining coral skeletal characters that are known to affect the light reaching their photosynthetic symbionts, but we are also examining how coral polyps interact with each other in a coral colony. Coral polyps are soft-bodied and live within hard calcium-carbonate corallites on the surface of the coral skeleton. They communicate with each other through soft tissue connections that are mirrored in their calcium-carbonate skeletons.
This ability to communicate integrates individual polyps into the colony for common defense and collaborative response to injury or disease, sharing of nutrients, and division of labor. But not all coral colonies are highly integrated. Some species are essentially independent polyps living in close proximity with clone-mates and other species are heavily reliant upon other polyps in the colony for their daily energy requirements. It is thought that highly integrated coral colonies may be incapable of isolating damage during a bleaching event and therefore be more susceptible. To assess how integrated each species is, and how that may be related to bleaching susceptibility, we examined skeletal characters that are reflective of the tissue connections between polyps in a colony.
The growth form of a colony can be an indication of coordination among polyps as they must be able to detect their position within a colony to build complex symmetrical forms. We divided growth form into three categories: massive or encrusting or columnar, laminar, and branching or digitate (see figures below, accompanied by their integration score and species name in parentheses). The first category is tied to lower colony integration while corals in the final category require greater coordination and communication among polyps to build complex forms.
0 = Encrusting (Montipora hispida)
0 = Columnar (Madrepora radula)
0.5 = Laminar (Echinopora concinna)
1 = Branching or Digitate (Madrepora corymbosa)
Polyp budding, or cloning, can occur within a corallite or external to the corallite wall. Those occurring within the parent corallite have a more intimate connection, often sharing the same “stomach” and “circulatory system”, for an extended period. This difference is reflected in the skeleton though the position of the corallites. The new polyp and its corallite tends to be smaller than its neighbors and may appear as though it is part of another corallite (within the parent corallite) or has been squeezed into the available space between corallites (external to the parent corallite). The following two figures show the difference between extracalicular and intracalicular budding (extracalicular species received a "0" for their integration score while intracalicular received a "1").
Colony formation explains the arrangement of the corallites on the surface of the coral skeleton. We divided formation into five categories; listed from lowest to highest colony integration level, they are: phaceloid or solitary, plocoid, cerioid, meandroid, and hydnophoroid. Phaceloid and solitary corals have corallites that are physically unattached by live tissue. Plocoid colonies also have corallites with their own walls and are separated by some distance. Cerioid colonies have fused corallite walls and are therefore directly adjacent to eachother. Meandroid colonies have corallites without walls and are arranged in a linear series, making it nearly impossible to determine where one corallite begins and the next in the series ends. Lastly, in hydnophoroids, corallites are grouped around conical hillocks and therefore the polyps abut multiple others (rather than two, at most, in meandroid corals).
0.25 = Plocoid (Acropora palmata)
0.5 = Cerioid (Coeloseris mayeri)
0.75 = Meandroid (Agaricia agaricites)
1 = Hydnophoroid (Hydnophora exesa)
Polymorphic polyps in coral colonies are those which have more than one type of polyp and indicate some degree of division of labor within the colony. If a colony has one polyp type, then the polyps provide the colony with many of its needs: food, protection, etc. In other words, they are generalists. If a colony has many polyp types, then the polyps may specialize on certain tasks while relying on their colony-mates for other vital functions. As a result, the polyps must communicate with each other for the colony to survive. Therefore, polymorphism is associated with higher colony integration.
1 = Yes Polymorphism (Acropora cervicornis). Red arrows indicate two different polyps
Based on these traits and others, we assigned our eighty-eight coral species an integration score. A higher score was tied to higher colony integration. With these traits, we are testing the hypothesis that greater integration is related to greater bleaching susceptibility in corals. By mapping the integration scores and bleaching responses onto a phylogeny (a map of the evolution of coral species) we can also correct our analyses for trends that may be caused by the family history of the coral species rather then reflecting the true association between integration and bleaching susceptibility. More details and results to come!
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