Skeletal morphology and material properties of a fragmenting gorgonian coral

Among species in which fragmentation is an important mode of asexual reproduction, mechanisms that facilitate fragmentation should be advantageous. The Caribbean gorgonian Plexaura kuna readily fragments on reefs of the San Blas Islands, Panama, to form aggregations of genetically identical colonies. To understand the mechanics of fragmentation and the role of structural properties, the force required to break branches was measured on live colonies in the field, and morphology, flexibility, and strength of the axial skeleton were measured in laboratory experiments. Young's modulus, used as a measure of stiffness, was higher in P. kuna than in many other gorgonians and significantly varied among clones, ranging from 2.859 to 6.073 GN m^-2. This level of variation indicates that drag should vary among clones due to differences in their flexibility. Of the morphological traits measured, coplanar constrictions influenced the force required to break a branch while anti-planar constrictions and branch anisotropy did not. The modulus of rupture, a measure of the strength of the branch, significantly varied among clones of P. kuna, ranging from 50.87 to 92.58 MN m^-2. Using hydrodynamic theory we predict that the 55% variation in skeletal strength is sufficient to produce variation in fragmentation rate among clones. The stiffness of the skeleton of some colonies may prevent bending and shedding of drag resulting in failure due to the weakness of the skeleton. Clonai variation in flexibility and strength provides a mechanism through which selection may act to favor fragmentation and clonal growth.