Morphology & Constraints
Morphological integration in Caribbean octocorals
Phenotypic traits can vary independently, but usually are constrained by networks of developmental, physiological and functional interactions of varying strengths. Phenotypic integration refers to the tendency for traits to covary due to a variety of mechanistic and non-exclusive associations. These can include, for example, shared or closely linked genetic control (i.e pleiotropy or linkage) or developmental mechanisms (e.g. hormonal control, growth regulation). Phenotypic integration may also be driven by traits sharing a common resource base, which favors allocation trade-offs for this resource, or by traits that share functional roles. Because groups of integrated traits evolve as a functional unit, patterns of integration can limit the range (i.e. constrain) the amount of phenotypic variation that can be created through evolutionary change.
While there has been a long history of investigation of trait covariation in flowering plants and vertebrates, less is known about the integration of morphological traits in invertebrates, such as octocorals. Octocorals represent an interesting model system to investigate how trait covariation constrains the evolution of morphology because, like many sessile, benthic invertebrates, they are modular organisms. Sessile organisms spend the majority of their lives attached to the substratum, so one of the primary ways they interact in beneficial ways with their environment is through morphological plasticity. This is especially true for modular organisms, colonies of which classically grow indeterminately through the repetition of modules (e.g. polyps) and branches. As a result, the overall size and shape of modular organisms can be highly responsive to local environmental influences, resulting in close matching between colony morphology and environmental conditions, even within species.
The purpose of this study is to investigate the extent to which inter- and intra-specific variation in octocoral morphologies is constrained by covariation among traits (i.e. integration), including how traits scale with colony size (i.e. allometry). This study is led by Hannah Nelson (me) with Peter Edmunds (CSUN). The work is conducted in the Virgin Islands National Park at the Virgin Islands Environmental Resource Station. This research is part of the St. John Long-Term Coral Reef Dynamics (NSF-LTREB) project.