The ecological consequences of phenotypic plasticity in the colonial hydrozoan, Hydractina symbiolongicarpus

For most benthic, colonial marine organisms, morphology determines ecology. Across phyla there is a limited repertoire of forms that are often linked with a suite of ecologically relevant traits, such as growth rate or competitive ability, such that these different morphologies can be viewed as representing alternate life strategies. Colonies of the encrusting hydrozaon, Hydractina symbiolongicarpus, show the full range of classic benthic morphologies. In this species, competition between genetically distinct conspecific individuals is achieved by an active battle for space. Success in these battles is strongly linked to the morphology of the colony, with those that produce sparse, sprawling colony forms consistently outcompeting compact, sheet like colony forms under laboratory conditions. This mechanism, coupled with the fact that colony morphology is highly heritable under constant conditions, led to the hypothesis that intraspecific competition acts as a selective force against sheet like colonies. However, in natural populations, the full range of morphologies is always observed. In this thesis, I tested a series of hypotheses to address this apparent paradox. First, I found that natural populations do not exhibit directional morphological selection in response to intraspecific competition by tracking a cohort of new recruits through the critical period of competition. Then, by propagating parents and offspring for several families and using clonal replicates of each genotype to construct reaction norms across a gradient of dissolved oxygen levels, I found that these hydroids are morphologically plastic in response to hypoxia and the shape of the reaction norm over a gradient of dissolved oxygen levels is heritable; though the heritability is different in each environment. In a final experiment, I demonstrated that a temporary exposure to hypoxia can permanently change the developmental trajectory of the colony and consequently change its competitive ability. The general conclusion of these three experiments was that morphological plasticity can ameliorate the apparent selective force of intraspecific competition. Also, because the production and ultimate success of a particular morphology by a given genotype is strongly dependent on the environment, selection for a given genotype is strongly context dependent. So, the presence of heritable morphological plasticity in these hydro ids may represent a mechanism of maintaining both genetic and morphological diversity in the population