Abstract
For individuals living in environmentally heterogeneous environments, a key component for adaptation and persistence is the extent of phenotypic differentiation in response to local environmental conditions. In order to determine the extent of environmentally induced morphological variation in a natural population distributed along environmental gradients, it is necessary to account for potential genetic differences contributing to morphological differentiation. In this study, we set out to quantify geographic morphological variation in the moon jellyfish Aurelia exposed at the extremes of a latitudinal environmental gradient in the Gulf of Mexico (GoM). We used morphological data based on 28 characters, and genetic data taken from mitochondrial cytochrome oxidase I (COI) and nuclear internal transcribed spacer 1 (ITS-1). Molecular analyses revealed the presence of two genetically distinct species of Aurelia co-occurring in the GoM: Aurelia sp. 9 and Aurelia c.f. sp. 2, named for its divergence from (for COI) and similarity to (for ITS-1) Aurelia sp. 2 (Brazil). Neither species exhibited significant population genetic structure between the Northern and the Southeastern Gulf of Mexico; however, they differed greatly in the degree of geographic morphological variation. The morphology of Aurelia sp. 9 exhibited ecophenotypic plasticity and varied significantly between locations, while morphology of Aurelia c.f. sp. 2 was geographically invariant (i.e., canalized). The plastic, generalist medusae of Aurelia sp. 9 are likely able to produce environmentally-induced, "optimal" phenotypes that confer high relative fitness in different environments. In contrast, the non-plastic generalist individuals of Aurelia c.f. sp. 2 likely produce environmentally-independent phenotypes that provide the highest fitness across environments. These findings suggest the two Aurelia lineages co-occurring in the GoM were likely exposed to different past environmental conditions (i.e., different selective pressures) and evolved different strategies to cope with environmental variation. This study highlights the importance of using genetics and morphometric data to understand jellyfish ecology, evolution and systematics.