Genome complexity and repetitive DNA in metazoans from extreme marine environments. As genomics converges with ecology and evolution to identify the fundamental linkages between genome structure and function, genome and transcriptome complexity will need to be measured in organisms from more diverse habitats, most often in the absence of complete sequence data. Here, we describe the complexity of ten genomes measured by a novel, high-throughput fluorescence-based kinetic hybridization assay. We applied the Shannon information index, H, and a related, fluorescence-adjusted index, H(f), as unique metrics of the hybridization kinetics to complement the conventional rate constant, k. A strong, positive relationship was present between H(f), and the repetitive DNA content of five eukaryotic genomes previously determined by Cot kinetic analyses (Onchorynchus keta, Ilyanassa obsoleta, Bos taurus, Limulus polyphemus, Saccharyomyces cerevisiae). This relationship was used to characterize the complexity of previously unstudied genomic samples in five metazoan taxa from three marine environments, including deep-sea hydrothermal vents (Alvinella pompejana), the temperate subtidal (Streblospio benedicti), and Antarctic coastal bays (Sterechinus neumayeri, Odontaster validus, Tritonia antarctica). Contrary to the predictions of nucleotypic theory, Antarctic invertebrates consistently had the lowest quantities of repetitive DNA in conjunction with low metabolic rates and highly protracted rates of cell division and larval development. Conversely, hydrothermal vent species with rapid cell division and growth do not have significantly different genome sizes or particularly low amounts of repetitive DNA as compared to non-vent, deep-sea taxa. Furthermore, there appears to be a positive correlation between the temperature at which the most abundant repetitive sequence classes anneal and habitat thermal stability. Thus, our study reveals a potential shift in repetitive sequence representation between these extreme environments that may be related to genome function in species living at these different thermal regimes.