Microbiomes and viromes are at the nexus of healthy hosts and environments, but the fundamental laws that govern host-associated microbial assembly remain unresolved. Chondrichthyan fish (sharks, rays, and chimaeras) are ancient extant vertebrates providing a pivotal point in host-associated microbiome evolution, displaying a broad geographic distribution, and diverse biophysical and physiological traits. My lab focuses on the microbes and viruses on the epidermis because sharks and rays have opposing epidermal features that may influence the microbial characteristics. Sharks have high dermal denticle topography and thin mucus layers, while rays have reduced dermal denticles and high levels of mucus. Shark microbiomes are highly shared, species specific, and show functional redundancy. In contrast, ray microbiomes are more flexible with less shared microbes and a broader range of functions. We predict the high level of topology of the dermal denticles is a major filtering component in microbiome assembly. On the species sampled to date, there is no difference in microbiome between host sex or size. Microbes for both sharks and rays carry a high levels of heavy metal related genes and low abundances of nitrogen fixation genes. Shark bioaccumulate heavy metals and use nitrogen and urea as osmolytes, suggesting an interconnection between hosts and the microbiome functions.