One of the classical paradigms of marine biology is that low-latitude oceanic domains are “biological deserts.” This perspective, initially borne from global primary and secondary productivity mapping, has changed profoundly with increased deep-water-column sampling and observation in the last two decades. While biomass per volume is indeed low in much of the deep-pelagic domain, the global biomass of deep-pelagic fishes, for example, when taken in toto, is massive. Likewise, biodiversity at a “point in the ocean” can also be extremely high due to the dynamic nature of the pelagic domain. This is particularly true of deep marginal seas. In the Gulf of Mexico (GoM), for example, well over half of all known fish species spend all or part of their lives in the pelagic domain. In this presentation we summarize the results of a large-scale sampling and analysis program to determine the factors and drivers of high fish diversity in a quasi-oligotrophic water body. We found that certain mesopelagic fish families (Stomiidae, Myctophidae) are particularly speciose in the GoM, with the former being the largest fish family in that water body (not a coastal reef fish family). Numerous undescribed species discoveries highlight the historic under-sampling of the bathypelagic zone. Leptocephalus larvae of eels can be large contributors to fish biodiversity (one of eight fish species in the pelagic GoM is an eel). Likewise, juveniles of deep-benthic and demersal species are recurrent members of the deep-pelagic fauna. And again likewise, juveniles of reef fishes are conspicuous members of the oceanic ichthyofauna, meaning that the pelagic domain may be an important reservoir of recruits to the highly dynamic (and highly disturbed) coastal domain. The latter three findings highlight the importance of horizontal (coastal-oceanic) and vertical connectivity as a primary driver of biodiversity in the pelagic realm. Lastly, the integration of genetic barcoding and alpha taxonomy is essential for deep-pelagic biodiversity assessment.