Current changes in the oceans, including global warming and ocean acidification, are partially caused by human activity, unlike earlier episodes of change throughout geological history. Understanding and forecasting the responses of marine organisms to these changes is top priority for scientists, managers and policy makers. Yet, relatively little is known of the effects of ocean change on marine zooplankton. Ocean change affects species diversity and distributions, but different zooplankton taxa may not be equally affected. This thesis aims to fill this knowledge gap by contributing information regarding the taxonomy, genetic diversity, and biogeography of several selected marine zooplankton groups, providing baseline information that is needed to track the effects of ocean change on marine zooplankton. The study organisms in this thesis represent two groups of planktonic gastropods: pteropods (sea butterflies and sea angels) and heteropods (sea elephants), and a group of crustaceans: the hyperiid amphipods. Pteropods are uniquely suitable for the study of long‐term evolutionary processes in the open ocean because their aragonite shells provide a good fossil record. They have been proposed as bioindicators to monitor the impacts of ocean acidification. Heteropods are another group of pelagic gastropods that independently colonized the pelagic. They are visual predators that prey upon shelled pteropods. Shelled heteropods have received little attention relative to pteropods, but are probably equally vulnerable to the effects of ocean acidification. Hyperiids represent a highly diverse and abundant group and are often commensals and parasitoids of gelatinous plankton. They play unique and important ecological roles in pelagic foodwebs. The major questions that are being addressed in this thesis are: (1) How can closely related pteropod species be distinguished? (2) When did current pteropod biodiversity evolve? (3) Which pteropod, heteropod, and hyperiid amphipod species are where in the Atlantic Ocean?