These two examples illustrate the kinds of changes to marine biodiversity that may result from anthropogenic effects. The Chinese clam example indicates how transoceanic movement of exotic species in ballast water carried during the ubiquitous human activity of commercial shipping, coupled with natural twists of fate (California's 1986 flood and the succeeding drought) can change an entire ecosystem. In this case, a legacy of studies of the invaded region of San Francisco Bay, as well as a keen awareness of potential effects of biological invasions in this region, facilitated the detection and documentation of the effect. The whaling story remains speculative, however, because of the infancy of understanding of deep-sea biodiversity patterns and processes in general, and because of a lack of appreciation for the wide range of scales over which the activities of humans may impact the marine environment.
In fact, the biodiversity of the marine realm is so sorely undersampled and underdescribed that identifying changes, irrespective of cause or ecological consequence, is not possible for most habitats most of the time. There are vast numbers of undescribed species in familiar habitats, such as coral reefs and temperate bays and estuaries; there are environments, like polar regions and the deep sea, which are so undersampled that hundreds of new species are added with each expedition to a new area; and new biological habitats that contain novel assemblages continue to emerge, such as the whale skeleton community described above. Furthermore, the application of molecular-genetic techniques for describing marine species is changing the recognition and description of the diversity of life in the oceans. Yet, whereas molecular-genetic techniques now permit the discovery of many new organisms in the microbial realm, as well as the recognition of previously unrecognized multispecies complexes of metazoans (multicellular organisms), the systematic and taxonomic sciences have declined precipitously, resulting in fragmentary and incomplete bridges between molecular and morphological systematics. Consequently, practical, straightforward species identification---that is, recognition on a morphological basis of metazoan taxa now regarded as genetically distinct---is compromised just when species-specific information is needed most for documenting biodiversity.
In the following, important issues, opportunities and critical research needs in marine biodiversity are discussed for the three fundamental levels of biodiversity---genetic, species and ecosystem diversity. Each section begins with new findings which give a flavor of the depth and breadth of current ignorance, and ends with suggestions for future research. The final section, on issues in taxonomy, is a cross-cutting concern for all studies of marine biodiversity.