Which groups of protostomes are thought to contain the most undiscovered species?

It is perhaps easier to list the groups of protostomes for which a nearly complete inventory of living species has been done than to list all the groups for which many new species remain to be described. Among the insects, the best-studied group, in terms of species, is the butterflies, which are widely collected and studied. There are still many species of other lepidopterans (such as moths), however, remaining to be discovered. As we explored in Investigating Life: How Many Unknown Species?, some other major insect groups, such as beetles, contain many undescribed species.

New species discovery and description rates remain high for almost all other major groups of protostomes. Second to the insects, and perhaps even rivaling the insects in undiscovered diversity, are the nematodes. Although known nematode diversity is only about one-fortieth of known insect diversity (in terms of number of described species), the taxonomy of nematodes has been much more poorly studied than that of insects. Some biologists think there are likely to be species-specific parasitic nematodes specializing on most other species of multicellular organisms. If so, then there may be as many species of nematodes as there are of plants, fungi, and other animals combined.

Most of the other diverse groups of protostomes also contain many as yet undetected species, judging from the rate of new species descriptions. In particular, flatworms (especially the parasitic flukes and tapeworms), marine annelids, mollusks, crustaceans, myriapods, and chelicerates all contain large numbers of undescribed species.

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Future directions

Biologists have now placed almost all known species into a comprehensive tree of life (see Appendix A). This achievement allows the development of new technology for automated species discovery and identification. Once we can identify an organism, it is then possible to connect it to all the known information about that species. It should soon be possible to construct a hand-held device that will isolate DNA from any sample, and then to sequence a series of target genes that can be used to place an unknown sample within the tree of life. This process results in either identification of a known species, or placement of an undescribed species with its closest relatives in the tree. This technology is expected to revolutionize the study of biodiversity and its applications.