Currently, between one-third and two-thirds of marine species may be undescribed, and previous estimates of there being well over one million marine species appear highly unlikely. More species than ever before are being described annually by an increasing number of authors. If the current trend continues, most species will be discovered this century.

LETTERSUndercover. Many Alpheidae shrimps live deep in the reef and are impossible to collect nonlethally. Published by AAAS

Recent advances in deep-sea exploration technology coupled with an increase in worldwide biotic surveys, biological research, and underwater photography in shallow water marine regions such as coral reefs, has allowed for a relatively rapid expansion of our knowledge in the global diversity of many groups of marine organisms. This paper is part of the PLoS ONE review collection of WoRMS (the Worldwide Register of Marine Species), on the global diversity of marine species, and treats the pennatulacean octocorals, a group of cnidarians commonly referred to as sea pens or sea feathers. This also includes sea pansies, some sea whips, and various vermiform taxa. Pennatulaceans are a morphologically diverse group with an estimated 200 or more valid species, displaying worldwide geographic and bathymetric distributions from polar seas to the equatorial tropics and from intertidal flats to over 6100 m in depth. The paper treats new discoveries and taxa new to science, and provides greater resolution in geographic and bathymetric distributions data than was previously known, as well as descriptions of life appearances in life and in situ observations at diverse depth.

An illustrated dichotomous key and synopses of the 32 genera of living pennatulacean octocorals are presented, which incorporate new morphological and distributional data from the examination of recendy collected material. In addition, a key to the 15 extant families, lists of valid genera, synonyms, and a table of comparative characters are also included. Lasdy, a revised classification and phylogenetic considerations are presented. Preliminary investigations indicate that the traditional higher classification scheme of the Pennatulacea is inadequate for reasons of paraphyly and intermediate taxa, that tend to negate precise distinctions between some of the nominal higher taxa. Of the approximately 436 described species of sea pens worldwide, only 186 (or 43%) are estimated to be valid. In addition, several undescribed species have recently been discovered, and others will no doubt be discovered in the future. It is therefore estimated that the extant pennatulacean fauna of the world comprises approximately 200 species in 32 genera.