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, 11 (10), e0162564
eCollection

The Global Diversity of Hemichordata

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The Global Diversity of Hemichordata

Michael G Tassia et al. PLoS One.

Abstract

Phylum Hemichordata, composed of worm-like Enteropneusta and colonial Pterobranchia, has been reported to only contain about 100 species. However, recent studies of hemichordate phylogeny and taxonomy suggest the species number has been largely underestimated. One issue is that species must be described by experts, and historically few taxonomists have studied this group of marine invertebrates. Despite this previous lack of coverage, interest in hemichordates has piqued in the past couple of decades, as they are critical to understanding the evolution of chordates-as acorn worms likely resemble the deuterostome ancestor more closely than any other extant animal. This review provides an overview of our current knowledge of hemichordates, focusing specifically on their global biodiversity, geographic distribution, and taxonomy. Using information available in the World Register of Marine Species and published literature, we assembled a list of 130 described, extant species. The majority (83%) of these species are enteropneusts, and more taxonomic descriptions are forthcoming. Ptychoderidae contained the greatest number of species (41 species), closely followed by Harrimaniidae (40 species), of the recognized hemichordate families. Hemichordates are found throughout the world's oceans, with the highest reported numbers by regions with marine labs and diligent taxonomic efforts (e.g. North Pacific and North Atlantic). Pterobranchs are abundant in Antarctica, but have also been found at lower latitudes. We consider this a baseline report and expect new species of Hemichordata will continue to be discovered and described as new marine habitats are characterized and explored.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Deuterostome Phylogeny.
Consensus relationships among deuterostome taxa are shown. Current data provides high-support for Classes Pterobranchia and Enteropneusta as reciprocally monophyletic. In addition, phylogenomic evidence suggests the enteropneust family, Torquaratoridae, fall within the Ptychoderidae. This tree utilizes consolidated data from 16S +18S rRNA, and phylogenomic studies from multiple sources [15, 27, 28, 100].
Fig 2
Fig 2. Body Plans of Hemichordate Species present Throughout the World.
Commonly studied free-living acorn worms (enteropneusts) include members of the A) Ptychoderidae and B) Harrimaniidae. Enteropneusts have most often been found in coastal areas in shallow and deep waters. In contrast, extant pterobranchs C) Cephalodiscidae and D) Rhabdopleuridae often inhabit the deep sea and southern polar regions, but some also occur in warm shallow water. Pterobranch species are colonial and individuals are connected to each other via long, branched stalks. Redrawn from Rychel and Swalla (2009) [65].
Fig 3
Fig 3. Images of Hemichordate Species found around the World.
A) Ptychodera flava (the Hawaiian acorn worm), has been found in many different marine ecoregions of the world. It was the first described hemichordate species [2]. B) Saccoglossus bromophenolosus has been found in the waters of Maine and Washington state. This species was apparently introduced from Maine to Washington due to the oyster industry in the early 1900s [45]. C) Zooid of the pterobranch Cephalodiscus gracilis, a species found in Bermuda. D) Rhabdopleura normani zooids living within a coenecium. Images: A) and B) photo credit Billie J. Swalla, C) and D) photo credit Kenneth M. Halanych.
Fig 4
Fig 4. Extant Hemichordate Species belonging to Enteropneusta or Pterobranchia.
The number of species within each A) family and B) each genus are shown. Note that recent taxonomic revisions have placed some species that were formerly the sole members of their own families into other families. Torquaratoridae and Ptychoderidae are shown as separate families to depict diversity within these groups; however, data support Torquaratoridae falling with Ptychoderidae [28]. See S1 Table for a comprehensive list of valid hemichordate species.
Fig 5
Fig 5. A Timeline of Hemichordate Species Discovery.
A) The cumulative number of new enteropneust and pterobranch species descriptions per year is shown. B) The percentage of species described according to author. For example, ‘single-species authors’ indicates 37% of species were described by authors whom described only a single hemichordate in his/her career. Reports by Ritter alone (4) are not binned with Cameron et al. descriptions (e.g. [101] and [77], respectively). Only extant species are included.
Fig 6
Fig 6. Biogeographical Distribution of Enteropneust and Pterobranch species.
Depiction of the number of unique species reported in each geographic region. Geographic regions are adapted from the marine provinces of Spalding et al. 2007 [88]. These numbers are an underestimation of true species diversity, as there are manuscripts in preparation and many described specimens [33]. See S1 Table for detailed marine province information. Map image: Courtesy of VLIMAR [102]. Figure modified from source material in reference [88]. Original figure: http://www.marineregions.org/gazetteer.php?p=image&pic=64936.

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Grant support

This research was partly funded by NSF DEB-0816892 to BJS and NSF DEB-0816892 and ANT-1043745 to KMH. All the funding or sources of support received during this specific study had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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