The Global Diversity of Hemichordata

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.

Unexpected molecular and morphological diversity of hemichordate larvae from the Neotropics

“…This may not be surprising, because adult acorn worms are difficult to sample; they live in soft‐bottom habitats, can often withdraw into deep tubes or burrows, and they fragment easily during collection. The poor knowledge of hemichordate diversity is widely acknowledged (Cameron & Perez, ; Tassia et al, ); however, surveys of larvae might help to identify hotspots where taxonomists can focus additional efforts to collect adults.…”

“…The development of hemichordates, especially enteropneusts or acorn worms, has received significant research attention, because it provides information on the evolution of development in the animal lineage that includes chordates (Freeman et al, ; Gonzalez, Jiang, & Lowe, ; Gonzalez, Uhlinger, & Lowe, ; Green, Norris, Terasaki, & Lowe, ; Ikuta et al, ; Kaul‐Strehlow & Stach, ; Röttinger & Lowe, ; Tagawa, ; Tagawa, Humphreys, & Satoh, ; Tagawa, Nishino, Humphreys, & Sato, ). As a close relative of chordates, and as a group which may be similar to the deuterostome ancestor (Gerhart, Lowe, & Kirschner, ; Tassia et al, ), hemichordates offer us the chance to better understand the origins of chordates and help us to refine our ideas about evolutionary transitions in development. Nevertheless, the diversity of this group as a whole is poorly documented (Tassia et al, ), and the larval development of most species is largely unknown.…”

“…As a close relative of chordates, and as a group which may be similar to the deuterostome ancestor (Gerhart, Lowe, & Kirschner, ; Tassia et al, ), hemichordates offer us the chance to better understand the origins of chordates and help us to refine our ideas about evolutionary transitions in development. Nevertheless, the diversity of this group as a whole is poorly documented (Tassia et al, ), and the larval development of most species is largely unknown.…”

“…A recent review of hemichordate diversity showed unusual global patterns across biogeographic regions (Tassia et al, ). Along the Pacific coast of the Americas, the lowest recorded diversity was found in the tropical region, with only one enteropneust species, Saxipendium coronatum Woodwick & Sensenbaugh 1985, reported in the literature for the Tropical Eastern Pacific, compared to 16 in the northeastern Pacific (Tassia et al, ). This pattern is likely the result of relative differences in sampling effort, and it remains to be seen whether it also reflects species diversity.…”

“…This pattern is likely the result of relative differences in sampling effort, and it remains to be seen whether it also reflects species diversity. Along the Atlantic coast of the Americas, 10 hemichordates (three pterobranchs and seven enteropneusts) have been reported for the greater Caribbean, with far fewer in the adjacent temperate regions (Tassia et al, ). The enteropneust species reported for the Caribbean include: Meioglossus psammophilus Worsaae, Sterrer, Kaul‐Strehlow, Hay‐Schmidt, & Giribet , , Mesoglossus caraibicus ( van der Horst 1924) , Balanoglossus gigas Müller in Spengel 1893 , Balanoglossus jamaicensis Wiley 1899, Glossobalanus crozieri van der Horst 1924, Ptychodera bahamensis Spengel 1893, and Schizocardium brasiliense Spengel 1893.…”