Phylum HEMICHORDATA

Hemichordata

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Phylum HEMICHORDATA

Hemichordates

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31 December, 1997 - C. Burdon-Jones, Queensland Museum, Brisbane, Queensland, Australia

Introduction

The phylum Hemichordata comprises three classes: the Enteropneusta, Pterobranchia and Graptolithina. A fourth class, the Planctosphaeroida, was established provisionally by Van der Horst (1936) and included as an addendum to the Stomocordés by Dawydoff (1948) and as a separate class by Hyman (1959). It is no longer considered to be a useful taxon (see Species Incertae Sedis).

The very apt name for the phylum was proposed originally as Hemichorda by Bateson (1885, 1886) who recognised their chordate-like characters, both developmental and anatomical, and proposed that they be included with the Tunicata and the Cephalochordata as subphyla of the Chordata together with the Vertebrata. Since then their taxon ranking has been raised to that of phylum in most major works and texts (for example Hyman 1959; Beklemishev 1969; Berry 1987; Willmer 1990; Benton 1993; Ruppert & Barnes 1994; Ruppert 1997).

The Hemichordata, Tunicata and Cephalochordata are often referred to as the Protochordata, an informal group, all members of which share homologies with the ancestral chordate. Although they have pharyngeal gill slits and a dorsal neurocord (confined to the mesosome), Barrington (1965) singled out the Hemichordata from the Protochordata because they do not have a notochord, a chordate character claimed for the other two groups—but they continue to be grouped together in discussions on the evolution and phylogeny of the Chordata.

The Hemichordata is now regarded as a unique phylum characterised by some basic chordate and invertebrate features, and as a link between the Chordata and the Echinodermata (Hyman 1959). A currently held view is that echinoderms, all protochordates and chordates evolved from a hemichordate-like ancestral stock, essentially pterobranch in character (see Willmer 1990; Christoffersen & Araújo-de-Almeida 1994). An alternative, somewhat controversial view is embodied in the calcichordate theory which also cites a pterobranch as the ancestral stock (Jefferies 1986; Jefferies et al. 1996).

Recently, a new classification of the hemichordates was proposed by Nielsen (1995) and Nielsen et al. (1996), following a cladistic analysis of major animal groups and the perceived monophyly of the Enteropneusta and Pterobranchia, each of which is accorded full phylum status. The Enteropneusta, Tunicata, Cephalochordata and Vertebrata form a clade—the Cyrtotreta, quite separate from the Pterobranchia. This new ranking is refuted by Ruppert (1997) who contends that the enteropneusts and pterobranchs share two unique apomorphies, viz. the collar pores (and ducts) and the stomochord, which places both classes in a monophyletic clade, the Hemichordata.

The origins and relationships of the phylum have also been analysed on the basis of molecular evidence derived from amino-acid sequences in certain proteins, using 18S rDNA sequence analysis (Turbeville et al. 1994; Halanych 1995, 1996). Some of their data support a close alliance of pterobranchs and enteropneusts and monophyly of the Hemichordata, however, other data tend not to do so (but see Jefferies 1986; Jefferies et al. 1996; and Ruppert 1997).

The foregoing, and other studies of morphology, microstructure, molecular embryology and phylogeny now being undertaken, will lead to a fuller understanding of the relationships within the Hemichordata and with the ancestral chordate, and may lead to a new classification. However, the traditional, hitherto widely held evolutionary classification, as presented below is retained in this work.

Table 1: Hemichordata classification in Burdon-Jones (1998) and used in this work. Note that no rank higher than family has been recognised in the class Enteropneusta; several fossil genera previously included in the order Graptolithina are now included with the Cephalodicidae or the Rhabdopleuridae (Chapman et al. 1995) and further rearrangement of genera and species is anticipated. The class Planctosphaeroidea and the family Protoglossidae are omitted for reasons given in the text.

Class: ENTEROPNEUSTA Gegenbauer, 1870

Family: Harrimaniidae Spengel, 1901

Family: Spengeliidae Willey, 1899

Family: Ptychoderidae Spengel, 1901

Family: Saxipendiidae Woodwick & Sesenbaugh, 1985

Class: PTEROBRANCHIA Lankester, 1877

Order: CEPHALODISCIDA Fowler, 1877

Family: Cephalodiscidae Harmer, 1905

Order: RHABDOPLEURIDA Fowler, 1877

Family: Rhabdopleuridae Harmer, 1905

Class: GRAPTOLINA Bronn, 1849

All the living hemichordates exhibit the essential embryological features of the Deuterostomia, and, with one exception, have a perforated pharynx. They are marine, benthic, soft-bodied, bilaterally symmetrical, enterocoelous, free living or colonial, inhabiting roughly U-shaped loosely coiled burrows in sand or mud, or collagenous coenoecia attached to, or ramifying over, a solid substratum.

The hemichordate body is archimeric, consisting of three regions of unequal size, structure and function, namely a pre-oral proboscis or prosoma, a short stalk attaching the proboscis to a collar or mesosoma surrounding the mouth and buccal cavity, and a trunk or metasoma. A diverticulum of the buccal cavity, referred to as the stomochord in many texts, extends through the proboscis stalk, and in close association with a heart vesicle, the pericardial sac and an excretory glomerulus, forms an axial complex within the proboscis coelom. In the Enteropneusta this is supported by a collagenous skeletal structure which anchors the proboscis to the collar.

Sexes are separate although hermaphrodites are known in some genera. Gonads are simple or lobed sacs, genital organs are lacking, and fertilisation is external. Development may involve a planula-like lecithotrophic larva with a short dispersal phase, or a tornaria larva which is planktotrophic and has a comparatively long dispersal phase prior to metamorphosis and settlement. Asexual reproduction by blastogeny or fragmentation is a significant part of the life history of some taxa. Extensive regenerative powers are reported for all the major taxa (Packard 1968; Petersen 1984). Viviparity has been described in one monospecific genus (Gilchrist 1925).

Hemichordates are widely distributed throughout the world from the polar regions to the tropics. The majority are found in the mid to lower intertidal zone or the shallow off-shore areas. However, some favour deeper muddier habitats on the continental shelf. Some have been recorded from abyssal depths (Zenkevitch 1963), and one in association with hydrothermal vents (Woodwick & Sesenbaugh 1985).

Sixteen genera and 94 species have been identified world-wide. Many more species have been recognised, but are yet to be described; others in the nomen nudum category are yet to be fully investigated.

Seven genera and 12 species (plus four or more undescribed species) are found in Australian waters, but only three of these species are endemic. They have been recorded from the shores and off-shore reefs and neighbouring islands of all states, but none has yet been recorded from the Northern Territory. Some, e.g. saccoglossids, tend to have patchy distributions, while ptychoderids can be fairly dominant members of the macro-endofauna, on shores and reefs.

Observations on live animals, their habitats and associated fauna are limited except for the most recently described species. Many of the earlier descriptions and illustrations are based on single, indifferently preserved, incomplete specimens, which bear little resemblance to the live animals. Although generic characters are often readily recognisable, the definition of specific features is more difficult and frequently requires a detailed study of serial sections of the new and type specimens.

In addition to those in standard texts, the most comprehensive accounts of the phylum are those of Van der Horst (1932, 1939, in German), Dawydoff (1948, in French), Van der Horst & Helmcke (1956, in German), Burdon-Jones (1957, in German), Hyman (1959, in English),
and Nishikawa (1986, in Japanese). Spengel's (1893) classic monograph (in German), on the 19 species known at the time, is still an invaluable source of anatomical detail and illustrations. For data on fossil representatives of the phylum, reference should be made to works by Kozlowski (1938, 1949, 1966, 1971); on Graptolithina to Bulman (1970), Rickards (1975, 1979), Andres (1977, 1980), Rickards & Dunican (1984), Mierzejewski (1986), Bengston & Urbanek (1986), Urbanek et al. (1992), Durman & Sennikov (1993), Rigby (1994) and Chapman et al. (1995), in which all recorded fossil rhabdopleuran pterobranchs are reviewed, tabulated, and genera are synonymised.

Van der Horst's treatise, published in six parts—1927, 1930, 1934, 1935, 1936 and 1939, remains the most extensive review and digest of the literature available on the phylum from 1825 to 1938. His standardised treatment of the systematics of the Enteropneusta is especially useful to anyone seeking to identify new material and while his treatment of the Pterobranchia is equally informative, reference should be made to Johnston & Muirhead (1951) and Markham (1971) for identification of specimens in this class. Markham's work contains the most recent systematic treatment of the genus Cephalodiscus M'Intosh, and a key and references to all the subgenera and species, with the exception of the recently discovered Cephalodiscus (Cephalodiscus) calciformis Emig, 1977 and C. graptolithoides (Dilly 1993).

Although Markham's key is designed to make identification possible on coenoecial characters alone, he also includes useful supplementary notes on the number of arms, pigmentation and size of the individual zooids. Dawydoff's (1948) account of the phylum is also a useful reference text, but it lacks Van der Horst's detailed systematic treatment of species.

Acknowledgements

The author is indebted for provision of laboratory space, specimens, photographs, loans of material and data on various expedition collections to Mrs L.M. Marsh, Western Australian Museum, Dr P. Hutchinson and Dr P. Berents, Australian Museum, Mr W. Zeidler, Mr I.M. Thomas, and Ms K. Gowlett-Holmes, South Australian Museum, Dr S. Boyd and Dr T. Stranks, Victorian Museum, Dr P. Arnold, Museum of the Tropics, Townsville, Mr W. Dowd, Museum of the School of Biological Sciences, James Cook University.

I also thank Prof. W. Stephenson, and Dr L. Newman, University of Queensland, Prof. J. Morton, Dr M. Miller, University of Auckland, Dr K. Probert, Portobello Marine Station, Prof. Claus Nielsen, Zoological Museum University of Copenhagen, Prof. J.S. Ryland, University College of Swansea, Prof. T. Nishikawa, Nagoya University, Prof. H. Casper, Zoologisches Statsinstitut, Hamburg, and Dr P. Gibbs, Plymouth Marine Laboratory, U.K., for specimens collected for me, or sent for identification, and for photographs and for data on distribution and habitats. For the translation of Kluge, 1948, the author is indebted to Dr K. Whibley, University College of Wales, Bangor, U.K.

For access to the Challenger, Terra Nova, Discovery, Siboga, Galathea and other expedition collections and data on type specimens, special thanks are due to Dr Sheila Halsey, Dr A.I. Muir and Miss M. Lowe, The Natural History Museum, U.K. For loans and data on collections held by their respective institutions the following are thanked: Dr S.S. Jervis, Fitzwilliam Museum, Cambridge, Miss M. van der Merwe and Ms M. Kalk, South African Museum (Van der Horst, collection), Dr L.G. Eldridge, Bishop Museum, Dr Cheryl Bright, National Museum of Natural History, U.S.A., Dr E. Willasen, Zoologisk Museum, Bergen, Dr M. Borel Best, National Naturhistorisch Museum, Leiden, Dr Flegra Bentivegna, Stazione Zoologica, Anton Dohrn, Naples, Prof. Å. Franzen and Dr K. Sindemark, Naturhistoriska Riksmuseet, Stockholm, Dr Michèle Bruni, Institut Océanographique, Monaco. For new records of Cephalodiscus species found in the Weddell Sea, by the Epos 3 Expedition 1989, the author is indebited to Prof. P.N. Dilly, Department of Anatomy, St George's Hospital Medical School, University of London (pers. comm. 1994).

For their support, encouragement and help in so many ways, the author wishes to express his gratitude to the Editor, Dr Alice Wells, to Dr P. Mather, Dr V. Davies, Dr L. Cannon, Dr J. Hooper and all my colleagues at the Queensland Museum and especially Mrs Lynette Dickfos and Miss Jennifer Mahoney for processing the manuscript, the staff of the library, Mrs K. Davie, Ms Victoria Harrison and Ms Meg Lloyd, for their patience and untiring effort tracing and organising loans and photocopies of obscure papers and records.

For assistance with preparation of the figures, the author is indebted to Dr A. Wells and Mrs C. Lambkin.

For the preparation of innumerable serial sections, without which the Enteropneusta listed in this work would not have been identified, the author is indebted to Mr Leigh Winsor, Histologist, School of Biological Sciences, James Cook University, where much of the basic research was undertaken, to Mrs A. Alexander, for her contribution as my research assistant, and to Ms Zeinab Harduwar, Queensland Museum. The work was funded initially by an ABRS grant, with support from the Australian research Grants Council, the JCUNQ, and personal sources, and its completion facilitated by the support provided by the Board and Director of the Queensland Museum, to the author as an Honorary Associate of the Museum.

Database Notes

The information on the Australian Faunal Directory site for the Hemichordata is derived from the Zoological Catalogue of Australia database compiled on the Platypus software program. It incorporates changes made to the work published on 14 May 1988 as (Burdon-Jones, C., 1998)

Limital Area

Distribution data in the Directory is by political and geographic region descriptors and serves as a guide to the distribution of a taxon. For details of a taxon's distribution, the reader should consult the cited references (if any) at genus and species levels.

Australia is defined as including Lord Howe Is., Norfolk Is., Cocos (Keeling) Ils, Christmas Is., Ashmore and Cartier Ils, Macquarie Is., Australian Antarctic Territory, Heard and McDonald Ils, and the waters associated with these land areas of Australian political responsibility. Political areas include the adjacent waters.

Terrestrial geographical terms are based on the drainage systems of continental Australia, while marine terms are self explanatory except as follows: the boundary between the coastal and oceanic zones is the 200 m contour; the Arafura Sea extends from Cape York to 124 DEG E; and the boundary between the Tasman and Coral Seas is considered to be the latitude of Fraser Island, also regarded as the southern terminus of the Great Barrier Reef.

Distribution records, if any, outside of these areas are listed as extralimital. The distribution descriptors for each species are collated to genus level. Users are advised that extralimital distribution for some taxa may not be complete.