Subphylum CEPHALOCHORDATA

<I>Epigonichthys australis</I> (Raff, 1912)

Epigonichthys australis (Raff, 1912)

Museums

Museum list

Regional Maps

Subphylum CEPHALOCHORDATA

Compiler and date details

1998 - B.J. Richardson, Centre for Biostructural and Biomolecular Research, University of Western Sydney, Hawkesbury, NSW

Introduction

Cephalochordates, also known as lancelets or acraniates, are small (‹50 mm) primitive chordates and are normally considered to be the sister group to the Vertebrata (but see Jefferies 1986; Chen et al. 1995). They show little evidence of cephalisation, are elongate, pointed at both ends and laterally compressed. A fold of skin extends along the dorsal midline, around the tail and anteriorly along the posterior part of the ventral surface. This fold, which is enlarged posteriorly, forms a continuous dorsal, caudal and ventral fin.

The notochord is homologous with the same structure in other chordates and consists of a series of cell discs or lamellae composed of muscle cells. A collagenous sheath surrounds the lamellae and by contraction of the muscles, a rigid axial skeleton can be produced. A hollow nerve cord is dorsal to the notochord. Cephalochordates will respond to light, tactile and chemical stimuli, however complex sense organs are absent. A single pigmented eye spot is found in some species. The animals are metamerically segmented with the muscles forming a series of myotomes.

The gonads, which vary in number from 20–40 in different species, are easily discernible externally. The sexes are similar and fertilisation is external. Spawning typically occurs at sundown. The larvae, which are similar to the echinoderm tornaria larva, swim actively using cilia. They develop into the asymmetrical, pelagic, amphioxoides larvae before transformation and settlement.

Cephalochordates are jawless, mucous filter-feeders similar to tunicates. Water is drawn in through the mouth, which is surrounded by buccal cirri, by ciliary action, then filtered through the gill slits and finally passes to the exterior through an atriopore.

The subphylum Cephalochordata was reviewed by Franz (1922) and Hubbs (1922). Only a single family, Branchiostomidae, is recognised, with two genera and about 23 species worldwide. The eight Australian species were revised by Richardson & McKenzie (1994). The only other review of the Australian fauna was by Whitley (1932). Other than passing observations, there have been no ecological or behavioural studies on Australian species. They are found in the coastal seas and bays around the Australian continent, except for the south-western section where they have never been recorded.

Database Notes

The information on the Australian Faunal Directory site for the Cephalochordata 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 1998 as (Richardson, B.J., 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.