Formerly included in the Gephyrea, the Priapulida have more recently been conceded phyletic rank. Lang (1953) has recently investigated the relationships of the Priapulida and concludes that they should be included in the same Class, or alternatively in the same Subclass as the Acanthocephala. Species occur in the colder waters of both hemispheres. There are only two genera in the phylum, Priapulus and Halicryptus, the latter being distinguished by the lack of caudal appendages. The recognised forms of Priapulus with their respective distributions are as follows:—
Priapulus caudatus Lamarck, 1816. Widely distributed in northern and Arctic seas, probably circumpolar (Theel, 1906).
Priapulus tuberculatospinosus Baird, 1868. Widely spread in shallow Ant-arctic seas. (For a more detailed account see later.)
Priapulus bicaudatus Danielssen, 1868. Deeper water than caudatus, with a more limited distribution pattern in the north Atlantic and Arctic.
Priapulus bicaudatus australis de Guerne, 1888. Off Patagonia and South Shetlands.
Priapulus horridus Théel, 1911. Off Uruguay.
Three species groups may be superficially distinguished by the nature of the caudal appendage. In P. caudatus and P. tuberculatospinosus there is a single caudal appendage, in P. bicaudatus the appendage is paired, in P. horridus the bladders of the appendage are replaced by hooks.
It has been the usual practice to quote P. tuberculatospinosus as a variety of P. caudatus and australis as a variety of P. bicaudatus. The continued use of the taxonomic term “variety” for forms found only in distinct geographical areas and there characterised by constant differences, even if slight, seems to be correlated with an interest in the distributional problem of bipolarity. The lower the grade of taxonomic difference between two forms, the greater the degree of relationship that can be evoked. In most other groups such differences would be accorded subspecific value and in the absence of other evidence, the writer would advocate that the two southern forms of P. caudatus and P. bicaudatus should be considered geographical subspecies of the northern forms. Théel (1911) has described and illustrated the differences between the northern and southern forms of caudatus as being mainly in the disposition and form of the dentary apparatus Lang (1951) has shown that in the larvae of the two forms the position of the tactile organs is considerably nearer the middle in tuberculatospinosus. This difference in addition to those stated by Théel, influences him in considering the two for ns specifically distinct. The differences cited, though superficially slight, stem from deep seated differences in morphological organisation and should therefore be considered of specific value.
Priapulus tuberculatospinosus Baird, 1868.
Priapulus tuberculato-spinosus Baird. Proc. Zool. Soc., 1868, p. 106.
Priapulus humanus Lamarck var. antarcticus Michaelsen, 1889. Jahrb. hamb. wiss. Aust., vol. 6, p. 10.
Priapulus caudatus Lamarck var. tuberculato-spinosus Baird; Stephen, 1941, Discovery Rep., vol. 21, p. 258.
Priapulus tuberculato-spinosus Baird; Lang, 1951. Archiv. Zool., Bd. 2, p. 567.
Stephen (1941) gives a full synonymy which will not be repeated here. The example from Cape Campbell in the Dominion Museum is comparatively large, 89 mm in total length. The trunk is considerably constricted throughout its length, apart from the posterior extremity and the introvert. This constriction gives the body a tripartite appearance which is probably due to contraction on preservation. The introvert is between one-quarter and one-fifth the total length, beset with low, bluntly rounded conical papillae arranged in 25 longitudinal rows. Constricted portion of trunk with 35 very distinct annulations. Caudal appendage of the usual single shrub-like form. (Fig. 1.)
Théel (1911) has shown that the form of the teeth of the dentary apparatus is the best diagnostic character for separating the two forms of caudatus. In this specimen (Figs. 2–5), the form of the teeth is exactly as described and figured by Théel for specimens of tuberculatospinosus Baird.
The specimen from Hicks Bay and that from off Cape Campbell in the Victoria University College Zoology Museum are of comparable size but lack the strong medial constriction.
New Zealand, Occurrences.
Benham (1932, p. 890) in a short note recorded a small specimen (about an inch in length), which he referred to the southern form of caudatus, from 8 fathoms in Islington Bay, Auckland. The writer can now add the following records:—
Off Hicks Bay, near East Cape, in 60 fathoms, A. S. Voss, December, 1949; off Cape Campbell in 40 fathoms, F. Abernethy, November, 1952 (Dominion Museum); off Cape Campbell (Zoology Museum. Victoria University College).
Text-Fig. 1.—Fig. 1—Priapulus tuberculatospinosus Baird, 40 fathoms off Cape Campbell.
Figs. 2–5—Teeth from different portions of introvert from anterson to posterior (Figs. 3–5 to same scale).
Distribution in Southern Seas.
The known distribution of this southern Priapulid forms an interesting pattern. On the Antarctic Continent it has been recorded from Commonwealth Bay, Victoria Land, Cape Adare and Graham Land. In the South American sector records exist for Patagonia. Magellan Strait, Tierra del Fuego, Falkland Islands, South Georgia, South Orkneys and South Shetlands. It has also been recorded from the Subantarctic Islands of Kerguelen and Macquarie. A few apparently anomalous low latitude occurrences have been noted, one from Southern Australia (Benham, 1016) and the four New Zealand records here noted. It does not appear, as yet. to have been discovered in the South African sector, nor at Bouvet, the Crozets or the New Zealand Subantarctic Islands. Adequate investigation may well show its presence in all these areas.
The distribution area is thus strictly comparable with that for a number of other forms with circum-Antarctic distribution, except that speciation is apparently slower among it Priapulids and there is therefore only one form involved.
Comparison with Powell's (1951) distribution maps for the molluscan genera Aforia and Fusitriton, both of which are also bipolar in distribution shows a striking similarity. Powell considers that “the bulk of the high latitude molluscan fauna could have been derived from the Americas” and postulates subsequent dispersal by means of comparatively shallow water ridges radiating from the Antarctic Continent or by lateral migration assisted by the West Wind Drift north of the Antarctic Circle, and the East Wind Drift to the south.
In theory three methods are available for the dispersal of such southern shallow water organisms as Priapulus:—
By migration along the comparatively shallow water radial ridges.
By transoceanic movement as larval stages.
By transoceanic movement, as adults attached to floating algae.
All three postulates are open to criticism. The actual continuity of the submarine ridges is open to doubt due to a lack of adequate soundings. In any case ridges can only be said to be less than 4,000 metres in depth. The theory of transoceanic movement of larval stages presupposes that the period of time as a floating larvae is long enough for ocean currents to transport the water masses over the required distance. Some evidence is available for mean speeds of the West Wind Drift (Dell, 1952), but little is known of the duration of larval stages for southern invertebrates. The number and type of animals that can be transported as adults attached to loose algae is restricted although this method has no doubt been quite effective in a number of cases.
Priapulids could avail themselves of methods (1) and (2) above. The life history of Priapulus is not fully known but it is believed that the gastrula probably develops directly into a loricate larva (Pickford, 1947) which from its general appearance, is adapted for a bottom living existence. Dispersal as a larva therefore appears most unlikely in the case of Priapulus, and it must be supposed that the southern distribution has been attained by migration along shallow water ridges. Whatever the method, it should be borne in mind that a number of other animals have achieved a similar distribution, presumably by similar means. The depths at which specimens of Priapulus tuberculato-spinosus have been collected range from 6 to 383 metres, which may be compared with 7 to 313 metres with one occurrence of 3,600 metres for southern species of Fusitriton.
In view of the known distribution of species of Priapulus it seems fairly probable that southern hemisphere populations have been derived from the cold water northern hemisphere stocks through the Americas. The method used to cross through tropical waters is not known, but the warmer water New Zealand and Australian occurrences at least indicate a greater temperature tolerance than might lave been assumed from the previously recorded cold water stenothermal distribution pattern.