Tuatara: Volume 7, Issue 2, December 1958
When the Russian research vessel Vitiaz visited Wellington in 1958 Dr. Georgei Beliaev exhibited specimens of Pogonophora, a group of marine worm-like animals now considered to comprise a distinct phylum, possibly related to Enteropneusta. Prior to the Russian visit no Pogonophora had been recorded from New Zealand, and a detailed account of them is still awaited.
The class Pogonophora was proposed in 1937 by Johansson to accommodate a peculiar worm-like animal Lamellisabella which previously had been described as a Polychaete. The present leading authority on them is Professor A. V. Ivanov of Moscow. In 1951 Ivanov recorded new types of Pogonophora from the North Pacific, and in 1955 gave a general account of the characters of the group. He also pointed out that another problematic animal, Siboglinum weberi, was a Pogonophoran; it had been collected by the Siboga in 1899-1900 in deep water off Malaya, and was thus the earliest example known to science.
Opinion is at present divided as to how these animals should be oriented. In this article I follow Ivanov and Ulrich, who regard the nerve cord as dorsal and the tentacles as ventral. The reverse orientation is preferred by Caullery, Dawydoff and Jagersten.
Pogonophora appear to be sedentary, non-colonial, tubicolous marine coelomates with an elongate, bilaterally symmetrical, cylindrical body divided into three regions or segments. The first segment, or prosoma, is very short, and contains an unpaired coelomic pouch which extends into either a single tentacle or, more usually, a tuft of tentacles. The second segment, or mesosoma, is also short; it contains a pair of coelomic pouches. The posterior segment, the trunk or metasoma, is very long and contains a pair of coelomic pouches, each housing a gonad. There is no alimentary canal, mouth or anus, the function of nutrition and respiration being performed by the vascular tentacles of the prosoma. There is a well-developed vascular system and a dorsal nerve-cord. The life-history includes a free-swimming larva within which a vestigial, solid endodermal mass has been recognised.
Although one species, Siboglinum ekmani Jagersten, occurs off the Norwegian coast in water as shallow as 98 fathoms, most species are known from the deep-sea trenches of the North Pacific, where they are sometimes exceedingly abundant. Most species are no more than 1 mm. wide and several centimetres long, but large forms reach a length of about a metre.
The tube is long and cylindrical. It appears to be a secreted structure of a brittle, membranous substance, characterised (in Siboglinum) bypage 44
alternating brownish and translucent transverse rings; distally the tube is thinner and more transparent, with transverse bars of darker material at wide intervals. The body-wall is covered by a hypodermis which secretes a chitinous cuticle, save on the tentacles, where the ectoderm cells form a ciliated epithelium. Within lie muscle-fibres, apparently longitudinal for the most part.
The vascular system is a closed one. Blood flows forwards in the median ventral vessel, enters the heart, circulates through the afferent and efferent tentacular vessels, and then passes into the median dorsal vessel, where it flows backwards. Short blind vessels supply the prosoma. In the trunk a pair of vessels supply the gonads, and at the posterior end of the body the ventral and dorsal vessels unite in a rete. Some genera have a contractile pericardial vesicle beside the heart.
Nutrition: The absence of an alimentary canal was a puzzle to the earlier investigators. Caullery (1914, 1944) thought that the individual he had received from the Siboga expedition must have been part of a colony whose nutritive zooids had not been collected. Later, Dawydoff (1948) concluded that there was no digestive system; but it was left to Ivanov (1952) to infer that the tentacles were probably the nutritive organs. In 1955 Ivanov suggested that the tentacles arrange themselves in such a way as to form a tube, into which the food material is taken and digested by enzymes secreted by gland-cells on the tentacles. In Lamellisabella a tube of this type is a permanent feature, as the tentacles are coalesced in that genus. An alternative suggestion by Jagersten (1957) is that Pogonophora may be saprozoic, the tentacles merely absorbing soluble substances produced as a result of bacterial activity on dead material on the sea-floor.
The nervous system resembles that of the Enteropneusta. There is a ganglion on the dorsal side of the prosoma. From it arise a pair of lateral commissures which supply the tentacles, and a median dorsal nerve-cord which runs backwards into the other segments.
LEGEND TO FIGURES
Fig. 1: Birsteinia witjasi Ivanov. Anterior end viewed from the right side (after Zenkewitch, 1954).
Fig. 2: Schematic diagram of Pogonophoran anatomical features, simplified from Ivanov (1955). On left, ventral aspect; on right, lateral aspect.
Fig. 3A: Siboglinum ekmani Jagersten. Three larvae and part of female parent.
Fig. 3B: S. ekmani. Larva enlarged. (3A and 3B after Jagersten, 1957.)
Reproductive system: The sexes are separate. The paired gonads lie in the metasoma. In the male the testes occupy the whole posterior part of the body. From each testis a long, ciliated gonoduct runs forwards to the anterior part of the metasoma, where it opens ventrally. In the female the ovaries lie in the anterior part of the metasoma, the short oviducts running posteriorly to open about midway along its length. The eggs are relatively large and yolky.
The larval form (Figs. 3a and 3b) has been desbribed by Jagersten (1957) for Siboglinum ekmani. There is a temporary pelagic stage, swimming by means of an anterior and a posterior ring of cilia. The voluminous, yolk-laden intestine lies within, and an anterior invagination may be the vestige of the mouth. Each of the segments carries paired setae similar to those of annelids. No tentacle is developed at this stage. Three such larvae were observed in the distal part of the tube of an adult female, and it was evident that they would have been extruded when next the parent extended her tentacle (there is only one tentacle in Siboglinum).
Up to 1956 five families of Pogonophora had been defined by Ivanov, eighteen species being arranged in eight genera, as follows: Fam. Siboglinidae (Siboglinum); Fam. Oligobrachiidae (Oligobrachia, Birsteinia); Polybrachiidae (Polybrachia, Heptabrachia); Lamellisabellidae (Lamellisabella); Spirobrachiidae (Spirobrachia). Kirkegaard (1956) has since added a ninth genus, Galathealinum.
Although Hartmann (1954) has suggested that Pogonophora may be no more than a heterogeneous artificial assemblage of aberrant Polychaeta, this view has not been supported. Ivanov himself (1954), after drawing attention to the distinctive characters shared by all known Pogonophora, considers that they should constitute an independent phylum (which he names Brachiata) diagnosed by the presence of vascular anterior tentacles, serving as respiratory and digestive organs.
Jagersten (1956, 1957) regards the Pogonophora as a distinct group, perhaps related to but not members of the Enteropneusta. Although he admits the presence in the larva of setae of the annelid type, he points out that such setae occur also in Brachiopoda, and rejects Hartman's suggestion that Pogonophora are Polychaetes. After his discovery of the larval stage Jagersten (1957) re-examined the problem. He concluded that since the larva is 3-segmented, like its adult, it cannot be a modified Trochophore; a further argument against annelid affinities. He notes the vague resemblance of the larva to that of some Enteropneusta but, lacking information on the range of types of larvae or their possible specialisation, he sees no grounds for believing Enterpneusta and Pogonophora to be page 47 closely allied. Jagersten thus favours treating them as a distinct phylum, at least in the meantime.
Acknowledgment: I wish to thank Dr. N. Danilow and Mr. S. Kustanowicz for translating passages from relevant Russian papers; Dr. Georgei Beliaev for drawing our attention to the Pogonophora; and Prof. G. Jagersten for data on Siboglinum ekmani.
Caullery, M., 1914—C.R.Acad.Sci. Paris, 158.
Dawydoff, C., 1948—Bull. Biol. France Belg., 82.
Hartmann, O., 1954—Syst. Zool. 3.
Ivanov, A. V., 1951—Doklady A. N. 76.
—— 1955— Ibid. 100 (1) 175-177.
Jagersten, G., 1956—Zoolog. Bidrag fr. Uppsala, 31, 211.
—— 1957— Ibid. 32, 67.
Johansson, K. E., 1937—Zool. Anz. 117.
Kirkegaard, J. B., 1956—Galathea Rpt. 2.
Zenkewitch, L. a., 1954—Union Intern. Sci. Biol., Ser.B., Pub. 16.