Among the animal oddities of New Zealand is a peculiar type of parasitic worm, Gyrocotyle, living in the spiral valve of Chimærid fishes. Gyrocotyle is a member of the Phylum Platyhelminthes and because of complete absence of a digestive system it is included in the Class Cestoidea (tapeworms). Along with a few other genera this parasite is classified in a small, rather unique subclass of cestodes, the cestodaria. These worms, unlike most cestodes, are unsegmented and without a distinct ‘head’, or scolex. For some years all unsegmented cestodes were considered as Cestodaria but most of these actually belong in the Order Pseudophyllidea, one of the large subdivisions of true tapeworms, and are not closely related to Gyrocotyle or true Cestodaria. True Cestodaria are not only unsegmented but have a larva provided with ten hooks rather than six as in the true cestodes. The arrangement and number of sex pores are also different in the Cestodaria.

The most common species in New Zealand is the type species of the genus, Gyrocotyle rugosa Diesing, 1850. Two species, G. fimbriata Watson, 1911, and G. urna (Grube and Wagener, 1852) occur in European and American waters. G. rugosa from the elephant

Legend
Diagram of Gyrocotyle rugosa. Ventral view. ac, acetabulum; mp, male papilla; ov, ovary; ros, rosette; t, testes; up, uterine pore; ut, uterus; v, vitellaria.

page 50 fish, Callorhynchus milii, seems to be a species of the Southern Hemisphere. It is at present less completely known than the two northern genera. G. urna also occurs in the elephant fish in New Zealand.

Gyrocotylids are large, dull pink to creamy white worms, 32 to 75 millimetres in length. At one end is a conspicuous ‘rosette’ formed by ruffles or undulations of the body wall. This rosette forms the border of a funnel-shaped cavity which opens through a smaller dorsal pore. In most species, the thin lateral margins of the body are thrown into folds or undulations but these are absent in G. rugosa. In the body wall on each side of an anterior sucker-like organ (the ‘acetabulum’) are a number of large spines. Other, body spines occur in G. fimbriata and G. urna but these are reduced in G. rugosa to minute points at the tips of small papillæ.

Strange as it may seem, there has been much debate as to which end of this worm is the anterior. Lynch (1945), who has recently studied two species of Gyrocotyle very thoroughly, concludes that the rosette is posterior and the acetabulum is anterior. This conclusion seems confirmed by his discovery of larval hooks in the wall of the rosette of young adults. The ciliated larva swims with the ten hooks posterior in position.

The correct status of Gyrocotyle in the animal kingdom is in some question. The old view that it represents a connecting link between Trematoda and Cestoidea is no longer held. Paired anterior excretory pores; the anterior sucker; arrangement of sex pores; and the posterior haptor-like organ all suggest monogenetic trematodes. Complete knowledge of the life-cycle might show that its phylogenetic relationships might be nearer the Trematoda than to the Cestoidea. Lynch concludes that Gyrocotyle ‘can be considered a Cestode only provisionally’.

The life-cycle of Gyrocotyle is not known. The eggs of G. rugosa are very thin-shelled, operculated and are fully embryonated before deposition. They hatch almost immediately in sea water. Eggs of G. fimbriata and G. urna require two weeks in sea water before they hatch. Recently-hatched larvae of G. rugosa are covered with cilia and swim very rapidly as if excitedly searching for some host. A remarkable phenomenon is the common occurrence of early ‘post-larval’ stages, some of which are barely larger than the ciliated lycophore, both within the tissues of adult worms and in the intestinal mucus of the host. The invasion of these larvae into the tissues of adult Gyrocotyle is probably accidental and abnormal, but their occurrence in the spiral valve of the host may be significant. It suggests a direct life cycle. How Gyrocotyle is transmitted from one elephant fish to another is unknown. Almost all intestinal helminths enter their definitive host by way of food. There is at present no evidence regarding what intermediate host, if any, is involved in the life cycle of Gyrocotyle. The life cycle of Amphilina, a cestodarian from the body cavity of Ganoid fishes, involves an amphipod as intermediate host. Amphilina, however, is not very closely related to Gyrocotyle.

The three species of Gyrocotyle may be distinguished by the following characteristics:

G. fimbriata: Rosette large, diameter averaging 82% (62% to 112%) of greatest body width; undulations of body numerous, averaging 31 (18 to 60), and highly developed; body spines present; left testicular field about 33% of body length; vitelline follicles extending to anterior border of rosette; lateral coils of uterus less than half body width; eggs when deposited containing an early embryo. Hosts: Hydrolagus colliei and Chimæra monstrosa.

G. urna: Diameter of rosette averaging 45% (35% to 60%) of greatest body width; lateral undulations of body few, averaging 15 (8 to 30); body spines present; left testicular field 11% (7% to 15%) of total body length; vitelline follicles extending only to dorsal pore of funnel; lateral coils of uterus less than half body width; eggs when deposited containing an early embryo. Hosts: Hydrolagus colliei; Chimæra monstrosa; Chimæra ogilbyi; Callorhynchus milii.

G. rugosa: Rosette 33% to 50% of greatest body width; lateral undulations of body lacking; vitelline follicles not reaching anterior border of rosette; lateral coils of uterus more than half body width; eggs when deposited containing a fully-developed lycophore. Host: Callorhynchus milii.

There is an extensive literature on Gyrocotyle. A complete bibliography can be derived from the following more or less monographic papers.