Introduction

Bucephalus longicornutus (Manter, 1954) is of considerable economic importance because its sporocysts infect the New Zealand mud-oyster, Ostrea lutaria Hutton, 1873, which is extensively fished in Foveaux Strait and marketed throughout New Zealand. This report of part of its life history confirms experimentally, for the first time, the main features of the life history of a marine member of the family Bucephalidae Poche, 1907.

Members of the family Bucephalidae, commonly referred to as bucephalids or gasterostomes, constitute an aberrant group of digenetic trematodes which, in their adult stage, infect the pyloric caeca, intestine, and more rarely the stomach and body cavity of both marine and fresh-water teleosts. The diagnostic feature of the family is that the mouth is on the mid-ventral surface and leads into a saccular intestine. Other morphological features are essentially the same as for the prosostome digenea (vide Dawes, 1946).

The presence of the ventral mouth and sac-like gut have served to link the family Bucephalidae with the Turbellaria, and the family is considered by some authors (e.g., Yamaguti, 1958) to stand apart from other digenetic trematode families in the Order Gasterostomata. On the other hand, the method of formation of the excretory bladder, the type of cercaria and its method of penetration into the second intermediate host has led La Rue (1957) to believe that the family page 2 has closer affinities with a number of other digenetic trematode families including the Strigeidae, Brachylaemidae, Fellodistomidae, Azygiidae and Schistosomatidae. Dawes (1946) stated (p. 508) that "the phylogeny of the Trematoda is a problem which cannot be solved in the present state of our knowledge" and this still holds true today. Thus the merits of either view given above cannot be evaluated at the present time.

The life history of members of the group is complex, involving three parasitic and two free-living phases, and has been determined for three fresh-water species only. A similar life history is postulated for marine species on the basis of work by Tennent (1906), and Carrere (1937), and stages recovered from marine hosts by various authors. The adults are generally intestinal parasites of predaceous fish. They produce eggs which hatch in water (or possibly within the rectum of the host) liberating free-swimming miracidia. These are small, pear-shaped organisms, and unique among trematode miracidia in that the cilia are borne on rod-like appendages. If they penetrate a suitable bivalve mollusc they develop into long, much branched germinal sacs or sporocysts in the visceral mass and eventually cause "parasitic castration" of the bivalve. Germinal cells in the sporocyst wall are proliferated into the sporocyst lumen and give rise to gasterostomate cercariae which are characterised by long, contractile furcae. After liberation from the sporocyst the cercariae commence a short free-living phase which culminates in attachment to small fish, and each becomes a metacercaria if it successfully penetrates the skin of the fish and encysts. When infected fish are eaten by predaceous fish the metacercariae excyst and develop into sexually mature adults.

The systematics of the family is based entirely on adult morphology. To date, some 140 adult species distributed among 12 genera are known. Hopkins (1956) has suggested that the generally accepted system of classification at the generic level may not be a natural one and the excretory system has much more phylogenetic significance than has been attributed to it in the past. However, until such time as excretory system details of all species are known, the only feasible system to follow is that set out by Yamaguti (1958).

The general uniformity of internal structure found among the adult members of the Bucephalidae has necessitated the use of minor characters to separate species. Judging by the variability of egg size and the relative positions of internal organs exhibited by some species, the validity of established species using such characters is questionable.

Yamaguti (1958) cited authors who have referred bucephalid metacercariae to various adult species of bucephalid. However, this latter practice is questionable because metacercariae are immature and, although they may show generic characters, they do not show the specific characters used to differentiate between adults. The adult status of a metacercaria must be determined experimentally.

To date, 23 bucephalid cercariae have been described. Of these, the adult status of three fresh-water North American species has been determined experimentally and that of one fresh-water European species has also been established on the basis of a solitary recorded species in Europe. The remainder cannot be assigned their adult status under the present system of classification since adult characters are not exhibited by cercariae. Nevertheless, many authors have placed cercariae in adult genera without experimental evidence to substantiate their views.

Many bucephalid cercariae are inadequately described for comparative purposes. Bucephalopsis haimeanus, for example, has been reported from 14 different species of bivalve molluscs. Regarding this, Hopkins (1950, p. 12) stated "On the basis of what is now known about the host specificity of trematodes in the precercarial page 3 stages, it seems most probable that each of the bucephalid cercariae reported from a different molluscan host will eventually be found to be a distinct species."

Hitherto, bucephalid sporocysts have been recovered from eight commercially exploited species of marine bivalve molluscs, and it is perhaps surprising to find that the life histories of all of these are unknown although they may at times affect seriously the host stock. It is hoped, therefore, that the present study will stimulate further life history work on this important group of trematodes.