Review of Previous Life History Studies

Von Siebold (1848) was the first to suggest a possible relationship between the cercaria Bucephalus polymorphus von Baer, 1827, from European fresh-water bivalves, and the adult trematode B. fimbriatus (von Siebold, 1848) (synonym Gasterostomum fimbriatum), from European fresh-water predaceous fishes, on the basis of the ventral mouth and sac-like intestine in each case.

Wagener (1858) outlined the probable life history of B. polymorphus. He pointed out that the sporocysts occurred in fresh-water bivalves; cercariae were formed in the sporocyst and when mature were liberated and became free-swimming; encysted, immature bucephalids occurred on the gills of small cyprinid fishes and B. fimbriatus occurred in fresh-water perches. Ziegler (1883) also suggested that B. fimbriatus was a synonym of B. polymorphus.

Only one species of bucephalid is recognised at present from European freshwater fishes and, therefore, there is no reason to doubt the validity of the suggested life history of B. polymorphus. In fact, Poche (1907) formally declared B. fimbriatus a synonym of B. polymorphus.

Rudolphi (1819) described several bucephalid species from marine predaceous fish and Lacaze-Duthiers (1854) described the first marine sporocysts and cercariae. When Maddox (1867) made the first discovery of encysted immature bucephalids in a marine fish, it appeared likely that marine bucephalids followed a similar life history to that postulated for the fresh-water B. polymorphus.

Tennent (1905, 1906, 1909) was the first to attempt to prove experimentally the connections between the various stages in the bucephalid life history. His work was based on the sporocysts and cercariae of B. cuculus McCrady, 1874, the sporocysts of which infect Crassostrea virginica (Gmelin). However, Tennent disregarded McCrady's name for the cercaria by calling it B. haimeanus Lacaze-Duthiers, 1854, even though McCrady had shown that it differed from B. haimeanus. The close similarity between the known bucephalid cercariae on the one hand, and adults on the other, led Tennent to believe that all forms were merely "physiological varieties" of the one species. It is clear from the systematic work of many earlier and subsequent authors on the group that this opinion held by Tennent is erroneous.

Hopkins (1954) critically reviewed Tennent's work and stated (p. 354) that "Tennent found immature encysted bucephalids (metacercariae) in Menidia and adults in Strongylura and assumed they were the same species as the oyster cercariae. He never obtained experimental evidence of a connection between these forms. He did prove that encysted forms in Menidia would excyst, live and grow when fed to predaceous fishes but he never completed these experiments ... Tennent did prove that some unknown gasterostome in Lepisosteus was capable of infecting oysters and developing in oysters at least to the sporocyst stage." However, Tennent did not describe the adult from Lepisosteus, assuming it to be the same species as other bucephalid adults previously described.

Hopkins (1954) has shown that Strongylura harbours three species of bucephalids whose excretory systems exclude any possibility that they might develop from the page 4 cercaria B. cuculus. He also described Rhipidocotyle lepisostei from Lepisosteus spatula and on the basis of the excretory system and Tennent's work, states that this species could possibly develop from B. cuculus. However, no infection experiments have been made to confirm this probable relationship.

Levinsen (1881) described the cercaria B. crux from the marine mussel Modiolaria discors. This cercaria was distinct from B. haimeanus in having a posterior, sucker-like extension of the tail-stem. Odhner (1905) considered that this was a stage in the life history of Prosorhynchus squamatum Odhner, 1905, and furthermore, he stated that immature specimens of this species had been found encysted in Cottus scorpio by Levinsen but identified by this author as G. armatum Molin.

Cole (1935) described the cercaria B. mytili (which resembles B. crux) and, although he was apparently unaware of Levinsen's B. crux and Odhner's suggested relationship between B. crux and P. squamatum, he considered that the identity of B. mytili "with a species of Prosorhynchus might yet be established."

Chubrik (1952) pieced together various stages in the life history of P. squamatum. He followed Odhner by assuming that a cercaria of the B. crux-type developed into a species of Prosorhynchus.

There is no experimental evidence to substantiate any of these views. Hopkins (1954, p. 355) stated "So far, there is no way to tell which genus of the Bucephalidae a given cercaria belongs to until the life cycle is worked out by means of experimental infections."

Carrere (1937) discovered bucephalid metacercariae in Atherina sp. from the Mediterranean Sea. These developed experimentally into a new species, Dolichoenterum lamirandi, in Labrax lupes and Hyla arborea. This, along with Tennent's observation that an unidentified bucephalid in Lepisosteus sp. could infect oysters and develop to the sporocyst stage, constitutes the only published experimental proof of a connection between the various stages in the postulated life history of marine bucephalids.

Yamaguti (1958) stated that the life history of the marine bucephalid Prosorhynchus uniporus Ozaki, 1924, was determined experimentally by Ozaki (1954). This work has not been found by the author despite an intensive search of the literature.

Three life histories of North American fresh-water bucephalids have been determined experimentally. They are Rhipidocotyle papillosum (Woodhead, 1929), and B. elegans Woodhead, 1930, determined by Woodhead (1929, 1930); and R. septpapillata Krull, 1934, determined in part by Krull (1934), and completely by Kniskern (1952).