Biology of a New Cercaria for Posthodiplostomum minimum (Trematoda: Diplostomidae)

Abstract

Snails, Physa halei Lea, from Madison County, East Texas, shed strigeid cercariae in large numbers. Adult Posthodiplostomum minimum (MacCallum, 1921) Dubois, 1936, were recovered from chicks force-fed metacercariae obtained from both natural and experimental infections of fishes. Trematodes recovered in the present study were identical to type specimens and to those reported by other workers. Cercariae differed from ones previously described mainly in the absence of caudal bodies at any stage of development and in flame cell number. Other differences included body annulations and associated spines, rows of spines on the oral sucker, flagellets on the tail stem, and body size range. Previous studies on metacercarial specificity between fish families and among members of the same family, and three former cercarial descriptions, have suggested the occurrence of several physiological strains or subspecies of P. minimum. Findings in the present study help support this belief. Posthodiplostomum minimum (MacCallum, 1921) Dubois, 1936, has been investigated intensively with plentiful data accumulating in the literature about the metacercaria and adult. During these studies, three different strigeid cercariae have been considered by various workers as the larval type for P. minimum: Cercaria multicellulata H. M. Miller (1923, 1925), C. louisiana E. L. Miller (1935, 1936), and C. minimum J. H. Miller (1954). These cercariae differ in size, arrangement and number of spines and setae, musculature in the tail stem, and presence or absence of caudal bodies. All three types have produced experimentally metacercariae identified as those of P. minimum: Ferguson (1943), J. H. Miller (1954), Hoffman (1958). J. H. Miller considered C. louisiana to be incorrect and C. multicellulata to be confused with another closely related strigeid. Further, C. minimum was regarded as the only cercaria proved experimentally to be the correct one for P. minimum. The present paper describes still another cercaria of Posthodiplostomum minimum, which has been studied experimentally to obtain metacercariae and adults for comparison with type specimens and descriptively for comparison with the three other reported cercariae. MATERIALS AND METHODS One hundred and twenty-five snails, Physa halei Lea, collected in the autumn of 1966 from three small ponds 3 miles west of Madisonville, Madison Received for publication 24 April 1967. * Supported by grants from the Penrose fund of the American Philosophical Society and the Society of Sigma Xi. County, Texas, were isolated in finger bowls where 90% shed strigeid cercariae spontaneously. All larvae were studied intensively while alive under light coverslip pressure both unstained and with supravital neutral red stain. Following live study permanent slides were prepared using either hot 5% formalin or alcohol-formalin-acetic fixative, Semichon's acetocarmine stain, oil of cloves clearing agent, and balsam mountant. Metacercariae and adults obtained during the investigation were studied and made into permanent mounts as described above. A Wild M20 phase-contrast microscope equipped with a camera lucida was employed in all observations and in preparation of illustrations. All larval measurements from specimens heat-killed in 5% formalin are expressed in microns. EXPERIMENTAL STUDIES Eight 21/2-inch fingerlings, four each of Lepomis macrochirus and L. microlophus, from a group of hatchery-reared uninfected fish were placed with about 700 cercariae in an 8-gal aquarium at 25 C. Cercariae were attracted to fishes and actively attempted penetration, causing fish to display violent finfanning and brushing against aquarium walls. Extensive hemorrhaging occurred around the fins of fishes at points of cercarial penetration. One L. macrochirus succumbed on day 20 and two L. microlophus on days 22 and 33, respectively. Controls were unaffected. Postmortem examinations of all four L. macrochirus showed a metacercarial infection of 91 to 147, average 115, and four L. microlophus 5 to 76, average 27. Fish not dead from infection were killed and examined periodically. Morphological changes, but little size change, were present in larvae examined on day 8. By this date, larvae were less tapered posteriorly and were