Character Divergence in Two Species of Trematodes (Pharyngostomoides: Strigeoidea)

Abstract

Two species of Pharyngostomoides (Ph. procyonis Harkema, 1942 and Ph. adenocephala Beckerdite, Miller, and Harkema, 1971), co-occur in raccoons (Procyon lotor) in the southeastern USA (Beckerdite et al., 1971, Proc. Helm. Soc. Wash. 38: 139-156). The species were separated by Beckerdite et al. on the basis of several characters; Ph. adenocephala was large, spatulashaped, pinkish in color when alive, the forebody and hindbody were usually on a different plane (forebody flexed dorsad), conspicuous glands were present posterior to the pseudosuckers, and no ejaculatory pouch was present; Ph. procyonis was small, ovoid in shape, ivory-colored when alive, the forebody and hindbody were usually on the same plane, the glands posterior to the pseudosuckers were absent or inconspicuous, and an ejaculatory pouch was present. Both Pharyngostomoides species have similar life cycles and use the same intermediate hosts (Beckerdite et al., 1971, loc. cit.; Miller, 1981, Parasitology 82: 335342). Both species were collected from a sample of 10 raccoons from southern Ontario (Butterworth and Beverley-Burton, 1981, Proc. Helm. Soc. Wash. 48: 24-37). Identifications were confirmed by comparing specimens with type and paratype specimens of Ph. procyonis (No. 44850, 44851) and Ph. adenocephala (No. 71586, 71587) obtained from the U.S.N.M. Helminth Collection, plus additional specimens from the U.S.N.M. Helminth Collection or from G. Miller (North Carolina State University). Measurements of gravid adult specimens showed that the size relationship reported by Beckerdite et al. (1971, loc. cit.) was reversed in specimens from Ontario; Ph. procyonis was significantly larger, not smaller, than Ph. adenocephala (Fig. 1). Body sizes of the larger species (Ph. adenocephala-southeastern USA, Ph. procyonis-Ontario) were not significantly different and the body sizes of the smaller species (Ph. procyonissoutheastern USA, Ph. adenocephala-Ontario) were not significantly different (Fig. 1). Thus, not only was the body size relationship reversed but a distinct different in size was maintained. A difference in body size (or sizes of feeding structures) of a pair of species which is greater in sympatric than in allopatric populations is referred to as a character displacement (divergence) (Grant, 1975, Evol. Biol. 8: 237-337). In our study the two species of Pharyngostomoides are regionally sympatric; however, two recent surveys of parasites of raccoon have reported only one species present. Pharyngostomoides adenocephala was reported from raccoons in Saskatchewan (Hoberg and McGee, 1982, Can. J. Zool. 60: 53-57) and Ph. procyonis from raccoons in Tennessee (Bafundo et al., 1980, J. Parasitol. 66: 134-139). We examined specimens from both studies; in each case the specimens were of the small body size. The size difference in sympatric as opposed to allopatric populations suggests character divergence. The process most often cited as the cause of character divergence has been competition; however recent criticism by Wiens (1982, Ann. Zool. Fennici 19: 297-308) and in references cited by him suggest that there is little evidence to support this claim. Reproductive isolation of congeners, climatic differences, predation and resource characteristics have all been considered to be valid alternate explanations for character divergence (Grant, 1975, loc. cit.). Of these, only competition or reproductive isolation are feasible explanations for the observed maintenance of a distinct size difference between the two Pharyngostomoides species in the presence of a size reversal. Beckerdite et al. (1971, loc. cit.) have observed that when the smaller species from the southeastern USA, Ph. procyonis, occurred in high numbers, the larger species, Ph. adenocephala,