SOUTHERN OYSTER DRILL AS A PREDATOR 



89 



There are no data to explain why some juvenile 

 laughing gulls should become naturally infected, 

 while others of the same age in the same colony 

 and free of natural infection could not he infected 

 experimentally. Possibly some signiticant, but 

 unknown, dilference between naturally encysted 

 metacercariae and those which have encysted in 

 the 4-inch bowls of sea water prevents the latter 

 from infecting laughing gulls. Neither is it pos- 

 sible to explain the observed resistance of least 

 terns. Perhaps least tenis are less susceptible to 

 Parorchu infection than were the common and 

 roseate terns used by Stunkard and Cable (1932). 



Encysted metacercariae were given per anum to 

 a single herring gidl on two occasions 18 days 

 apart. It became infected after the second dose 

 and yielded 19 mature worms at autopsy. 



Table 7. — Susceptibility of marine birds to expcrimentaj 

 P. acanthus infection by encysted metacercariae given 

 orally 



' Fed two metacercariae naturally encysted on a Xanthid crab; all other 

 birds fed metacercariae which had encysted on bottom of a glass bowl. 



Transfer of adult worms was successfiil only 

 once. A single sexually mature worm was trans- 

 ferred from a laughing gull to a young least tern 

 and was recovered alive when the tern died a week 

 later. Three other transfers failed to establish 

 infections in the recipient : Herring gull to herring 

 gull, 13 immature worms; herring gull to herring 

 ^11, 58 immature worms; and herring gull to 

 ring-billed gull, 16 mature worms. It is not known 

 (vhy these transfers failed to infect. Possibly, the 

 recipient gulls possessed a local immunity wliich 

 laused rejection of the inocula. It is also remotely 

 possible that the recipients could have gained im- 

 munity via a pre-existing infection since they were 

 wild birds. Rut since the birds were all still im- 

 nature, it seems unlikely that such infections 

 ;ould have been lost quickly. Further, direct vis- 

 lal examination of the cloacae made repeatedly 

 Jver a period of several weeks prior to the trans- 

 ■ers failed to reveal any Parorchis infection and 

 lone was found at autopsy. 



To obtain a measure of the intensity of infection 

 which might be expected to develop in host birds, 

 9 gulls were given known numbers of encysted 

 metacercariae orally, and the numbers of adult 

 worms developing in resultant infections were 

 determined. Table 8 shows that only about one- 

 half of the birds became infected. In successful 

 infections generally only a few of the encysted 

 metacercariae developed into adult worms. 



The yield of adult worms was also low among 

 six other gulls, not shown in table 8, which earlier 

 were fed large, but undetermined numbers of 

 Parorchis cysts in order to obtain worms and eggs 

 for use in drill infection studies. One herring gull 

 yielded five worms. Two of five ring-billed gulls 

 yielded one worm each ; the other three yielded two 

 worms each. 



LARVAL HOSTS 



The results of drill-infection experiments are 

 given in table 9. Nearly 60 percent of all experi- 

 mental drills became infected. The small number 

 of rediae recovered in serial sections of infected 

 drills indicates that most of the miracidia failed 

 to penetrate drills and consequently, the intensity 

 of infection in individual drills was very low. 

 This very low intensity of the experimental in- 

 fections might be interpreted to suggest that re- 

 peated exposure to infection would be required in 

 order to build up intensities comparable to those 

 found in individual wild drills. However, the 

 observed incidence of natural infections was so 

 low that it appears more likely that the high in- 

 tensities observed in natural infections are the 

 result of multiple exposure, i. e., simuJtaneou.^ 

 exposure to many miracidia, rather tlian of re- 

 peated, or sequential exposures. 



Table 8. — Relation of P. acanthus adults recovere-d to en- 

 cysted metacercariae given orally in gun infection ex- 

 periments 



' Metacercariae naturally encysted on Xanthid crab leg. 



