Life history of Coitocaecum parvum Crowcroft, 1944 (Trematoda) from Canterbury.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The identity and life history of a trematode of the genus Coitoaaecum , commonly found in freshwater fishes in Canterbury, has been examined. Adults collected from naturally and experimentally infected fish closely resembled C. parvum Crowcroft, 1944 and an account of C. anaspidis Hickman, 1934, given by MacFarlane (1939). They differed from the latter in the position of the genital pore, shape of the cirrus sac and ovary, and in their size. Differences between the adults from Canterbury and the types of C. parvum were considered insufficient to regard them as separate species. The life history of C. parvum from Canterbury was completed through experimental infections of laboratory-reared hosts. Adults were recovered from the common bully, Gobiomorphus cotidianus, sporocysts and cercariae from the snail, Potamopyrgus antipodarum, and metacercariae from the mysid Tenagomysis chiltoni. Coitocaecum parvum were also found in field collections of freshwater fish (G. cotidianus, G. breviceps, Galaxias maculatus, Retropinna retropinna, Anguilla spp.), snails (P. antipodarum) and crustaceans (T. chiltoni, Paracalliope fluviatilis). Abbreviation of the life history of C. parvum was found to be possible through the production of viable eggs by encysted progenetic metacercariae. Eggs of C. parvum metacercariae from mysids and amphipods hatched and were capable of successfully infecting laboratory-raised snails. A mechanism that may account for the release of eggs of progenetic metacercariae from within the confines of the cyst wall and tissues of the second intermediate host was examined. Metacercariae were found to excyst both on death of the mysid host and when treated with fluid from the hepato-pancreas of the host. Some of the possible factors affecting the appearance of progenesis in C. parvum metacercariae have also been examined. Progenetic cysts were present in mysids during most of the year but prevalence was highest from winter to early spring and in summer. High levels in winter and spring were attributed to the gradual maturation of cysts accumulated by long-lived overwintering mysids, while high levels seen in warm summer months were probably due to the rapid maturation of cysts in spring generation mysids. The appearance of progenesis in C. parvum metacercariae appeared largely independent of environmental water temperature although warm temperatures may have enhanced development. Sexual maturation and related hormonal changes in hosts were not considered important factors in stimulating egg production in c. parvum. The prevalence and intensity of infection with progenetic cysts did not vary between male and female hosts. Progenetic cysts were also present, although not in equal numbers, in juvenile mysids. The prevalence and intensity of progenetic infections in mysids increased with an increase in the size of the host. Although progenetic cysts were most frequently recovered from large, mature mysids, they were also occasionally found in small hosts. Physiological changes that may have occurred as the hosts aged and approached their death did not seem important in stimulating the appearance of progenesis in metacercariae. Progenesis in C. parvum was considered a natural result of continued growth by the metacercariae during prolonged confinement within the second intermediate host.