The development and morphogenesis of Camallanus cotti Fujita, 1927 (Nematoda: Camallanidae), with notes on its phylogeny and definitive host range.

The freshwater fish nematode Camallanus cotti Fujita, 1927 (Spirurida: Camallanidae) is naturally distributed in East, South and Southeast Asia. During the past three or four decades the species has been disseminated to Europe, North America, Australia and Hawaii, mainly due to the extensive ornamental fish trade. In the present study, its development and larval morphogenesis is described. Newborn first-stage larvae are ingested by copepods and within hours reach the haemocoel, where the worms moult twice. At 22 degrees C ambient temperature, fully-developed third-stage larvae appear on day 11 p.i. After transmission into the fish intestine two further moults occur before the adult stage is reached. At 23 degrees C water temperature, the final moult is initiated around day 33 p.i. in males and between 34 and 42 days p.i. in females. Morphologically the young and older adults are similar, but the buccal capsule of both young males and young females is not yet fully sclerotised. Based on the close similarity in the development and adult morphology of the buccal capsule in C. cotti and three congeners from Europe and North America, we suggest, in general agreement with earlier workers, that the Camallanus Railliet & Henry, 1915 originated in tropical Asia and subsequently radiated to species when adapting to new climatic and/or host-related conditions in the Old World and the New World. C. cotti may thus represent a lineage of speciation which did not apparently involve extensive changes in relation to their natural geographical distribution or definitive host range.