ABSTRACT: Piscine coccidia differ from terrestrial-host coccidia in having a soft, membranous oocyst wall. In contrast, however, to the structural and developmental conformity observed among the highly evolved and specialized monoxenous eimeriid coccidia of avian and mammalian hosts, piscine coccidia demonstrate extreme diversity in developmental sites, sporocyst morphology, macrogamont organization and oocyst wall formation. Sporozoites and some merozoites of several piscine coccidia contain refractile bodies, while those of others contain crystalline ones. Some are obligatorily heteroxenous, while in others transmission is mediated by paratenic hosts. Structural and developmental variability among piscine coccidia could imply a polyphyletic origin, but it could also be an evidence for a lower degree of evolutionary specialization. Many of the structural and developmental features found in piscine coccidia occur in other lower vertebrate and invertebrate-host coccidia, as well as in the heteroxenous cyst-forming coccidia of higher vertebrates.

KEY WORDS: Coccidia . Fishes . Ultrastructure . Development . Transmission . Diversity . Phylogeny

Published in DAO Vol. 22, No. 1 (1995) on May 4
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1995