Abstract

This study reports on the ultrastructure of the digestive gland of the queen conch Strombus gigas Linné, 1758. This analysis was related to the intense and generalized sporozoan infection first detected in individuals from San Andres Island (Colombia). Three cell types occur in the epithelium of the blind-ending tubules of the digestive gland according to histological and TEM observations: digestive cells (1); cryptic cells (2), and vacuolated cells (3). The columnar digestive cells (1), characterized by large alcian blue positive granules, represent the most conspicuous secretory cells composing the digestive acini of S. gigas. Strong positive in situ hybridization of eukaryotic ribosomes indicated that triangular shaped cryptic cells (2) are usually very active. This was confirmed by the larger content of ribosomes and REG in their cytoplasm observed with TEM compared with the other cell types of the digestive gland. These data suggested that such cryptic cells are probably secretory active cells rather than immature cells involved in a process of replacement of eliminated digestive cells. The vacuolated cells (3) are not typical gastropod excretory cells. They appear as the most frequently infested cells by Apicomplexa parasites which have been found in the digestive gland of every sampled organism in the queen conch population of Guadeloupe as well as described in the San Andrès Colombian island. Various stages of the parasite were identified using TEM and SEM, indicating that this parasite apparently complete its entire cycle within the same host and tissue. Apicomplexa-like trophozoites were embedded in the digestive gland cells and attached to host cell wall by their conoid end. Several stages were detected: trophozoites, sporocysts, and gamonts containing macrogametes or microgametes. Further analyses are in progress to check for the presence of such parasites in the digestive gland of other Strombus species of the Caribbean area. Moreover, new investigations are designed to improve our knowledge of the parasite phylogeny according to 18S rDNA gene sequence analysis.

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