Pigmented corpuscles in the midgut gland of Pomacea canaliculata and other Neotropical apple-snails (Prosobranchia, Ampullariidae): a possible symbiotic association.

Address correspondence to: Dr. Alfredo Castro-Vázquez, Fisiología Normal, Facultad de Ciencias Médicas, Universidad Nacional de Cuyo. Casilla de Correo 33, (5500) Mendoza, ARGENTINA. Fax: (+54-261) 449 4117; E-mail: [email protected] / [email protected] Received on November 29, 2000. Accepted on November 13, 2001. Pigmented intracellular corpuscles (“spherioles”) have been known to occur in the midgut gland of ampullariid snails for many years (Meenakshi, 1955; Andrews, 1965; Lutfy and Demian, 1968). Andrews (1965) described them in her morphological account of the midgut gland of P. canaliculata (Lamarck, 1822), and referred to them as “greenish spherules” and “brown concretions” (in this paper, they will be referred to as C and K corpuscles, respectively). She also adscribed a digestive-excretory function to C corpuscles and an excretory function to K corpuscles, since she found them associated with two distinct cell types in the gland acini: C corpuscles to the supposedly digestive (columnar) cells, and K corpuscles to the supposedly excretory (triangular or pyramidal) cells. However, we were intrigued by the “cellular” appearance of C corpuscles, and by the hard multilamellar structure of K corpuscles, that we could only dissolve in hot sodium hydroxide solutions and found that it contains proteins. Therefore, the fecal loss of these physiologically valuable material was intriguing, particularly if the snail ́s need of an adequate nitrogen and carbon balances were considered. So, we envisaged the possibility that they could be indeed the vegetative and kystic forms, respectively, of a prokaryotic symbiont that was living and reproducing within the glandular cells, and that was also eliminated to the external environment, where it could also undergo a free-living cycle. The results of our first efforts to test this hypothesis are presented here.