Occurrence of an unusual branchial mycoplasma-like infection in cockle Cerastoderma edule (Moliusca, Bivalvia)

Unusual cytoplasmic structures were observed and described in gill epithelial cells during an occurrence of high mortality of cockle Cerastoderma edule obtained from an estuarine region of central Portugal (Aveiro) in 1991 and 1992.Ultrastructural studies on gaping cockles revealed a gill infection characterized by the presence of numerous long rod-shaped structures (RSS) regularly distributed, singly or in groups, within the cytoplasmic cisternae of gill epithelial cells.The RSS of 0.5 to 4 pm length and a uniform width of 0.9 pm presented a well organized structure in the basal portion of the host cell.Each RSS showed a homogeneous and electron-dense material delinuted by a tri-laminar membrane devoid of cell wall.Lysed ultrastructural aspects of the host cell were observed.Due to their ultrastructural organization these RSS were identified as mycoplasma-like microorganisms and their pathogenic activity in the mortality of the cockle is discussed.