Fine Structure of Marteilia sydneyi sp. n.: Haplosporidan Pathogen of Australian Oysters

Abstract

A new species of oyster pathogen, Marteilia sydneyi, from Australian oysters, Crassostrea commercialis, is described incorporating light and electron microscope observations. The pathogen is a haplosporidan which exists as a plasmodium in the oyster hepatopancreas. Upon sporulation, 8 to 16 uninucleate sporangial primordia are internally cleaved (endogenously budded) from each plasmodium; thus conversion to a sporangiosorus occurs. Each sporangium enlarges and internally cleaves into 2 or 3 spore primordia each of which, in turn, internally cleaves into 3 uninucleate sporoplasms of graded sizes, the largest containing the smaller 2 in a vacuole and the intermediate-sized one containing the smallest in a vacuole. The spore wall is continuous without an orifice or operculum. Wolf (1972) recorded the presence of a haplosporidan in Sydney Rock oysters (Crassostrea commercialis Iredale and Roughley) from Moreton Bay, Queensland, Australia, in which he noted spores in the hepatopancreas. In subsequent papers Grizel et al. (1974) and Mix and Sprague (1974) noted its close similarity to Marteilia refringens Grizel, Comps, Bonami, Cousserans, Duthoit, and LePennec, a pathogen of Ostrea edulis Linne in French estuaries. Grizel et al. (1974) suggested that M. refringens is one of the lower fungi; however, Perkins (1976) noted haplosporidan characteristics in the form of haplosporosomes and in the mechanism of spore formation from sporangia. The organism has, therefore, been placed in the Class Haplosporea. The present paper shows that the pathogen of C. commercialis is a haplosporidan and is in the same genus as the pathogen of Ostrea edulis, popularly known as the agent of Aber disease (Alderman, 1974). The pathogen has been found only in subtropical and tropical regions of the eastern Received for publication 14 October 1975. *Contribution No. 744, Virginia Institute of Marine Science, Gloucester Point, Virginia 23062. Australian coast, not in the colder waters south of Richmond River in Australia. The other species of Crassostrea, C. echinata Quoy and Gaimard, from the eastern coast of Australia is also parasitized by a species of Marteilia probably identical to the one described herein (Wolf, unpublished). MATERIALS AND METHODS Blocks of hepatopancreas, 1 to 4 mm8 in size, were obtained from Moreton Bay, Queensland oysters (C. commercialis) and fixed in 6% glutaraldehyde for 5 to 7 days during transit from Australia to the U.S.A. Subsequent treatment has been described (Perkins, 1976). Light micrographs were obtained from histological slides of oysters fixed in Davidson's fluid and stained with Harris' hematoxylin and eosin. Conclusions concerning spore structure were based on electron microscope observations of serial sections through 2 spores as well as observations of individual sections. Serial sections were mounted on a Formvar substrate on Mason and Morton slot (2 X 1 mm) discs.