Rhabdamoeba marina was isolated from coastal waters off the Isle of Cumbrae, Scotland. It is worthy of attention for two reasons. Firstly, it is an example of a marine amoeba that is easily overlooked in fresh or enriched samples despite its widespread distribution within these waters. Secondly, the validity of the genus Rhabdamoeba has been questioned mainly because the original description by Dunkerly (1921) was brief and based solely on preserved material. The present redescription of this isolate, based on light and electron microscopy, removes the confusion surrounding the status of the genus. Amoebae are between 5.0 and 25.0 mu m in length, and are essentially stationary apart from the occasional extension of a long, active, feeding pseudopodium up to 50 mu m from the cell body. The pseudopodia of stationary cells are distinctive, appearing as knob-like or foot-like extensions around the cell body. Small papillae at the tips of these pseudopodia are evident at high magnifications by Light and scanning electron microscopy. Diagnostic features of note at the ultrastructural level include a cell surface without a glycocalyx and a nucleus with a central nucleolus (i.e., vesicular nucleus) which is common in small amoebae. The cytoplasm contains Golgi cisternae and the mitochondria have tubular cristae. The life cycle of this protist includes a flagellated stage that has not previously been reported.
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