A Redescription of Rhabdamoeba marina, an Inconspicuous Marine Amoeba from Benthic Sediments

Abstract

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 jim in length, and are essentially stationary apart from the occasional extension of a long, active, feeding pseudopodium up to 50 jLm 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. Additional key words: naked amoebae, taxonomy, flagellates, flagellated buds, ultrastructure It is now generally accepted that heterotrophic protozoa, especially flagellates, are one of the major groups regulating bacterial abundance in marine systems (e.g., McManus & Fuhrman 1986). Recent studies have shown that amoebae can also be numerically important on surfaces, in the water column attached to flocs and in benthic habitats. Indeed, within the benthos, naked amoebae may be one of the major consumers of bacterial production (Butler & Rogerson 1996). Methods for studying amoebae usually rely on cultivation-enrichment techniques to amplify species present (Darbyshire et al. 1996). Direct observation of field material rarely reveals many amoebae since most are slow-moving, transparent, and attach firmly to surfaces. Even after enrichment cultivation, some amoebae are easily overlooked. A recent series of papers has focused on these types in an attempt to alert microbial ecologists to their existence and to increase our a Author for correspondence. E-mail: arogerso @ ocean.nova.edu understanding about the diversity of marine amoebae (Anderson & Rogerson 1995; Anderson et al. 1996, 1997; Hannah et al. 1996). Many amoebae are overlooked by virtue of their small size; for example, a recent study has shown that 61% of all the naked amoebae in marine benthic sediments are <10 jIm in size (Butler & Rogerson 1995). Others are overlooked because they are difficult to culture and attain only low densities after enrichment cultivation in the laboratory, and yet others are both small and slow-moving like Parvamoeba rugata ROGERSON 1993, a relatively common coastal isolate with a mean size of only 3.1 tim (Rogerson 1993). The present paper describes the features of an inconspicuous, slow-moving amoeba assigned to the genus Rhabdamoeba DUNKERLY 1921. This genus was first erected for a small unusual amoeba found in samples originating from the Plymouth Marine Laboratory, England, UK. The original description of the genus by Dunkerly (1921) was brief and based solely on observations of preserved material, and also failed to This content downloaded from 157.55.39.127 on Sun, 26 Jun 2016 06:54:25 UTC All use subject to http://about.jstor.org/terms Rogerson, Hannah, & Anderson identify a flagellate stage. Furthermore, the validity of the genus was questioned by Page (1983), who commented that Rhabdamoeba may be a small, smooth form of the testate amoeba, Trichosphaerium SCHNEI-