The ultrastructure of Gymnosphaera albida Sassaki, a marine axopodiate protozoon.

Abstract

Gymnosphaera albida has been found on the sponge Sycon ciliatum in the Menai Straits, North Wales, during the months of May to December. It commonly adopts a sedentary mode of life when cultured, settling with its body in contact with the substratum and its axopodia radiating upwards and outwards all round. At times it floats freely. When sessile it can displace itself, but not by rolling. It is a voracious carnivore. The largest seen had a body size of 510 mum X 320 mum. The body of Gymnosphaera is divided into three zones: a central medulla, a cortex and a superficial reticulated pseudopodial layer. The medulla is finely vacuolated and contains an axoplast at its centre. The cortical cytoplasm contains many nuclei, Golgi bodies, polysomes, mitochondria, osmiophilic globules, lipoid spherules and vacuoles of various kinds, but no zooxanthellae. The superficial reticulated pseuopodial layer contains osmiophilic globules and occasional mitochondria. Axonemes radiate from the axoplast to the axopodia, along which osmiophilic globules are generally in motion. In between the cortex and the reticulated pseudopodial layer there is a narrow, extracytoplasmic capsular wall (Sassaki's line), consisting of a microfibrillar coagulum. The wall is a labile structure, perforating locally to allow the passage of food vacuoles or faeces and vanishing completely in certain conditions. It is evaginated to form a sleeve around the base of each axopodium. The cortex is completely penetrated by a system of clefts, the lumen of which opens here and there into the space containing the capsular wall. The clefts are distinct from the endoplasmic reticulum, cisternae of which are commonly found near the surface of the cytoplasmic tracts. Some of the cortical vacuoles contain organic refractive crystals. The crystals have the shape of crossed rodlets, each rodlet having a thermostable component ensheathing a thermolabile component. Their properties are described. The nuclei are enveloped in a thin layer of cytoplasm, connected by narrow bridges to the adjacent cytoplasmic strands. They generally contain several peripherally arranged nucleoli, each bearing a number of nucleolar organizers. Near the centre of the nucleoplasm there is usually a "central chromatin body". The vacuoles of the medulla are of two kinds, one equipped with a fibrous coat. In the vicinity of an axoneme the coat commonly connects with the microtubules and their cross-bridges. The axoplast has a central "hyalosphere" exhibiting a fibrogranular matrix. No tripartite organelle is present therein. The axoplast shell consists of the proximal ends of the axonemes, each enveloped by a fibrous sheath, the fibres coursing around adjacent axonemes, binding them together. The shell thickness is a constant fraction (1/2.5) of the axoplast diameter. The axonemes consist of bundles of parallel microtubules arranged in transverse section in a pattern of alternating rows of hexagons, the microtubules being joined together by 12.3 nm long cross-bridges...