The ultrastructure of Heleopera petricola an agglutinate soil amoeba; with comments on feeding and silica deposition

Abstract

The ultrastructure of H. petricola is described from cultured specimens. Siliceous shell plates are bound by a continuous layer of organic cement which is thicker in the posterior half of the body. The cytoplasm is divided into two distinct portions: anteriorly it is vacuolar and composed of numerous pseudopodial strands; posteriorly it is a compact mass containing the organelles, two or three contractile vacuoles, a typical 'ovular' nucleus with several nucleoli that occupies a central position in a region of dense endoplasmic reticulum, and numerous dictyosomes. Prey is captured by a loose arrangement of pseudopodial strands and digestion occurs within specific areas of compact cytoplasmic strands. It is suggested that the cyst wall of naked amoebae is ruptured by specific enzymatic activity and the contents digested after penetration by cytoplasmic strands. Predated siliceous body plates, enclosed in membrane bound vacuoles are modified by the addition of silica to their surfaces. Some organic particles, like bacteria, may also be coated with biogenic silica. Comparisons are made both with the predation of fungal spores by giant naked amoebae and silica biomineralization in another testate amoeba, Netzelia tuberculata.