Genus Saccamoeba Frenzel, 1892, emend. Bovee, 1972
Diagnosis: Locomotive form active but not eruptive; medium-sized, active limax amoeba with a vesicular nucleus; uroid a villous or papillate knob, with a bulging contractile vacuole; usually with cytoplasmic crystals; floating form with irregularly rounded pseudopodia, without long granular pseudopodia. Cysts reported in some species.
Type species: Saccamoeba lucens Frenzel, 1892.
Ecology: Freshwater and marine.
Key to the species (Page, 1988):
Cyst-forming; L 30-75 µm, L/B > 4; nucleus 4.5-8.5 µm (mean 6.5 µm); no cytoplasmic crystals; endocytic bacteria in all known strains; cysts with circular or slightly oval outline. usually sticky coat, diameter 12-19 µm; cup-like surface structures c. 25 nm in diameter; mitochondria often elongate.
|–||No cysts known; distinct cytoplasmic crystals||2|
|2||Crystals shaped like a croissant|
|–||Crystals otherwise shaped|
|3||Numerous small (1-3.5 µm) crystals; amoebae broad||4|
|–||Usually fewer than 30 crystals; amoebae broad or slender|
L to 175 µm, some not exceeding 130 µm; pseudopodia sometimes flattened in slow locomotion: distinct hyaline cap in rapid advance; wrinkled villous-bulb uroid; nucleus 3.5-6.5 µm; one to several contractile vacuoles.
L to 120 µm; hyaline cap reduced in rapid advance; uroidal villi on clear disc or bulb; nucleus 10-15 µm; one contractile vacuole.
Broad, L 70-100 µm; smooth or finely papillate bulbous uroid; nucleus 6-7 µm; usually fewer than 20 crystals, 5-15 µm, often appearing cubical or plate-like.
Slender (L/B > 4), L 35-85 µm; villous bulb common; nucleus c. 6-11 µm; bipiramidal crystals to about 3.5 µm, seldom more than 30 present, sometimes lost after years in culture; endocytic bacteria in known strains; cup-like surface structures c. 40 nm in diameter; mitochondria never elongate.
Remarks: This genus contains the largest hartmannellids and bears the most resemblance to the Amoebidae, especially in the frequent occurrence of crystals and the frequent possession of a villous-knob uroid, which sometimes occurs on some Amoebidae. It is most like Trichamoeba, from which it has been distinguished by having a vesicular rather than a granular nucleus. With the finding that at least one member of the Amoebidae has a central nucleolus, this basis of distinction is somewhat weakened. However, no undoubted Saccamoeba has a floating form with long, granular pseudopodia. The finding of cup-like surface structures similar to those of other hartmannellids strengthens its place in this family. The possession of endocytic bacteria may be a generic characteristic; they have been found in two strains of S. limax and three strains of S. stagnicola. It should be noted that one of the infected strains of S. limax was Scottish and one American. Since Saccamoeba is aerobic and has mitochondria, the bacteria do not play the same role as in Pelomyxa.
It is still a good guide that a medium-sized, not eruptive but active limax amoeba with a vesicular nucleus, cytoplasmic crystals, a villous or papillate uroidal knob, and a bulging contractile vacuole is likely to be a Saccamoeba. If it has long floating pseudopodia, however, it is probably a Trichamoeba, but electron microscopy may be required to decide. The occasional absence of cytoplasmic crystals must also be kept in mind. (Page, 1988)