Protozoa diurnal

Arcella

Arcella hemisphaerica

Meet the golden dome-builder of the microscopic world. Arcella hemisphaerica is a tiny architect that carries its house on its back, patrolling your backyard ponds and mosses in search of its next meal.

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Quick Identification

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Size

40–70 micrometers (0.0015 to 0.0027 inches) in diameter

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Colors

Transparent to light yellow when young, turning golden-brown or amber as the shell ages and incorporates iron

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Key Features

Hemispherical dome-shaped shell (test)
Central circular opening called an aperture on the underside
Blunt, finger-like pseudopodia used for locomotion
Smooth, chitin-like surface texture

When You’ll See Them

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Activity pattern diurnal

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Peak hours Active 24/7 as long as moisture is present, though most easily observed during daylight hours under a microscope.

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Season Year-round, with population spikes during warm, wet summer months.

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Diet A micro-predator that feeds on bacteria, small green algae, and other tiny protists by engulfing them with its pseudopodia.

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Habitat Freshwater environments including ponds, slow-moving streams, Sphagnum moss, and even damp backyard garden soil.

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Behavior

Arcella hemisphaerica is a testate amoeba, meaning it lives within a protective, self-constructed shell. Unlike the 'blob' shape people often associate with amoebas, Arcella is remarkably symmetrical and looks like a tiny golden umbrella or dome under the microscope. It moves by extending lobopodia—thick, blunt arms—through a hole in the bottom of its shell, pulling itself along surfaces like pond weeds or grains of sand.

These micro-architects are fascinating because they can regulate their buoyancy. By producing tiny carbon dioxide bubbles within their shell, they can float up through the water column to find better feeding grounds or sink back down to the safety of the sediment. They are solitary hunters but are often found in high densities where bacteria and organic decay are prevalent.

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Camera Tips

Capturing Arcella hemisphaerica requires a departure from traditional trail cameras. To find these 'backyard monsters,' you will need a digital microscope or an AI-powered smart microscope. The best way to get a shot is to create a 'wet mount' slide. Squeeze a handful of damp moss or take a dropper-full of water from the very bottom of a backyard pond, where the sediment is rich. Place a drop on a slide and look for the characteristic golden-brown domes.

Because Arcella are slow-moving, time-lapse photography is your best friend. Set your camera to capture an image every 2 to 5 seconds. When played back as a video, you will see the amoeba's 'arms' rhythmically reaching out and pulling the shell forward in a way that is invisible to the naked eye. To get the best contrast, use 'oblique illumination'—angle your microscope light slightly from the side rather than directly from below—to highlight the three-dimensional curve of the hemispherical shell.

If you are using an AI identification system, focus specifically on the aperture (the hole at the bottom). A clear shot of the circular opening and the way the shell curves away from it is the primary diagnostic feature for this species. If the specimen is flipped upside down, you can see the internal organelles through the opening, which makes for a stunning and educational image. For the most vibrant colors, look for older specimens which have accumulated more iron, giving them a deep, burnt-orange hue that pops against green algae backgrounds.

Similar Species

Species that look similar or are commonly confused with Arcella.

Arcella vulgaris

The shell of A. vulgaris is much flatter and pancake-like, whereas A. hemisphaerica is shaped like a tall dome or a half-sphere.

Difflugia

Difflugia builds its shell out of tiny grains of sand or minerals, giving it a rough, 'rocky' appearance compared to the smooth shell of Arcella.

Frequently Asked Questions

Arcella hemisphaerica are active whenever they are submerged in water. However, they are easiest to photograph during the day when consistent light sources are available for microscopic observation.

You don't need to do much! Simply maintaining a small pond, a birdbath with a bit of organic debris, or even a patch of healthy, damp moss will provide the perfect habitat for these tiny creatures.

They primarily eat bacteria and single-celled algae. They use their flowing arms to trap food particles and pull them inside their shell to be digested.

Yes, they are incredibly common and can be found in almost any suburban environment that stays consistently moist, including garden soil and gutter debris.

Look for the shell. Unlike 'naked' amoebas, Arcella has a distinct, amber-colored dome. You can tell it from other Arcella species by its perfectly hemispherical shape—it is taller and more rounded than the flatter Arcella vulgaris.

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