A. tenebrosa is radially symmetrical around the oral-aboral axis, with the mouth as the oral axis and the opposite end of the mouth (the base), the aboral axis. Radial symmetry is when a vertical section along the axis or through any radius produces mirror-image halves. Being an opportunistic feeder, radial symmetry allows the animal to detect prey or predators from all directions.

A. tenebrosa has a polyp form which is cylindrical in shape with a crown of tentacles arranged in six circles (subclass: "hexa"corallia) around the circumference of the oral disc and surrounds the mouth which occurs in the center of the oral disc. The tubular column is usually reddish-brown in color and the simple, tapered tentacles are usually blood red. The tentacles are responsible for feeding and contain nematocysts that are capable of paralysing the prey so that they can be pushed by the tentacles towards the mouth. Specialised blue structures called acrorhagi lay near the bases of the tentacles and are solely used for offense (Abel, 1954). Nematocytes (stinging cells) occur in dense concentrations inside these structures and are used when fighting over territories on inter tidal substrates (Bigger, 1982). During aggression, acrorhagi are inflated and discharge nematocysts causing necrotic lesions on the epidermis of their opponent.

The column wall is generally smooth, undifferentiated and is divided into two regions by a fold which is referred to as the parapet. The region above the parapet is a thin-walled, neck-like introvert (capitulum) whereas the column wall below the parapet is usually thicker. When disturbed or exposed to air, the oral disc is retracted together with the tentacles and capitulum into the column and the parapet covers the opening. An expansion of the base of the column forms the pedal disc which functions as a site of attachment onto rocks or other substratum.