Colonial ascidian with zooids completely embedded forming elaborate thick encrusting cushions (10 - 20 cm in diameter and easily up to 5 cm thick).
Branchial aperture, often adored with white pigment collar, are arranged around central common cloacal openings.
Colonies may be white, grey, red or orange with distinct white rings or not.
Surface without epifauna, sand or shell particles.
Sand particles and shell fragment may be present in varying density in the internal test.
Branchial sac with 10-12 rows of 12-17 stigmata, stomach with 20-30 folds (some may be incomplete).
Gonads in posterior-abdomen with male follicle posterior to the ovary.
Two to three embryos may be present in cloacal cavity.
Larval (trunk 0.5-0.6 mm long) have three stalked adhesive papillae alternating with four long finger-shaped ampullae.
Millar, R. H. 1962 Further descriptions of South African ascidians56:113-221
Monniot, C.; Monniot, F.; Griffiths, C. and Schleyer, M. 2001 Ascidians of South Africa.108:1-141
Shenkar, N.; Gittenberger, A.; Ruis, M.; Rocha, R. M.; Swalla, B.; et al. 2017 Ascidiacea World Database. Atriolum marinense Kott, 2001. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=250052 on 2019-03-12