Abstract.Deltaic populations ofConopeum seuratifrom diverse environmental settings have variable phenotypes related to strong competitive regimes and to dissolved oxygen minima. Under extremely stressful conditions, the poorly canalized phenotype of C.seuratidisplays new additional features such as kleistozooids (kleistós= closed) and chaotic celleporoid layers. Kleistozooids are clearly distinguishable polymorphs characterized by the presence of a compact, imperforate frontal membrane and by a strongly reduced lumen of the “opesium”. They commonly aggregate in clusters of regularly‐arranged individuals that store a large quantity of proteic material. The inferred function of these cell‐like structures is to supply nutrients for basic colony activities. Celleporoid layers are, possibly, a developmental relaxation of the colony ofC. seuratifor a more efficient use of space in competitive interactions.Both celleporoid layers and kleistozooids reveal the magnitude of intracolony variability of a widespread opportunistic bryozoan
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