Monster or multiplacophoran: A teratological specimen of the chiton Acanthopleura granulata (Mollusca: Polyplacophora) with a valve split into independent and symmetrical halves

Teratological specimens deviate from the conserved form of their species. In doing so, they serve as natural experiments that refine our knowledge of developmental mechanisms and the natural limits of phenotypes. Here, we describe a specimen of the West Indian Fuzzy Chiton Acanthopleura granulata (Gmelin, 1791) with a fifth valve split into two halves. Using micro-CT to non-invasively visualize the external and internal morphology of this specimen, we find that the half valves are symmetrical and independent from each other and from any of the other valves. The presence of girdle-like tissue between the split valves suggests that this shell abnormality arose in early development and was not the product of damage to the adult animal. While the present specimen of A. granulata is clearly abnormal for its species, its split valve may provide some insights into the developmental pathways that would underlie macroevolutionary transitions to multi-plated chiton forms known from the fossil record.