A crab with three eyes, two rostra, and a dorsal antenna-like structure

•We describe a crab with three eyes, two rostra, and a dorsal antenna-like structure.•An inspection of internal features revealed a brain with enlarged protocerebrum.•The putative causes for this kind of malformation are discussed.•A scenario combining a conjoined twin with a regeneration event seems most likely.

We describe a malformed specimen of the freshwater crab Amarinus lacustris from New Zealand. With three eyes in a horizontal row, two rostra, and a dorsal antenna-like structure, the pattern of malformation of this animal is unique and has not been described before. A careful inspection and description of external and internal structures, in particular the central nervous system, were carried out. These revealed, in addition to the external abnormalities, a retarded brain with a hypertrophied and backwards bent protocerebrum connected with all three eyes and putatively with the dorsal antenna-like structure. Based on these data, a variety of hypotheses about the causes for this kind of malformation are discussed. A scenario combining a conjoined twin (Duplicitas anterior) based on the duplication of the embryonic anterior head lobes and a regeneration event leading to the replacement of an eye by an antenna shows the best fit to the observed patterns.