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Fifty years ago, it was reported that individuals of the Pacific boreal limpet Problacmaea (= Erginus) bear a penis for copulation and brood their young in the pallial cavity. These observations were based on light microscopy, but now we reveal new details of the reproductive biology of Erginus (Problacmaea) puniceus with electron microscopy. Gametogenesis is fundamentally similar to other Patellogastropoda, but there are some key differences. Oocytes develop in the ovary, dorsal to the foot in the posterior half of the body, before passing through the oviduct to the pallial cavity, where they are fertilized and brooded. Development is direct with embryos being brooded to the crawl-away juvenile stage. However, sometimes fertilization occurs internally with embryos beginning development in or near the gonad. Free sperm, which were likely autosperm (self) but could have been allosperm (cross), were found near developing oocytes in the gonad. Spermiogenesis results in the formation of an enta-quasperm with a cap-like acrosome, a bullet-shaped nucleus and spherical mitochondria in the midpiece. Copulation was not observed, but we confirm the presence of a penis with a dorsal penial groove that lies below the right cephalic tentacle in males, which is typical of functional penises of some molluscs. Males were always smaller, but intermediate-sized individuals were hermaphroditic to different extents. The largest individuals were entirely female, and their penises were smaller and often lacked the penial groove. Thus, E. (Problacmaea) puniceus appears to be a protandrous hermaphrodite, passing from male to female phase. Remnants of the testis persist into the female phase, suggesting that they are protandrous hermaphrodites with overlap, possibly allowing for self-fertilization to occur on occasion. However, the largest females lacked any testis remnants, suggesting instead that the species may be protandrous sequential hermaphrodites. [ABSTRACT FROM AUTHOR]