How to stay attached—Formation of the ricefish plug and changes of internal reproductive structures in the pelvic brooding ricefish, Oryzias eversi Herder et al. (2012) (Beloniformes: Adrianichthyidae)

Teleost fishes show an enormous diversity of parental care, ranging from no care toviviparity with maternal provisioning of embryos. External brooders carry theirdeveloping eggs attached to their bodies. This requires the formation of novelmorphological structures to support attachment. The pelvic brooding ricefish Oryziaseversi evolved such a structure, called the “plug.” The plug anchors attachingfilaments from the fertilized eggs inside the female reproductive system, allowingthe female to carry the embryos until hatching. Using histological sections and µ‐computed tomography scanning, we show that the plug is formed by several typesof interstitial cells, blood capillaries, and collagen fibrils that encapsulate the end ofthe attaching filaments in the anterior part of the gonoduct. Even 15 days after theloss of the protruding attaching filaments, the plug remains. In addition, thedeveloped plug contains multinucleated giant cells that are derived from fusingmacrophages. We thus hypothesize that the ricefish plug, which is vital for eggattachment in O. eversi, evolved due to an inflammatory reaction. We assume that itforms similar to a foreign body granuloma, as a reaction to irritation or injury of thegonoduct epithelium by the attaching filaments. Our study further corroborates thatpelvic brooding entails a complex set of adaptations to prolonged egg‐carrying in thefemale reproductive system. During brooding, for instance, ovulation in the ovary issuppressed and the anterior part of the gonoduct is characterized by an intricate,recessed folding.