How to stay attached

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