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)

Abstract

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 µcomputed 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.

Ovarian changes and development of the branchial placenta occurring in Jenynsia lineata (Cyprinodontiformes, Anablepidae)

“…The success of viviparity requires the regulation of the local immune and inflammatory response (Mor et al, 2017). No other references on immunotolerance responses in viviparous teleosts were found; however, there are studies that describe the characteristics of the leukocyte infiltrate and its implication in the immunological responses in the ovary during reproductive season (Gardiner, 1978; Koya et al, 1997; Schüller et al, 2022). Here, we observed that, during the nongestation stage, a variety of lymphocytes, EGCs, fibroblasts, MMCs and tissue debris was found in the lamina propria of some ovarian folds.…”

A landmark‐free analysis of the pelvic girdle in Sulawesi ricefishes (Adrianichthyidae): How 2D and 3D geometric morphometrics can complement each other in the analysis of a complex structure

“…Here, females carry the cluster of fertilized eggs on their ventral side until the embryos hatch. Pelvic brooding comes with a set of adaptations that include a novel egg‐anchoring tissue (Hilgers et al., 2022 ; Iwamatsu et al., 2008 ; Schüller et al., 2022 ), shorter ribs that create a ventral concavity into which the fertilized eggs are placed (Kottelat, 1990b ; Popta, 1905 ; Spanke et al., 2021 ), and elongated pelvic fins that cover the eggs and ventral concavity (Figure 1 ). Adaptations to pelvic brooding are thus female‐biased, resulting in a pronounced sexual dimorphism.…”