Embryonic suckling and maternal specializations in the live-bearing teleost Zoarces viviparus

The European eelpout follows an aplacental viviparous reproductive strategy, in which gestation lasts 4–5 months. During the last months of development yolk reserves are depleted, and embryos depend on an external source of nutrients. Here we provide evidence for novel specialized physiological, morphological and behavioural adaptations, which we propose as the responsible mechanisms for the exchange of nutrients and gases between the maternal organism and her embryos. Ovarian follicles contain an internal glomerulus-like structure within the distal tip of each follicle. Ultrastructural examination indicated a capacity for steroid synthesis and secretion. Gel electrophoresis demonstrated a protein size distribution in the follicular fluid different from that of the maternal serum, and that ovarian fluid is devoid of protein. From vascular casts and histological sections the follicle was reconstructed. The glomerulus has a central canal that is exteriorized at the tip of the follicle, allowing passage of follicular fluid. Oxygen measurements across the ovary of near-term females showed a strongly hypoxic ovary lumen, yet ovarian fluid adjacent to follicles was oxygen saturated. As another novel observation, embryos were seen engaged in suckling on follicles. We hypothesize that embryos use the follicles on the ovarian wall as placental analogues and that they use their mobile jaw apparatus to attach themselves and apply suction.