Nutrient Transport in the Male Pregnant Seahorse, Hippocampus abdominalis

Abstract

Male seahorses have the most complex brooding structure known among oviparous brooding vertebrates, where males internally fertilise and incubate embryos in an enclosed brood pouch until live birth, a process analogous to viviparity. The inner tissue layer of the brood pouch represents the male component of a functional placenta, analogous to the female placenta in viviparous species. The aim of this thesis was to identify the range of nutrients supplied via patrotrophy in the male-pregnant pot-bellied seahorse, H. abdominalis, to test the hypothesis that wide-ranging parentotrophy has convergently evolved in male brooding and viviparous vertebrates, irrespective of sex or gestating organ. I begin with a review of the widespread methodological inconsistencies in how teleost literature assigns provisioning strategies based on a parentotrophy index (PI) threshold. I recommend consistent methodology for obtaining and interpreting a PI, and propose alternative methods for identifying provisioning strategy. Next, protein, calcium, magnesium, and phosphorus content were measured in newly fertilised embryos (NFE) and neonates using mass spectrometry. A significant increase in all measured nutrients was observed in neonates compared to NFE, providing the first evidence that a male brooding vertebrate has convergently evolved wide-ranging mechanisms for patrotrophy, analogous to matrotrophy in viviparous vertebrates. Lastly, I characterise an acellular layer closely apposed to the pregnant brood pouch of H. abdominalis, using light microscopy and immunohistochemistry. The layer is only present during pregnancy, is composed of acid mucins and is partially composed of oviductal glycoprotein 1. Using an extreme example of convergent evolution, my work further supports the hypothesis that viviparity, or the internal incubation and close association of parental and embryonic tissues with the release of live young, and parentotrophy consistently co-evolve across taxa and sex.