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The consequences of placentation on the swimming performance of pregnant livebearing fish

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Livebearing and swimming performance in fish

Bearing live young as a reproductive strategy has both advantages and disadvantages. One major drawback is a reduction in the ability to move freely, which can adversely affect feeding and avoidance predators.

Climate Change and Environment

Some livebearing organisms have evolved adaptations that enable them to minimise the disadvantages associated with this life history strategy. The project SWIM (The consequences of placentation on the swimming performance of pregnant livebearing fish) compared locomotory performance between closely related livebearing species with and without a placenta. The research team had previously proposed that the placenta was a life history adaptation that evolved in livebearing fish to reduce adverse effects on locomotory performance during gestation. The placenta regulates embryo nourishment and waste product removal. As this temporary organ has evolved many times throughout the animal kingdom it suggests that it must offer the organism an adaptive advantage. This hypothesis was tested in a biomechanical study using the fish family Poeciliidae. The project aimed to discover whether species with a placenta have a lower reproductive burden and smaller distension of the abdomen than species without a placenta. It also investigated whether these differences decline over the course of gestation and how this differs between placental and non-placental species. In addition, it was determined whether placental species have improved swimming performance compared to closely related, non-placental livebearing species. SWIM combined expertise from the field of bio- and fluid mechanics with evolutionary ecology. A state-of-the art swim arena was built to film the startle response of female fish using three high-speed video cameras positioned at right angles to one another. The films were analysed using novel in-house automated tracking software. The Fish Tracker software enabled the precise 3D tracking of suitable numbers of fish. It recorded instantaneous and mean swimming speed and acceleration related to centre of mass, tail-beat amplitude and tail-beat frequency. The results showed that livebearing fish exploit their full 3D environment to make escape manoeuvers. Project work has yield a large quantity of high-quality data that will lead to a number of publications in peer-reviewed scientific journals.


Livebearing, swimming performance, placenta, Poeciliidae, evolutionary ecology

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