De Renquin served under Captain Callany. He discovered a Mermaid called Amfetriti whom he named Amy Ivans and took aboard the ship, hoping to sell her. He also capture her daughter and hid her in a wall compartment. In 1901, Callany was planning his retirement, at which point he told De Renquin he would hand over the ship to him. De Requin was poisoning Amy with small doses of mercury.
While the Doctor was elsewhere, De Renquin tried to have Peri drink mercury, to prevent her telling the Doctor of Amy's secret. They fought and Amy's wheelchair was knocked over, revealing her true form.
Amy died and Peri escaped with Galliteya. She confronted De Requin, who threatened her into compliance with a gun. He then tried to force Callany into preparing him a lifeboat to escape. Callany refused and told him there was none. He then fired at Callany, killing him. He began to laugh insanely and descended into madness.