The second half of the nineteenth century saw a significant rise in the interest in the natural history of the deep sea, in part triggered by the Challenger Expedition. This resulted in a change in awareness of the importance of the oceans, which is excellently illustrated by the work of pioneers such as French physiologist Paul Regnard, and notably by his 1891 book “Recherches expérimentales sur les conditions physiques de la vie dans les eaux”. This paper focuses on the much-neglected work of this physiologist.

As a student of Paul Bert (1833-1886), Regnard had studied the effects of air pressure on various terrestrial animals, and through his friendship with Prince Albert I of Monaco (1848-1922), he was drawn to questions regarding life in the oceans. When he was given the opportunity to study the effects of hydrostatic pressure in the laboratories at Sorbonne and Le Havre, he focused on the investigation of the effects of hydrostatic pressure on terrestrial life forms so as to assess the physiological adaptations required for life at depth in a marine environment.

Regnard’s work covered an array of experiments which documented the unusually intimate relationship of marine organisms with the medium in which they lived. Familiar physiology was found to fail at pressures found at depths of 4,000 meters, where yeast became latent and muscle tissue failed to respond to electrical stimuli. It was thus evident that deep sea life required significant physiological adaptations.

Regnard criticized the neglect of interest into these physiological adaptations, which ignored the importance of marine life on this planet. Like other nineteenth-century naturalists, he drew attention to the general discrepancy between biologists’ interest in terrestrial and marine life. Although the latter constituted the main domain of life, it was barely understood and rarely investigated.

The figure of Regnard sheds light on the alternating fortune of deep sea biology throughout its uneven history. The general tendency to understand biological phenomena from a terrestrial point of view has been repeatedly interrupted by an awareness of this bias. While Regnard’s work represents a high point in this awareness, the relative historical obscurity into which his work has fallen illustrates the subsequent decades’ declining interest in deep sea biology. This repeating pattern of alternating biases deserves our attention, as a general trait of the history of modern deep sea biology.