A ship-based jacuzzi for deep-sea invertebrates

By Stephane Hourdez

All species on Earth are affected by the temperature they encounter in their environment. Ectotherms are especially sensitive to temperature variations as their internal temperature follows that of their environment. As a result, these species’ distribution correlates with latitude on land and in coastal marine environments (e.g. temperate species will not occur in polar or equatorial regions). In the deep-sea, temperatures are much more homogenous, cold, and should not significantly affect species distributions. Deep-sea hydrothermal vents, however, are a notable exception. There, the hot hydrothermal fluid (up to 400˚C) exits the seafloor in focused areas and mixes with the deep-sea, very cold (2-3˚C), water. Depending on the proportion of hydrothermal fluid mixed with the seawater, the whole range of temperature is possible. If some microorganisms can grow at up to 113˚C, metazoans cannot withstand temperatures much higher than 50˚C. Over very short distances, species can experience sharp temperature variations. Other conditions (pH, oxygen concentration) near hydrothermal vents can also be very challenging and affect species distribution. Our observations have shown that there are distinct species assemblages and that their thermal environment is different. What is the role of temperature in the distribution of these species? Are other conditions important as well or is temperature the only driver?

During this cruise, we carried out experiments on a diversity of invertebrate species to determine their tolerance to temperature. We are working with deep-sea species and most would not survive at atmospheric pressure. We therefore need to reproduce the pressure to which they are exposed in situ (250 times the atmospheric pressure at 2500 m). The animals are placed into a pressure vessel (photo) through which sea-water flows to minimize oxygen depletion. We start the experiments at the temperature of the surrounding deep-sea and raise it by 1 ˚C every 10 minutes. The top window allows us to observe the animals and determine when they can no long withstand the conditions. Some species will die at 10˚C while others can withstand at least 20˚C more. These tolerances reflect values for short term tolerances, long term tolerances are roughly 8˚C lower.

Figure legend: Scaleworms (annelids of the family Polynoidae) from the Tica vent site on the East Pacific Rise in a pressure vessel at 250 times the atmospheric pressure.

Overall, the tolerances we measured correlate well with the in situ measurements made near the assemblages where their usually live. Some species, however, have higher tolerances than expected. This could mean that we have not properly documented their natural environment or that other parameters affect these species’ distribution.

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