Stephane Hourdez is a member of a very exclusive club; he's one of the few individuals that has made the perilous journey to the bottom of the ocean,visiting deep-sea hydrothermal vents; it's another planet down there' he says. Originally intrigued by the invertebrates that thrive in the toxic waters, Hourdez teamed up with Roy Weber to analyse the remarkable creature's haemoglobin and the protein's adaptations to the harsh environment. But Hourdez also noticed that several fish species seemed perfectly content to swim in the vent's noxious waters. Had the vertebrates made similar adaptations to protect themselves from the deadly conditions(p. 2693)?

But catching fish at such incredible depths is far from straightforward. Hourdez explains that the Thermarces cerberus he found at the Pacific vent were particularly tricky to catch, as they never leave the vent and tend to be well hidden among the vent's tubeworm aggregations. But with the help of François Lallier, the submarine's pilot, Hourdez was able to retrieve some fish from the depths. Fortunately, when he moved to the Lucky Strike'vent field in the Atlantic, Symenchelis parasitica were slightly easier to collect in traps because they preferred spending time away from the vent. Having returned both species to the surface, Hourdez quickly collected blood samples, ready to send them to Weber in Aarhus, Denmark, where he and his team set about extracting haemoglobin from the precious blood cells.

The team began by testing the vent fishes' haemoglobin distributions. Fish that live near to the surface usually carry several different forms of haemoglobin and are classified depending on the amounts of each haemoglobin that they produce. When Weber separated the different haemoglobin components in the deep-sea vent fishes' blood, he found that both fish had very different haemoglobin distributions, even though they both lived in the same hazardous environment. He also realised that Symenchelis' blood cells had the characteristic haemoglobin pattern of anoxia-tolerant species found at the surface. But had the fish's oxygen affinity increased to survive in the vent's hypoxic waters?

Weber began measuring the haemoglobin's affinity, and was pleased when Thermarces' haemoglobin turned out to have a much higher affinity than relatives from shallower waters, enabling Thermarces to survive in the vent's hypoxic waters. But when he looked at the characteristics of Symenchelis' haemoglobin, it was equally intriguing. The oxygen affinity of one of its haemoglobin components was only slightly affected by temperature variations, allowing the fish to transport oxygen regardless of the temperature of the water, which would be beneficial under the drastic temperature variations encountered in hydrothermal vent visits' he explains. However, the protein's oxygen affinity was relatively low, as if the fish were better prepared to spend time away from the anoxic vent, in the colder,oxygen-rich waters of the deep ocean. Which is something of a paradox, why would Symenchelis go into the vent's toxic waters when its haemoglobin isn't well adapted to the rigors of vent water? Because the vents are a deep sea oasis' explains Hourdez, supporting a thriving biomass in the otherwise empty waters. He believes that the fish trade off the perils of the toxic waters, in favour of the vent's rich dining opportunities.

Weber, R. E., Hourdez, S., Knowles F. and Lallier, F.(
2003
). Hemoglobin function in deep-sea and hydrothermal-vent endemic fish: Symenchelis parasitica (Anguillidae) and Thermarces cerberus (Zoarcidae).
J. Exp. Biol.
206
,
2693
-2702.