Due to the unique feeding method of Rimicaris exoculata, it is constrained to remain close to vent fluid so its “farmed” episymbiotic bacteria have access to the reduced chemical compounds necessary for chemosynthesis. This presents a problem for Rimicaris as vent fluid is extremely hot, reaching temperatures of 350ºC. Understandably, Rimicaris avoids the hottest part of the fluid and is instead found in the mixing zone between vent fluid and the colder (approx 2ºC) surrounding water. However the mixing zone itself is a thermally unstable environment regularly experiencing unpredictable temperature fluctuations. Indeed early measurements of temperature inside Rimicaris swarms gave readings varying from 10 to 40ºC (Gebruk et al, 1993). Sustained tolerance to temperatures approaching 40ºC would be amongst the highest of any organism yet measured prompting researchers to investigate the thermal tolerance of Rimicaris in detail.
However when lab based experiments on pressurized live organisms (Fig. 10) were conducted, researchers were surprised to find that Rimicaris did not tolerate sustained exposure to temperatures greater than 33ºC (Ravaux et al, 2003). Animals kept at temperatures higher than 33ºC died and even animals kept 25ºC displayed behavioral responses indicative of stress (escape response) and increased production of heat shock proteins (HSP’s). HSP’s are involved in the molecular response to thermal stress by assisting protein refolding and the removal of damaged proteins (Parsell & Lindquist, 1993). As the induced synthesis of HSP’s indicates an animal is subject to stress, the authors drew the conclusion that Rimicaris was typically found at temperatures below 25ºC. Similar findings have been coroborated in more recent studies (Cottin et al, 2010).
The apparent inconsistency between field measurements of temperature and lab based studies of thermal tolerance can be reconciled by considering the age of the oldest temperature measurements. Measuring the temperature in the swarm is difficult (due to crowding round the probe, fig 11) and older temperature measurements of the shrimps microhabitat approached 40ºC (Gebruk et al, 1993). However recent measurements have failed to record such temperatures (Desbruyeres et al, 2000; Geret et al, 2002; Schmidt et al, 2008) which when combined with experimental data strongly suggests that Rimicaris inhabits water less than 25ºC. In context this is a similar temperature tolerance to that of the Green Shore Crab Carcinus maenas (Cuculescu et al, 1998) and a lot less that the vent polychaete Alvinella pompejana which regularly inhabits water 42ºC and higher (Ravaux et al, 2013).
Given the surprisingly low thermal tolerance of Rimicaris, just how do the shrimps avoid poaching themselves in the lethal water temperatures located just centimetres away? As Gebruk et al, 2000 reported that 30% of specimens had scalded cuticles the answer may well be that individuals do occasionally enter areas of lethal temperature. Ravaux et al, 2013 suggested that by inducing a flow of cold water by moving its appendages, Rimicaris may be able to regulate (to some extent) its microhabitat. However the primary mechanism by which Rimicaris avoids superheated water is most surprising and to read more about it please click here.