Nature Blog Network

Thursday, July 12, 2007

The Eye of the Shrimp

*Not to be confused with the hit song by Survivor.

Photo is from here (in japanese)

The vent shrimp, Rimicaris exoculata (literally the Rift-shrimp deprived of eyes), swarms hydrothermal chimneys, with temperatures reaching over 350C, en masse in the darkness of the deep sea. It has a certain peculiarity in that its eyes are completely absent yet there is a high concentration of the visual pigment, rhodopsin, in a dorsal "eyespot" beneath a transparent cuticle of the carapace. Other vent shrimp, such as individuals in all other alvinocarid genera, do retain the eye, though it tends to be reduced and fused together. The anatomy of this eye has been studied in detail and several authors have hypothesized that it can detect "black-body" radiation, or thermal radiation given off by the vents at around 350C.

Figure is from Nuckley et al. 1996

Pelli & Chamberlain (1989) actually calculated out whether or not R. exoculata could see this black-body radiation given the concentration of rhodopsin it has and the radiation properties of 350C hydrothermal vents. Using alot of physics and math, their conclusion is:
"... 50pmol of rhodopsin present in the shrimp eye will yield approximately 1,600 thermal isomerizations per second. To detect the 350C vent, the shrimp would have to discriminate a signal-plus-noise isomerization rate of 1,800+1,600=3,400 per second from a noise-alone rate of 1,600 per second. A brief integration time of only 20ms would yield a decision variable with a signal-to-noise ratio of 6.4.[...] This would allow the shrimp to detect correctly the lethal hot water 99.9% of the time, and mistake ambient for hot water only 2% of the time."
They determined that the shrimp's peak wavelength was around 500nm, pointing out that the luminance of the vent is just below the threshold for a dark-adapted human. Nuckley et al. 1996 concluded from their examination of retina:
"The structural changes observed in the retina of Rimicaris sp. might be expected in an animal that is adapting to a dim-light environment... and are consistent with the changes we have observed in other hydrothermal vent shrimp. In general, an animal's eye continues to enlarge as the environment becomes dimmer and dimmer until the "quit point" is reached where exploitation of light in the environment becomes impossible [...]. In animals that adapt to the darkness beyond the quit point, the eye shrinks or disappears altogether. The eyes of the [alvinocaridid] shrimp that live on the sides of black smoker chimneys at sites along the Mid-Atlantic Ridge have all enlarged... however the eye of the predator Alvinocaris markensis, which lives at the base of these chimneys, seems to be in the process of evolving past the quit point [...]."
An interesting point also is that Rimicaris farms and harvests sulfide-loving bacteria under its carapace. The bacteria are where the temperatures are hot because that is where the sulfide is. Therefore the shrimp would have to make periodic excursions into hot water to get more bacteria to graze. This eyespot appears to be able to sense the thermal radiation given off by vents and use it to as a navigational cue utilizing the light-sensing organ in addition to using it to sense where it may get scorched! I have sense some alvinocaridids with what looks like burnt off tails. These alvinocaridids do not have the dorsal eyespot that Rimicaris does. Chamberlain 2000 reviewed vision in vent shrimp. He compared the image-forming eye of shallow water shrimp to the photodetection eye of vent shrimp, as well as comparing juvenile to adult vent shrimp. A very interesting insight was that
"the retinal structure of juvenile vent shrimp resembles much more closely that of the imaging eyes of surface-dwelling species... than it does the retinal structure of adult vent shrimp ...these data suggest that vent shrimp have evolved from ancestors that lived in a visual environment bright enough for them to use pattern vision, since the juveniles still show this kind of retinal structure,"

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