cultures grow on the inner surfaces of the gill chambers. The shrimp achieves gas
exchange for this on-board bacterial farm by holding position in turbulent mixing
flows along the vent walls. It seeks water at 20 to 40°C, which contains both sulfide
and oxygen, harvesting meals by scraping over the maxillae and gill chambers with
other mouthparts (Gebruk et al. 1993).
Fig. 15.7 Rimicaris exoculata, a bresiliid shrimp from black smoker chimneys on the
Mid-Atlantic Ridge. Appearance of the male shrimp.
(^) (After Williams & Rona 1986.)
The species name exoculata, meaning without eyes, is a misnomer. During
development, the eyes simplify from a fairly standard form of ommatidia and migrate
back below the upper carapace where they look out through transparent chitin (Van
Dover et al. 1989). The two retinal planes are ∼0.25 × 1 cm, a total of ∼50 mm^2 . All
of the optical focusing parts are lost, and the rhodopsin-packed rhabdomes
hypertrophy into a thick (100 μm) layer backed by white diffusing cells that return
uncaptured photons back to the rhabdome. The absorption spectrum peaks at 500 nm,
and White et al. (2002) have detected very dim green light emitted from the water
emerging from the vents, water hotter than the thermal tolerance of the shrimp. At
wavelengths shorter than 650 nm, this light exceeds the expected blackbody radiation
for vent temperatures. At ∼ 104 photons cm−2 s−1 sr−1 (sr = steradian), it is not visible
to people and is generated by an uncertain mechanism. Light emitted by bubbles
collapsing under extreme compression; energy release from crystal formation; and
chemiluminescence are suggested as possible sources by White and colleagues. The
function of light detection for the shrimp could be avoidance of scalding, finding the
way back to vents areas, or something else. It is very likely that different information
is gained from this light than would come from the more abundant blackbody
irradiance at longer wavelengths. However, it is also possible that evolution of the
Rimicaris pigment from that of a more ordinary deep-sea shrimp naturally favored the
shorter end of the visible spectrum. Chorocaris has a similar eye, although not so
large and not shifted back into the carapace. In addition to an unusual visual system,
likely involved in orientation to and on vents, Rimicaris have sulfide-sensing
dendrites under pores on the second antenna (Renninger et al. 1995). The intensity of
activity in these nerves is exponentially proportional to Na 2 S concentration, and thus