suitable for navigating to sulfide sources. Olfactory response to sulfide is not unusual,
you can smell it, but Renninger et al. did not find it in other shrimp.
(^) Nicole Dubilier and colleagues (Peterson et al. 2010) sequenced DNA encoding
SSU rRNA from numerous samples of the episymbiont bacteria living on R. exoculata
collected at four sites along 8500 km of the MAR from 36°N to 4°S, the entire range
of the species. They found two groups of bacteria, one of epsilon- and one of
gammaproteobacteria. The epsilon group divided into some closely related subgroups
that partially sorted along the ridge. All of those were related to known sulfide/sulfur-
oxidizing chemoautotrophs, including those of Alvinella. The gamma group was both
less abundant, although found on all shrimp from all sites (well, just three from each
site) and less diverse, but their closest metabolically characterized relative is also a
sulfur-oxidizing chemoautotroph. While there are other bacteria on the shrimp
surfaces, these two proteobacterial groups are the consistent dominants and appear to
be obligatory symbionts, at least for the shrimp. The Dubilier team also examined two
mitochondrial genes from the shrimp, finding only trivial variation along the 8500
km. They make the point that the chemistry of vent fluids along this stretch is not
consistent (Schmidt et al. 2008). At the south end, the vents are “basalt hosted”,
which makes them rich in sulfide and depleted in hydrogen and methane. At the north
end, vent fluids are heated in ultramafic rock, which leads to the opposite: low S2–,
high H 2 and CH 4 . The epibiont bacteria of the shrimp, however, almost certainly have
very similar chemical requirements. This can be termed an obligate relationship for
the shrimp or bacteria, which names the observation without explaining it. Possibly,
the two dominant proteobacterial strains carried by the shrimp are metabolic virtuosos
capable of several modes of chemoautotrophy, but probably not.
(^) Teams led by Bruce Shillito (e.g. Ravaux et al. 2003) have directly tested the high-
temperature tolerance of R. exoculata, which live on the walls of vents conducting
water at 350°C into the ocean just a few meters away. Shrimp were collected by a
suction system on an ROV, brought to the surface, and promptly returned to 230 atm
pressure in sealed, “endoscope”-equipped aquaria. After observation of their
survivorship at moderate temperature, the temperature was raised. Death occurred at
remarkably low temperatures, 33 to 37°C. In later work, Shillito and colleagues
characterized a 70-kilodalton, heat-inducible heat-shock protein (of the Hsp70 class)
that the animal accumulates above only 25°C, implying that its optimal habitat
temperature is somewhat less than that. Possibly, at many sites the best sulfide/sulfur
and oxygen concentrations are found at that temperature, which is higher than at the
East Pacific sites occupied by Riftia.
(^) According to limited studies (Ramirez Llorda et al. 2000), Rimicaris reproduction is
similar to that of other caridean shrimp. The male packages sperm in a spermatophore
and attaches it to the female gonopore. The sperm are transferred into holding
chambers in the female reproductive tract. Oocytes are present in the ovary at all