remains to be learned, as does the means for distributing the product from these
glandular tissues to the rest of the snail. Scaly-foot snails live, together with other
snails – Alviniconcha – around the base of smokers with both sulfide and oxygen
available to them. Warén et al. (2003) report a very small digestive system compared
to typical snails, suggesting strong dependence upon the endosymbionts for nutrition.
However, they also show a very elaborate radula structure, which must retain some
scraping function. Goffredi et al. report abundant filamentous episilon- and
gammaproteobacteria coating the scales, but it is difficult to see how the snail’s mouth
would reach them for ingestion. Unlike Alviniconcha and its relatives, there are no
bacteria in the gill.
(^) Not much is known about reproduction of the scaly-foot snail. Warén et al. report an
unusually anterior position for the ovary, which could mean anything, and they found
spermatophores in the female gonoduct. Apparently, males deliver sperm in packages.
Bathymodiolus and Calyptgena
(^) Unlike the scaly-foot gastropod, most of the mollusks sustaining chemoautotrophs
harbor them in gill filaments. That is true of the mussel genus Bathymodiolus, species
of which are found in widely dispersed venting areas, and the clam Calyptogena
magnifica of the East Pacific Rise. Gills are vascularized and expanded for a wide
area of contact with external fluid in any animal, thus locating symbionts there readily
achieves high rates of gas exchange. This localization has been shown both by
microscopy and by presence in these tissues of the carbon-fixing enzyme RuBisCO
(Felbeck 1981). Methanotrophic bacteria have been reported (Cavanaugh et al. 1992)
to live symbiotically in Bathymodiolus, apparently co-existing with sulfide-oxidizing
chemoautotrophs (Fisher et al. 1993).
(^) These bivalves are distinctive for their dependence on endosymbiotic,
chemosynthesizing bacteria, but vesicomyids (Calyptogena) are not restricted to
hydrothermal systems. They also occur at cold seeps where squeezing of subducting
sediments pushes sulfide, hydrogen, and other chemicals into the benthic boundary
layer. Fujiwara et al. (1998) made an interesting observation about C. soyoae and C.
okutani living in such a seep area in Sagami Bay, Japan. It may or may not apply to
vent bivalves. The two clams release both sperm and eggs freely into the water where
they must meet to join. This requires nearly simultaneous release. In the cold-seep
clams, that is achieved by sensing short-term temperature increases and by olfactory
sensing of gametes in the water. Fujiwara et al. set up a video-recording observatory
to watch C. soyoae over time (1.5 years) while also recording temperature.
Temperature increases of only ∼0.2°C (Fig. 15.8) induced sperm release, which
apparently induced egg release; at least all cases of egg release were preceded by
sperm release. Fujiwara et al. then induced spawning events using a heater inside a