MALACOLOGIA, 1985, 26(1-2): 253-271
A NEW MUSSEL (BIVALVIA, MYTILIDAE) FROM HYDROTHERMAL
VENTS IN THE GALAPAGOS RIFT ZONE
Vida Carmen Kenk 1 & Barry R. Wilson 2
ABSTRACT
A new subfamily, Bathymodiolinae, and new genus and species, Bathymodiolus thermophilus,
are described from material collected by the 1977 and 1979 expeditions to the hydrothermal
vents in the Galapagos Rift Zone. This large modioliform mussel has very unusual anatomy,
exhibiting extreme mantle fusion which restricts the incurrent aperture to a short byssal-pedal
gape in the ventral midregion. The gills lack food grooves ventrally; the free edges of the gills fit
axial ridges on the visceral mass and mantle lobes, thereby isolating the dorsal excurrent
chambers from the rest of the mantle cavity. The gut is short and different from that of other
mytilids in lacking a recurrent loop, the stomach is simple and lacks a deep sorting caecum,
dorsal hood and left pouch, and there are but three pairs of digestive ducts opening into the
stomach. The auricles of the heart have a broad connection to the longitudinal vein laterally
between the branches of the divided posterior retractor muscles in addition to the normal
connection anterior to these muscles. The kidney is very small.
Feeding is discussed in light of high densities of chemoautotrophic sulphur-oxidizing bacteria
in the environment and the possibility of a symbiotic relationship between the mussels and
bacteria.
INTRODUCTION
The discovery in 1977 of biological com-
munities surrounding hydrothermal vents in
the Galapagos Rift Zone at latitude 00.47°N
(Corliss & Ballard, 1977; Lonsdale, 1977;
Corliss et al., 1979; Enright et al., 1981;
Edmond, 1982) led to the Galapagos Rift
Biology Expedition in 1979 (Ballard & Gras-
sle, 1979; Galápagos Biology Expedition Par-
ticipants, 1979). Since the initial discovery,
additional submarine hydrothermal com-
munities have been described at 21 °N (Rise
Project Group, 1980) and 11-13°N (Des-
bruyères et al., 1982). The majority of speci-
mens collected on these expeditions are un-
usual organisms differing from known rela-
tives at generic or higher levels (Newman,
1979; Williams, 1980; Burreson, 1981; Fretter
et al., 1981; Jones, 1981; Krantz, 1981;
McLean, 1981; Desbruyères & Laubier, 1982;
Williams & Chace, 1982).
One of the most abundant and conspicuous
organisms collected at some of these hydro-
thermal vents is a large modioliform mussel.
Although the shell form is like that of the
mytilid genus Modiolus, anatomical study of
preserved specimens has revealed many dis-
tinctive features. This animal is described
here as a new genus and species and a new
subfamily is erected for it. The mussels were
abundant at several vent sites in the Galapa-
gos Rift Zone. The species is also present,
though apparently less abundantly, at the 1 1
BioStor
