Plexiglass® dome placed with a submersible over the clam bed; again, sperm were
produced first, then eggs. The observations of natural spawning have been elaborately
repeated (Fujikura et al. 2007). Of 213 sperm releases, 90 were followed by egg
release. Eggs were never released first. Males spray sperm in an arc by swinging their
siphons. Pulses of flow along the turbulence-inducing, rough surface of the clam bed
would mix the streams of sperm and eggs together, ensuring high fertilization rates.
Similar observations have not been reported from hydrothermal clam beds, but similar
signaling should be possible. There are pulses of warmer (and colder) water passing
over hot vent fields due to tidal and turbulent flow variations.
Fig. 15.8 Four examples of sperm release events (squares) by Calyptogena soyoae,
three of them followed by egg-release events (circles) during slight temperature rises
at the cold seep clam bed (1174 m depth) in Sagami Bay, Japan.
(^) (After Fujiwara et al. 1998.)
(^) Ova of Bathymodiolus spp. are planktotrophic, sustained by feeding, on what
exactly is uncertain, possibly on the enhanced bacterial content of vent plumes. They
are likely relatively long lived, extending transport. Larvae of vesicomyid clams and
many vent snails and annelids are more limited to the nutrition supplied in the egg
(“lecithotrophic”), and likely must settle sooner. This planktotrophic–lecithotrophic