In Potamocorbula amurensis time for development to the
straight-hinge larval stage is 48 hr at 15°C. Potamocorbula amurensis settles
at a shell length of approximately 135 um 17 to 19 days after fertilization.
Our observations of timing of larval development in P. amurensis support the
hypothesis of earlier workers that its route of initial introduction to San Francisco
Bay was as ve1iger larvae transported in ballast water by trans-Pacific
cargo ships. The length of the larval period of P. amurensis relative to water
mass residence times in San Francisco Bay suggests that it is sufficient to allow
substantial dispersal from North Bay to South Bay populations in concordance
with previous observations that genetic differentiation among populations of
P. amurensis in San Francisco Bay is low. Potamocorbula amurensis is markedly
euryhaline at all stages of development. Spawning and fertilization can occur
at salinities from 5 to 25 psu, and eggs and sperm can each tolerate at least a
lO-psu step increase or decrease in salinity. Embyros that are 2 hr old can tolerate
salinities from 10 to 30 psu, and by the time they are 24 hr old they can
tolerate the same range of salinities (2 to 30 psu) that adult clams can. The ability
of P. amurensis larvae to tolerate substantial step changes in salinity suggests a strong potential to survive incomplete oceanic exchanges of ballast water and
subsequent discharge into receiving waters across a broad range of salinities
