I could not find much info on the subject. It appears that crabs move to shallower water during periods of low dissolved oxygen.
Excerpt from http://www.mvcommission.org/doc.php/Blue%20Crab%20Report.pdf?id=18
'15. L. Pihl, S. Baden, R. Diaz (91) Effects of periodic hypoxia on distribution of
demersal fish. Marine Biology 108: 349-360
Several species studied migrated out of deeper (over 10m) water during hypoxia and
returned after the event ended.
Blue crab survived in 14 to 25% Oxygen saturation.
Hypoxia below 2ppm or 28% sat for 6, 14 and 6 day periods was monitored during
Adult crabs found at all depth ranges sampled in June. First hypoxic event did not
change their presence. During second event crab density decreased in sampling sites.
No significant changes in blood haemocyanin found except for deepest sampling zone
during and after the second hypoxic event.'
Lots of other interesting information in the report worth reading.