A relatively large comma-shaped cavity (or sometimes fishhook shaped as in Alexandrium catenella and A. tamarense) on the apical pore plate (Po) of Alexandrium spp. It is a diagnostic feature of the apical pore complex (APC).
Alternative forms for foramen : cavity, comma.
Alexandrium catenella-SEM of Po with foramen and aap. Po=apical pore plate; aap=anterior attachment pore