“Minimum Dimension” means the minimum dimension of an organism based upon the dimensions of that organism’s body, ignoring e.g., the size of spines, flagellae, or antenna. The minimum dimension should therefore be the smallest part of the “body”, i.e. the smallest dimension between main body surfaces of an individual when looked at from all perspectives. For spherical shaped organisms, the minimum dimension should be the spherical diameter. For colony forming species, the individual should be measured as it is the smallest unit able to reproduce that needs to be tested in viability tests.
Source: Resolution MEPC.173(58), Guidelines for ballast water sampling (G2), 10 October 2008, International Maritime Organization. Regulatory Guidance