The immunoglobulin isotype found both in cartilaginous and bony fishes is
In immunology, antibody structure dictates that immunoglobulin fold with antiparallel beta-sandwich provides antigen binding. Therefore IgM is correct as it demonstrates immune recognition architecture, supported by mechanistic studies and conserved across related systems.
Ref: Owen, Punt and Stranford, Kuby Immunology, 8th Edition, Chapter 4, Antibody Structure, covers immunoglobulin domains, beta-sheet organization and antigen binding detailing immune recognition.
