Abstract The J-Chain is involved in multimerization of IgM. The oldest animals with Ig isotypes, the cartilaginous fish, express IgM in two forms: a pentameric (19S) form, attached to J-chain, and a monomeric (7S) form. By in situ hybridization adult nurse sharks have splenic IgM-producing plasma cells that are either J-chain positive, and presumed to secrete pentameric IgM, or are J-chain negative 7S secretors. Neonatal nurse sharks produce pentameric IgM, and all plasma cells express J-chain. In addition, some neonatal shark plasma cells, found in discrete splenic locations, express a germline joined isotype, IgM1gj. We hypothesize that during B cell development either 19S producers “switch” to become 7S producers by shutting down J-chain expression, or the two populations develop from separate lineages, with the 7S, J-chain-negative lineage arising later in development than the 19S and IgM1gj lineages. Identification of factors controlling J-chain expression, in shark B cells is necessary to clarify which B cell development pathway is correct. In mammals, Blimp-1 is required for development and maintenance of J-chain expression and immunoglobulin secretion in plasma cells. However, in situ hybridization data show that Blimp-1 is not expressed in neonatal IgM secreting cells, or in half of adult shark secretory cells, presumably the 19S producers. Our data imply that J-chain expression and antibody secretion functions of some shark plasma cells are not controlled by BLIMP1.