Seminal vesicle protein caCA12 in Corydoras aeneus inhibits sperm motility for sperm drinking

Seminal vesicle (SV) secretions enhance fertilization by regulating sperm motility and fertilization capacity, and by forming plugs that prevent mating with other males. Although SVs are rare in teleosts, certain species, such as Corydoras spp., do possess them. In Corydoras spp. and other species that exhibit sperm drinking or related behaviors, females attach their mouths to the males' genital pore to ingest semen, a reproductive behavior known as sperm drinking. However, the major proteins and functions of seminal vesicle fluid (SVF) in Corydoras remain unidentified. This study aimed to identify the SVF proteins in Corydoras aeneus and clarify the functions of the identified major SVF proteins. The SVF of this species was found to be highly viscous with a high protein concentration. Sperm motility was strongly suppressed in the presence of the SVF. We identified three SVF proteins - alpha-2-macroglobulin (A2M), carbonic anhydrase 12 (CA12) and lymphocyte antigen 6 (Ly6) - through RNA sequencing (RNA-Seq), LC-MS/MS and amino acid sequencing. Additionally, we found that the identified CA12, termed 'caCA12,' was degraded into about 10 kDa and 33 kDa polypeptides containing the CA domain. The 33 kDa polypeptide with the CA domain was found to inhibit sperm motility. The identified SVF proteins, including caCA12, may play a role in keeping sperm in an immotile state until they are close to the female ova, facilitating the remarkable sperm drinking reproductive process observed in C. aeneus.