C-type lectins (CTLs) are a large family of calcium-dependent carbohydrate-binding proteins that play crucial roles in innate immunity as pattern recognition receptors. Bivalve mollusks possess exceptionally diverse and expanded repertoires of CTLs, yet a systematic review integrating their structural, functional, and regulatory aspects has been lacking. This article provides a comprehensive synthesis of current knowledge on bivalve CTLs, analyzing their biosynthesis, complex tissue-specific expression under both normal and stressed conditions, and their multifaceted roles in immune defense and other physiological processes. Our analysis consolidates data on their diverse domain architectures, phylogenetic relationships, and the variability of key motifs within their carbohydrate-recognition domains. The results demonstrate that bivalve CTLs are not only critical for pathogen recognition, agglutination, and phagocytosis but also involved in processes like nutrition, development, byssus formation and biomineralization. However, a significant finding is that the detailed carbohydrate specificity for most bivalve CTLs remains poorly characterized, often limited to monosaccharide inhibition assays. In conclusion, while the immune role of bivalve CTLs is well-established, this review underscores a critical gap in understanding their fine glycan-binding profiles. Therefore, a shift in the focus of future research towards elucidating their structure and carbohydrate specificity is required for a full understanding of their biological functions and an assessment of their biomedical potential.

C1q domain-containing (C1qDC) proteins are widespread in invertebrates and often perform immune functions by binding pathogens via carbohydrate components of pathogen-associated molecular patterns. The MkC1qDC protein from the hemolymph of the horse mussel Modiolus kurilensis has previously shown significant biomedical potential, including the ability to agglutinate marine bacteria. In this study, MkC1qDC protein was tested using 15 bacterial strains, 4 of which were standard terrestrial strains and theother 11 were isolated from marine organisms. The results showed agglutination by MkC1qDC protein of all tested species. Smaller and more numerous agglutination complexes were characteristic of the standard strains of Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, and the marine bacteria Microbacterium spp. For all other species, large complexes measuring 10–50 μm in diameter were characteristic. Comparison of the agglutination complexes of all Gram-positive and Gram-negative strains in total did not reveal reliable differences, which demonstrates the broad effectiveness of MkC1qDC protein. To assess bacteriostatic activity, one representative culture from each of the four classes included in the study was selected: Pseudovibrio ascidiaceicola from the class Alphaproteobacteria; Acinetobacter johnsonii from the class Gammaproteobacteria; Lysinibacillus fusiformis from the class Bacilli; and Cutibacterium acnes from the class Actinomycetes. Bacteriostatic activity was observed against all four tested cultures, although the protein showed lower activity against L. fusiformis.These results indicate that MkC1qDC protein is an effective bacterial agglutinin valuable for biotechnological and biomedical applications.