Synechococcus is a highly versatile and adaptive genus of picocyanobacteria, consisting of over twenty clades with different characteristics. Many of these clades coexist within the same marine biomes or habitats. The diverse characteristics of these clades influence their grazing susceptibility and response to grazing pressure. This, in turn, affects energy transfer to higher trophic levels and influences interactions between different Synechococcus strains and other marine microorganisms. This experimental study aimed to evaluate grazing susceptibility of three marine Synechococcus strains to a heterotrophic nanoflagellate. Additionally, predator-induced formation of microcolonies in Synechococcus as a defence mechanism against grazing was investigated. Microcolony formation has previously been observed as a morphological response to grazing pressure in some freshwater Synechococcus strain. The experiments were conducted in both single and co-culture to determine whether differences in strategies observed in single culture would influence interactions between strains and the predator in co-culture. This has also been previously reported between two freshwater Synechococcus strains. Examination and quantification of both total abundance and microcolonies were performed through flow cytometry. The three marine strains in this study showed varying responses to grazing pressure. While two strains were grazed, one strain showed no signs of grazing in either single or co-culture. Additionally, the strains displayed different responses in terms of microcolony formation. One strain increased its rate of microcolony formation in the presence of the predator, while the other two did not. Furthermore, one of the strains that did not increase microcolony formation in single culture was observed to do so when co-cultured with the other strains, particularly with the strain that created the highest percentage of microcolonies. The insights gained through this study encourage further research into interactions and behaviours among and between different Synechococcus strains and clades, their predators, and other marine microorganisms within an ecological context.