Why do mantis shrimp have sixteen photoreceptor types but poor colour discrimination?

Mantis shrimp have many photoreceptor types mainly to sample a broad range of ultraviolet, visible, and polarization information, not to provide fine colour resolution. Their poor wavelength discrimination is most plausibly explained by a categorical or labelled-line decoder that identifies the strongest receptor channel rather than comparing activity ratios across overlapping receptors as humans do. Narrow tuning, receptor noise, and limited overlap may contribute but are not sufficient explanations by themselves, while proposed speed and neural-cost benefits remain speculative.