The secret world of shrimps: polarisation vision at its best

TL;DR

This paper reveals that the crustacean Gonodactylus smithii possesses a sophisticated polarisation vision system capable of measuring all Stokes' parameters, including circular polarisation, which enhances contrast and accuracy in polarisation detection.

Contribution

It demonstrates that crustaceans can measure all components of polarisation, including circular, providing a new understanding of animal vision capabilities.

Findings

Crustacean measures all Stokes' parameters for polarisation

Circular polarisation vision exists in crustaceans

Polarisation vision enhances contrast and detection accuracy

Abstract

Animal vision spans a great range of complexity, with systems evolving to detect variations in optical intensity, distribution, colour, and polarisation. Polarisation vision systems studied to date detect one to four channels of linear polarisation, combining them in opponent pairs to provide intensity-independent operation. Circular polarisation vision has never been seen, and is widely believed to play no part in animal vision. Polarisation is fully measured via Stokes' parameters--obtained by combined linear and circular polarisation measurements. Optimal polarisation vision is the ability to see Stokes' parameters: here we show that the crustacean \emph{Gonodactylus smithii} measures the exact components required. This vision provides optimal contrast-enhancement, and precise determination of polarisation with no confusion-states or neutral-points--significant advantages. We emphasise that linear and circular polarisation vision are not different modalities--both are necessary for optimal polarisation vision, regardless of the presence of strongly linear or circularly polarised features in the animal's environment.