title: Null point of discrimination in crustacean polarisation vision.
abstract: The polarisation of light is used by many species of cephalopods and crustaceans to discriminate objects or to communicate. Most visual systems with this ability, such as that of the fiddler crab, include receptors with photopigments that are oriented horizontally and vertically relative to the outside world. Photoreceptors in such an orthogonal array are maximally sensitive to polarised light with the same fixed e-vector orientation. Using opponent neural connections, this two-channel system may produce a single value of polarisation contrast and, consequently, it may suffer from null points of discrimination. Stomatopod crustaceans use a different system for polarisation vision, comprising at least four types of polarisation-sensitive photoreceptor arranged at 0, 45, 90 and 135 deg relative to each other, in conjunction with extensive rotational eye movements. This anatomical arrangement should not suffer from equivalent null points of discrimination. To test whether these two systems were vulnerable to null points, we presented the fiddler crab Uca heteropleura and the stomatopod Haptosquilla trispinosa with polarised looming stimuli on a modified LCD monitor. The fiddler crab was less sensitive to differences in the degree of polarised light when the e-vector was at -45 deg than when the e-vector was horizontal. In comparison, stomatopods showed no difference in sensitivity between the two stimulus types. The results suggest that fiddler crabs suffer from a null point of sensitivity, while stomatopods do not.
authors.affiliation: School of Biological Sciences, Bristol Life Sciences Building, Tyndall Avenue, University of Bristol, Bristol BS8 1TQ, UK Queensland Brain Institute, University of Queensland, St Lucia, QLD 4072, Australia Smithsonian Tropical Research Institute, Apartado 0843-03092, Panama City, Republic of Panama m.how@bristol.ac.uk.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: How
authors.affiliation: Smithsonian Tropical Research Institute, Apartado 0843-03092, Panama City, Republic of Panama.
authors.forename: John
authors.initials: J
authors.lastname: Christy
authors.affiliation: School of Biological Sciences, Bristol Life Sciences Building, Tyndall Avenue, University of Bristol, Bristol BS8 1TQ, UK.
authors.forename: Nicholas W
authors.initials: NW
authors.lastname: Roberts
authors.affiliation: Queensland Brain Institute, University of Queensland, St Lucia, QLD 4072, Australia.
authors.forename: N Justin
authors.initials: NJ
authors.lastname: Marshall
mesh_terms: D000818:Animals; D001522:Behavior, Animal; D003386:Brachyura / Q000502:physiology*; D003445:Crustacea / Q000502:physiology*; D005133:Eye Movements; D008027:Light; D017956:Photoreceptor Cells, Invertebrate / Q000502:physiology*; D014785:Vision, Ocular / Q000502:physiology*
keywords: Discrimination threshold; Fiddler crab; Mantis shrimp; Polarisation distance