Original Paper

Journal of Comparative Physiology A

, Volume 199, Issue 2, pp 99-113

First online:

Systematic variations in microvilli banding patterns along fiddler crab rhabdoms

  • Ali AlkaladiAffiliated withDepartment of Biological Sciences, Faculty of Science, King Abdulaziz UniversityARC Centre of Excellence in Vision Science, Research School of Biology, Bld 46, The Australian National University Email author 
  • , Martin J. HowAffiliated withQueensland Brain Institute, University of Queensland
  • , Jochen ZeilAffiliated withARC Centre of Excellence in Vision Science, Research School of Biology, Bld 46, The Australian National University

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Polarisation sensitivity is based on the regular alignment of dichroic photopigment molecules within photoreceptor cells. In crustaceans, this is achieved by regularly stacking photopigment-rich microvilli in alternating orthogonal bands within fused rhabdoms. Despite being critical for the efficient detection of polarised light, very little research has focused on the detailed arrangement of these microvilli bands. We report here a number of hitherto undescribed, but functionally relevant changes in the organisation of microvilli banding patterns, both within receptors, and across the compound eye of fiddler crabs. In all ommatidia, microvilli bands increase in length from the distal to the proximal ends of the rhabdom. In equatorial rhabdoms, horizontal bands increase gradually from 3 rows of microvilli distally to 20 rows proximally. In contrast, vertical equatorial microvilli bands contain 15–20 rows of microvilli in the distal 30 µm of the rhabdom, shortening to 10 rows over the next 30 µm and then increase in length to 20 rows in parallel with horizontal bands. In the dorsal eye, horizontal microvilli occupy only half the cross-sectional area as vertical microvilli bands. Modelling absorption along the length of fiddler crab rhabdoms suggests that (1) increasing band length assures that photon absorption probability per band remains constant along the length of photoreceptors, indicating that individual bands may act as units of transduction or adaptation; (2) the different organisation of microvilli bands in equatorial and dorsal rhabdoms tune receptors to the degree and the information content of polarised light in the environment.


Microvilli banding Rhabdom Polarisation vision Dichroism Fiddler crab