Fine structure of the lateral eyes in the tiny marine Antarctic mite Halacarellus thomasi : an ultrastructural study

Abstract

The lateral eyes of the tiny marine /-Vntarclic mite, Halacarellus iliomasi, consist of two entities in the same eye capsule. The oplical axes of the two eyes point in different directions and the two eyes are of different sizes- The larger of the two is covered by a circular lens of approximately 13 |im diameter, with considerably greater inner than outer convex curvature. The cover of the smaller eye consists of little else than a circular 10-).m'i-\vide transparent patch of cuticle. Both eyes have an everted retina with few (4-6) retinula cells, and their microvilli, which are oriented perpendicular to the light' path, have diameters of 55 nm. Screening pigments in the form of lipid-like, electron-opaque cell inclusions, measuring 1-2 um in size, are abundant on the proximal side of the retina. A small l-)im-thick muscle is in con­ tact with the eye capsule. On the basis of our structural data and comparisons with other mite species, we con­ clude that form, colour and polarization vision would play no role in the eye of /-/, ihouiasi. More likely, (he function of the eye is to monitor seasonal daylength changes to allow synchronization of reproductive activities.