Fine structure and adaptive variation of compound eyes in two species of infralittoral prawns (Palaemon, Caridea): New insights into imaging mechanisms of reflecting superposition eyes in decapod crustaceans

The main goal of this study has been to explore and compare the functional morphology and photoadaptive patterns of the compound eyes of two closely related prawn species both inhabiting different infralittoral visual environments. Using light and transmission electron microscopy we investigated light- and dark-adapted ommatidia of the light-resistant Palaemon elegans and the shade-preferring Palaemon xiphias. Ommatidia of both Palaemon species generally share the same cellular architecture, except for the irregular 8th retinula cell building up the distal rhabdom. This structure functions as UV-light receptor and potential light guide, providing dichroic vision and protection of the subjacent main (banded) rhabdom, formed by the remaining retinula cells 1-7, from harmful UV-radiation. As both the apical 4-lobe system of the 8th cell and the distal rhabdom are much stronger developed in P. elegans, we conclude that different light intensities in the respective photohabitats have led to noticeable micro-evolutionary adaptations at cellular level. In contrast, the main (banded) rhabdom, is capable of perceiving polarized light which is of special photo-ecological benefit for the diurnal P. elegans when populating shallow rock pools. The ommatidial ultrastructure of both species is very similar in the dark-adapted state. Many traits support reflecting superposition: such as (1) square corneal facet and crystalline cone, (2) the clear zone along main rhabdoms, (3) a mirror layer established by interommatidial pigment cells, and (4) the proximal tapetum established by reflecting pigment cells below the rhabdom. During light-adaptation, a massive turnover and shift of both organelles or whole cell bodies along the ommatidial optical axis enables the use of functional apposition optics at daytime in both study species. Some major differences in light-adaptation patterns and the assumed efficiency of functional apposition can be explained by adaptations to different light habitats. Our TEM study shows that shifting patterns of various pigment granules in interommatidial pigment cells, which occur over light adaptation, are species-specific. As a first measure to protect the main rhabdom from excessive light we identified the super-fast breakdown of a mirror layer around the cone's tip which is made of crystal granules and, thus, widens the aperture of ommatidia in superposition mode at night.