A new catoptric system in the eyes of penaeid shrimp
Zyznar, Edward Stanley, 1942-
In arthropod compound eyes the absorbing and reflecting pigment play an important role in light and dark adaptation. Absorbing pigments are believed to be melanins or ommochromes (Kleinholz, l96l), whereas the reflecting material by analogy with vertebrate systems, has long been considered to be guanine. More recently, it has been found to include a number of purines and the pteridine, xanthopterin (Kleinholz, 1959). The presence of two dark pigment layers and a single reflecting or tapetal layer is the condition existing in the compound eyes of many crustaceans thus far investigated. However, the positions these layers take during light and dark adaptation varies considerably. When in the light adapted state, the distal absorbing pigment envelops the crystalline lens or portions of the crystalline tract. This pigment is either movable or fixed in different species. Proximal pigment covers the light receptor unit and the tapetal layer is contracted in some species. Dark pigment prevents penetration of light from adjacent ommatidia and limits the intensity of light reaching the rhabdomes. As ambient illumination decreases, dark pigment is withdrawn from the retinula cells and, in some forms, reflecting material expands around the base of the rhabdome and retinula cells. Axial light and light from nearby ommatidia passing through the rhabdome is reflected back to the rhabdome enhancing visual senitivity. Preliminary observations showed the eyes of certain penaeid shrimp to be unusual in possessing three distinct reflecting layers. One of the new layers is situated at the distal-most part of the crystalline lens outside the distal absorbing pigment whereas the other is static beneath the basement membrane. A similar catoptric system is present in the caridean prawn, Palaemonetes vulgaris (Welsh, l930; Kleinholz, l936). Reflecting material caps the distal absorbing pigment and processes from the reflecting layer extend well below the basement membrane (Kleinholz, 1961). The function and importance of these layers were not discussed. Determinations of the chemical nature of the reflecting layers showed substantial quantities of cholesterol (Nicol and Attaway, unpublished) and pteridines (Nicol and Van Baalen, unpublished) to be present in shrimp eyes. Further study indicated that pteridines are the probable chemical constituents of the tapeta.