ORIGINAL PAPER Ontogenetic changes and environmental effects on ocular transmission in four species of coral reef fishes Received: 17 September 2002 / Revised: 12 March 2003 / Accepted: 25 March 2003 / Published online: 17 April 2003 Ó Springer-Verlag 2003 Abstract Filtration by the humors, cornea and lens limits the spectrum of light available for vision as blocking compounds prevent some wavelengths from reaching photo-sensitive cells of the retina. The visual ecology of fishes is dependent upon factors changing with size and/ or habitat. We predicted that ontogeny and habitat depth would affect ocular transmission for four fishes, Mulloidichthys flavolineatus, Parupeneus multifasciatus, Acanthurus triostegas, and Naso lituratus. We measured ocular transmission in specimens from a range of sizes (juvenile–adult) and capture depths (<3–37 m), and used the wavelength (nm) where transmission was reduced 50% as our comparative measure (T 50 ). We modeled lens transmission varying pigment concentra- tions and pathlength, and compared predicted versus measured results. P. multifasciatus, M. flavolineatus, and N. lituratus showed a significant increase in short- wavelength blocking with size. A. triostegas were con- stant across sizes, and showed a slight but significant effect with depth. Comparisons of predicted versus observed transmission values suggest that pigment con- centrations are held constant with age for all species, but species- and family-level differences emerge. The accu- mulation of blocking compounds in ocular tissues is a contributing means for balancing the costs and benefits of admitting short-wavelength radiation to the retina. Keywords Cornea Æ Lens Æ Light Æ Ultraviolet Æ Vision Introduction Vertebrate visual capabilities are limited in part by light transmission through pre-retinal materials—cor- nea, lens, and humors. Thus, ocular transmission pro- vides some indication of the visual capabilities of a fish (Losey et al. 2003; Siebeck and Marshall 2001). In particular, concentrations of ultraviolet (UV)-blocking compounds, especially mycosporine-like amino acids (MAAs), will partially determine whether an animal is capable of UV vision (Losey et al. 1999). Because UV radiation (UVR, <400 nm) is potentially damaging (Calkins 1980; Zigman 1983, 1995; Cullen and Mon- teith-McMaster 1993; Dillon et al. 1999; Sliney 2001), permitting limited quantities of such radiation to reach photosensitive cells may be indicative of a trade-off between the physiological cost and advantages in sen- sory perception. Both costs and benefits associated with UVR are likely to vary among underwater habitats, and according to the ecology of different species and different life his- tory stages within a species (e.g., Thorpe and Douglas 1993; Thorpe et al. 1993; Lythgoe et al. 1994). Different fish species filter short-wavelength light using differing types and quantities of blocking compounds (Douglas and Marshall 1999), but pathlength—the distance through the ocular media that light must travel to reach the retina—also affects light transmission (Douglas and McGuigan 1989; Douglas and Thorpe 1994). We ad- dressed three basic questions regarding ocular trans- mission in this study: (1) what are the ontogenetic patterns, (2) what is the role of differing depth envi- ronments, and (3) can we distinguish between differing (putative) concentrations of blocking compounds and the effect of varying pathlength? We also sought to compare ocular transmission with known photopigment sensitivities using microspectrophotometry (MSP) data from Losey et al. (2003). We investigated ocular transmission in four species of diurnal coral reef fishes, Mulloidichthys flavolineatus J Comp Physiol A (2003) 189: 391–399 DOI 10.1007/s00359-003-0418-y P. A. Nelson Æ J. P. Zamzow Æ S. W. Erdmann G. S. Losey Jr P. A. Nelson Æ J. P. Zamzow Æ S. W. Erdmann Æ G. S. Losey Jr Hawai’i Institute of Marine Biology and Department of Zoology, University of Hawai’i, Coconut Island, P.O. Box 1346, 96744 Kane’ohe, Hawai’i P. A. Nelson (&) Center for Marine Biodiversity & Conservation, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0202, USA E-mail: pnelson@ucsd.edu Tel.: +1-858-534-4186 Fax: +1-858-822-1267