Light and Shape: A Contribution to Demonstrate Morphological Differences in Diurnal and Nocturnal Teleosts Domitilla Pulcini, 1 * Corrado Costa, 1,3 * Jacopo Aguzzi, 2 and Stefano Cataudella 1 1 Laboratory of Experimental Ecology and Aquaculture, Department of Biology, University of Rome Tor Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy 2 Institut de Cie `ncies del Mar (CSIC), Passeig Marı´tim de la Barceloneta 37-49, 08003 Barcelona, Spain 3 Agritech Lab, CRA-I.S.M.A. (Council for Research in Agriculture - Agricultural Mechanization Research Institute), Via della Pascolare, 16, Monterotondo, 00016 Roma, Italy ABSTRACT Light intensity is an important environ- mental factor affecting the structure of fish assemblages during the day–night cycle. Light influences how organ- isms perceive their environment, modulating their intra- specific and interspecific relationships. The relationship between light intensity variations and biological cycles should be observed at the level of organismal morphol- ogy. In this study the relationship between activity rhythms, thus light intensity experienced by fish in the period of major activity and external morphology, have been investigated. The morphological traits of 97 selected fish species were compared in order to deter- mine the existence of a common morphological plan in agreement with their diurnal or nocturnal activity rhythm. Species sorting was performed by maximizing the diversity of activity rhythm, habitat choice, ecology, and trophic habits within the same family, to assess the importance of the day–night cycle on species morphology in relation to other environmental features. The morpho- logical characters selected for the geometric morphomet- ric analysis were body profile and the position of mouth, eye, pelvic, pectoral, dorsal, and caudal fin. The present analysis allowed different consensus forms for nocturnal and for diurnal species to be identified. Two-block Par- tial Least Squares analysis was then performed for the purpose of modeling the covariation between the form and two important external variables (ecology and activ- ity). J. Morphol. 269:375–385, 2008. Ó 2007 Wiley-Liss, Inc. KEY WORDS: fish; ecomorphology; nocturnal and diurnal rhythms; geometric morphometrics; shape The form of an organism constrains the use of resources and, at the same time, resource avail- ability contributes to the construction of the form. This occurs via evolution through tasks that increase the fitness of individuals (Chan, 2001). The environment is thus recognized as a powerful force in modeling organism morphology during on- togeny (Wainwright, 1991; Russo et al., 2007). According to ecological morphology, the form of species is related to their ecological role in the community (Motta et al., 1995a,b). The study of the form of organism apparatuses, in relation to each single function they perform, defines the framework of the functional morphology. This dis- cipline studies how a particular design has been favored by natural selection owing to its more effective energetic and/or anatomical functioning (Alexander, 1988). In this context, ecological mor- phology also includes functional morphology since the whole form of an organism is the result of all the selective pressures exerted by different physi- cal and biotic factors on all its structures. The mu- tual contribution of these environmental factors (physical and biotic) on the form of species is then analyzed, in accordance with Motta and Kotrschal (1992). It is therefore of interest to understand how the morphological variation among individu- als, populations, or species, corresponds to a varia- tion in their ecology (Leisler and Winkler, 1985; Sarda ` et al., 2005; Recasens et al., 2006; Costa and Cataudella, 2007). In marine ecosystems, the day–night cycle is one of the most powerful environmental agents driving the patterns of species activity in relation to their ecological role (Naylor, 2005). This is particularly evident for those organisms that inhabit the three dimensions of the water column, where light inten- sity is progressively reduced and its spectral qual- ity modified with increasing depth (Herring, 2003). Day–night variations determine marked changes in marine community structure over 24 h at differ- Contract grant sponsor: Ministero Italiano per le Politiche Agri- cole e Forestali; Contract grant number: Law 41/82. *Correspondence to: Dr. Domitilla Pulcini, Laboratory of Experi- mental Ecology and Aquaculture, Department of Biology, University of Rome ‘‘Tor Vergata’’, Via della Ricerca Scientifica, 00133 Rome, Italy. E-mail: domitillapulcini@fastwebnet.it or Dr. Corrado Costa, Laboratory of Experimental Ecology and Aquaculture, Department of Biology, University of Rome ‘‘Tor Vergata’’, Via della Ricerca Sci- entifica, 00133, Rome, Italy. E-mail: corrado_costa@libero.it Published online 30 October 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jmor.10598 JOURNAL OF MORPHOLOGY 269:375–385 (2008) Ó 2007 WILEY-LISS, INC.