135 Contributions to Marine Science 2012 FLUORESCENT PATTERNS IN SOME PORTUNUS SPECIES (CRUSTACEA: BRACHYURA: PORTUNIDAE) Wong Ze-Lin Department of Biological Sciences, National University of Singapore, 14, Science Drive 4, Singapore 117543. Ng Ngan-Kee Department of Biological Sciences, National University of Singapore, 14, Science Drive 4, Singapore 117543. Email: dbsngnk@nus.edu.sg S. L-M Teo Tropical Marine Science Institute, National University of Singapore, 18 Kent Ridge Road, Singapore 119227. Email: tmsteolm@nus.edu.sg Fernando J Parra-Velandia Tropical Marine Science Institute, National University of Singapore, 18 Kent Ridge Road, Singapore 119227. Email: tmsfjp@nus.edu.sg ABSTRACT. — Portunus is a large genus of swimming crabs, which has been observed to have fluorescing patterns in certain species, including commercially important ones. We examined these patterns for their potential usefulness as a diagnostic character in species identification. Specimens of various Portunus species were photographed under normal light and UV light. It was observed that fluorescent patterns varied inter- and intra-specifically. The patterns were consistent between males and females of each species, but not amongst juveniles. Fluorescent colour patterns were most pronounced in fresh specimens, and may be useful as a complementary taxonomic character for species identification. KEY WORDS. — Structural colour, fluorescence, iridescence, crustacean, Portunus Contributions to Marine Science 2012: 135–143 Date of Publication: 29 Sep.2012 © National University of Singapore INTRODUCTION The detection of electromagnetic (EM) radiation is important in the natural world as many organisms rely on their sense of sight to gather information about their environment and communicate with conspecifics and other organisms. It plays an important role in inter- and intra-specific communication, protection (camouflage and aposematism), sexual selection and foraging (Altshuler, 2001; Lim et al., 2007; Stevens & Marilaita, 2009). As vision is widely used, many marine organisms use colour as cues. With the exception of bioluminescence, most organisms require that electromagnetic radiation be reflected off its body to appear visible and in colour to others; this can be achieved by the use of microstructures (i.e. structural colours) or, the commonest way, by using pigments which absorb specific electromagnetic waves of the spectrum (normally between 100-1200 nm), and reflect also a specific wavelength with lower energy than that of the absorbed (Britton, 1983; Parker, 2005) producing a colour. The term fluorescence applies when the absorbed wavelength is in the ultraviolet band range of the spectrum (200-400 nm) and normally the re-emitted electromagnetic wave is of a longer, and often, visible wavelength; it has been observed in aquatic and terrestrial invertebrates such as corals, scorpions and spiders (Stachel et al., 1999; Mazel & Fuchs, 2003; Lim et al., 2007). However, besides all the information on animal interaction using fluorescence, the exact mechanism is poorly known or seldom discussed. The genus Portunus Weber, 1795, is a large genus of swimming crabs comprising 93 species (Ng et al., 2008), many of which are commercially important (FAO fishery statistics, 2012) and occur mainly in the Indo-west Pacific (Davie, 2002). Portunus pelagicus has been found to be