An interplay between short- and long-range interactions is a crucial element in a mathematical model of biological pattern formation formulated by Alan Turing in 1952 [11,12]. Turing, whose 100 th anniversary was commemorated earlier this year, formulated this mathematical model based on concentrations of two substances, an activator and an inhibitor. The activator activates its own synthesis and that of an inhibitor, which inhibits the activator, and both substances diffuse away from the source at different rates. Depending on which parameters are chosen, a regular periodic pattern of substance distribution can emerge. What is exciting about this model is that the pattern can basically arise from ‘nothing’, i.e. from very small fluctuations of initial concentrations. In that sense, it is appealing to think of the zebrafish stripes, which also have self-organising characteristics, as Turing patterns. Turing conceived his model as a purely mathematical system in one dimension, but simulations based on Turing models can give rise to an amazing variety of biological patterns, from sea shells to cats [12]. Such a general model is naturally appealing for biologists who often lament the lack of unified theories in their field, but the challenge is to identify how it is implemented in the real world. Obviously, Turing could not know about the principles and intricacies of cellular signalling. So, in the study of real-life Turing patterns, the abstract roles of his ‘activator’ and inhibitor’ need to be played by real molecules or cells. One of the most clear-cut incarnations of a Turing mechanism in the context of a periodic pattern was found in the spacing of hair follicles in mice, where the signalling molecule WNT is acting as an activator and its antagonist DKK as the inhibitor [13]. Sure enough, Turing patterns can also match with astonishing precision the colour patterns observed in zebrafish under various conditions [10]. However, it is not yet clear whether such an activator–inhibitor system is really at play here, and if so how it is implemented. It need not be as literal as in the case of mouse hair follicle spacing. Instead, the ‘activator’ could be a stimulation of proliferation, and the inhibitor could be the repulsion seen when melanophores and xanthophores bump into each other. Integrating the electrical properties of the pigment cells into a Turing model will be a challenge. But the idea that the stripes of zebrafish could be a Turing pattern come to life organised by membrane potentials — something rarely considered in the context of developmental pattern formation — is definitely an electrifying one. References 1. Parichy, D.M. (2003). Pigment patterns: fish in stripes and spots. Curr. Biol. 12, R947–R950. 2. Inaba, M., Yamanaka, H., and Kondo, S. (2012). Pigment pattern formation by contact-dependent depolarization. Science 335, 677. 3. Yamaguchi, M., Yoshimoto, E., and Kondo, S. (2007). Pattern regulation in the stripe of zebrafish suggests an underlying dynamic and autonomous mechanism. Proc. Natl. Acad. Sci. USA 104, 4790–4793. 4. Parchem, R.J., Perry, M.W., and Patel, N.H. (2007). Patterns on the insect wing. Curr. Opin. Genet. Dev. 17, 300–308. 5. van Eeden, F.J., Granato, M., Schach, U., Brand, M., Furutani-Seiki, M., Haffter, P., Hammerschmidt, M., Heisenberg, C.P., Jiang, Y.J., Kane, D.A., et al. (1996). Genetic analysis of fin formation in the zebrafish, Danio rerio. Development 123, 255–262. 6. Haffter, P., Odenthal, J., Mullins, M.C., Lin, S., Farrell, M.J., Vogelsang, E., and Nu ¨ sslein-Volhard, C. (1996). Mutations affecting pigmentation and shape of the adult zebrafish. Dev. Genes Evol. 206, 260–276. 7. Maderspacher, F., and Nu ¨ sslein-Volhard, C. (2003). Formation of the adult pigment pattern in zebrafish requires leopard and obelix dependent cell interactions. Development 130, 3447–3457. 8. Watanabe, M., Iwashita, M., Ishii, M., Kurachi, Y., Kawakami, A., Kondo, S., and Okada, N. (2006). Spot pattern of leopard Danio is caused by mutation in the zebrafish connexin 41.8 gene. EMBO Rep. 7, 893–897. 9. Iwashita, M., Watanabe, M., Ishii, M., Chen, T., Johnson, S.L., Kurachi, Y., Okada, N., and Kondo, S. (2006). Pigment pattern in jaguar/ obelix zebrafish is caused by a Kir7.1 mutation: implications for the regulation of melanosome movement. PLoS Genet. 2, e197. 10. Nakamasu, A., Takahashi, G., Kanbe, A., and Kondo, S. (2009). Interactions between zebrafish pigment cells responsible for the generation of Turing patterns. Proc. Natl. Acad. Sci. USA 106, 8429–8434. 11. Turing, A.M. (1952). The chemical basis of morphogenesis. Phil. Transact. Roy. Soc. London B 237, 37–72. 12. Kondo, S., and Miura, T. (2010). Reaction-diffusion model as a framework for understanding biological pattern formation. Science 329, 1616–1620. 13. Sick, S., Reinker, S., Timmer, J., and Schlake, T. (2006). WNT and DKK determine hair follicle spacing through a reaction-diffusion mechanism. Science 314, 1447–1450. Florian Maderspacher is Current Biology’s Senior Reviews Editor. E-mail: florian.maderspacher@ current-biology.com DOI: 10.1016/j.cub.2012.03.032 Sensory Ecology: Giant Eyes for Giant Predators? Mathematical models suggest the enormous eyes of giant and colossal squid evolved to see the bioluminescence induced by the approach of predatory whales. Julian C. Partridge In the American Museum of Natural History, a striking diorama (Figure 1) depicts a battle between one of the world’s largest mammals and its second largest invertebrate: in the darkness of a deep ocean, a sperm whale wrestles a giant squid. Although this interaction has never been witnessed, these species have captured the human imagination for millennia, and their putative combat for centuries. In stories and myth sperm whales (Physeter macrocephalus) and giant squid (Architeuthis spp.) are conjured as terrible and terrifying animals, easily provoked to attack both seafarers and their ships. Such attacks on ships may have occurred, but attacks by whales on squid are certainly much more common: giant squid are undoubtedly important components of the diet of sperm whales, squid beaks often being found in sperm whale guts, and the skin of sperm whales often baring scars from giant squids’ formidable suckers. Indeed, predation of giant squid by sperm whales can be considered the culmination of an approximately 30 million year evolutionary arms race between cephalopods and whales. This race is marked by an interesting sensory imbalance, in which whales depend on reflected sound to find Current Biology Vol 22 No 8 R268