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Chameleon of the Sea 4 Br J Ophthalmol 2005;89:4 Br J Ophthalmol: first published as 10.1136/bjo.2004.057240 on 21 December 2004. Downloaded from Cover illustration.............................................................................. Chameleon of the sea mbush predators possess the in its attack. Over these short distances of advantage of surprise and, when a few body lengths, its speed is a Acoupled with speed, are devastat- humbling 4 metres/s. This is approxi- ingly successful. Speed and accuracy mately 200 body lengths/s and makes require adept sensory mechanisms, the sandlance’s strike for all the world most commonly vision, and excellent like the strike by the chameleon’s tongue. vision usually requires large eyes espe- Coupled with an oral ‘‘cage’’ that allows cially among the vertebrates. But evolu- thefishtoengulfpreyifitisnotexactly Figure 1 Note the corneal lenticle, the size of on target or if the prey should move tion has found creative, often unique, the lens, and the convexiclivate fovea. (From solutions for the smaller vertebrates that Collin SP, Shand J. In: Collin SP, Marshall NJ, erratically, this species rarely misses. survive in the highly competitive world eds. Sensory processing in aquatic Interestingly, it can, and does, target of predation. The sandlance, Limnichthyes environments 2003:139–69. Courtesy of prey monocularly because of its unique fasciatus, has just such a unique solution. Springer, New York.) optics, which resemble those of a L fasciatus is no more than 20–40 mm chameleon and are substantially differ- in length and hides in the loose sand of and thus creates monocular parallax. ent from our own. The monocular the shallow reefs of the Indo-Pacific. Most vertebrates must move their head movements of L fasciatus contain the Using creative adaptations, this benthic or even their body to gain monocular familiar rapid saccades to an eccentric predator will explode from its camou- distance cues through parallax. Such anatomical position to locate potential flaged hide among the coral rubble in a animals as a robin or pigeon bob their prey, but what follows that saccade is an brief and extremely rapid sally to attack heads do this, but the sandlance merely independent monocular drift back small planktonic prey. moves its eye to achieve the same degree towards a more neutral position. In Most fish have all the refractive power of parallax. Because of this unique most animals, the oculomotor system of their eye concentrated into a round system, it can function perfectly well, has fast saccades to prevent a slow crystalline lens (BJO covers May 2002 monocularly, as an ambush predator in blurring or destabilisation of the image, and Oct 2004) with little or no refractive a three dimensional world and because but the slow drift in the eyes of the effect from the cornea since the refrac- of the nature of its visual system can sandlance is a distinct departure from tive index of the cornea and seawater dissociate its optokinetic response from the norm and probably occurs because is approximately the same. L fasciatus, one eye to another. of a passive relaxation of the extraocular though, has a very thick cornea repre- The almost exclusively cone retina eye muscles (Fritsches KA, Marshall J, senting approximately one seventh of consists of a peculiar matrix of a single Curr Biol 1999;9:292). the diameter of the approximately cone surrounded by four double cones. This remarkable little fish should 1 mm long eye, as can be seen from The ganglion cells are tightly packed remind you of another ambush preda- the figure on this page. This fish, unlike with a foveal density of 150 000 cells per tor, the chameleon, because of numer- http://bjo.bmj.com/ any other, has a cornea with an embed- mm2 which is the highest among all the ous similarities such as camouflage, ded lenticle that functions as a lens with fish species studied and rivals the highly rapid accurate targeting of small mobile approximately 180 dioptres of accommo- specialised retinas of birds (Collin SP, prey, corneal accommodation, telescopic dative potential that can be unleashed Collin HB, J Comp Neurol 1988;278:226). optics with a forward movement of the in a fraction of a second by the use of The fovea is convexiclivate, meaning axis of rotation of the eye, monocular a striated retractor cornealis muscle. that it is steep walled with a deep range finding, and extreme non-syn- Corneal accommodation is seen in many central depression as can be seen in chronous ocular movement. Now if L birds, but is unique to this fish in the the figure. This anatomical arrangement fasciatus could only change colour! on October 2, 2021 by guest. Protected copyright. piscine world (Pettigrew JD, Collin SP, probably provides image magnification I R Schwab, S P Collin, J D Pettigrew J Comp Physiol A 1995;177:397). because of the difference in the index of University of California, Davis, Sacramento, The crystalline lens is peculiar as well, refraction between vitreous and retina CA, USA; [email protected] at least for a fish. The lens of the combined with the steep curvature of sandlance is flattened. The combination the sides of the foveal pit. of a corneal lenticle with a flattened, but The eyes move independently, and hyperopic, lens creates a magnified alternately (to avoid the simultaneous image on the fovea. Although the optics acquisition of different prey by each eye are complicated and beyond the scope of that could lead to the fish theoretically this essay, the nodal point is moved splitting itself down the middle). The Pictures by S P Collin, Department of forward and separated from the axis of animal burrows slightly below the sand Anatomy and Developmental Biology, and rotation allowing monocular parallax. with only its eyes exposed, and waits, J D Pettigrew, The Vision, Touch and Hear- This displacement of the nodal point and scans with both eyes––indepen- ing Research Center, School of Biomedi- cal Sciences, University of Queensland, and separation of the nodal point from dently. Once a planktonic prey species, Brisbane, Australia. Visit the website for a the axis of rotation create a significant such as a copepod, comes into striking video of this creature (http://bjo.bmjjournals. displacement of the image on the retina, distance, L fasciatus will waste little time com/supplemental). www.bjophthalmol.com.
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