About the Cover
Cover:
Two striking features of the open ocean are the lack of obvious physical features and the extraordinary clarity of the water, in which visibility extends to about 100 meters. Animals that live in this pelagic realm are, of course, extremely vulnerable to being seen, identified as food, and eaten. To meet the challenge of predation, oceanic animals have evolved various tactics for hiding in an environment that precludes hiding. Effective adaptations include counterillumination, countershading, and mirrored sides; but the most apt mechanism is surely whole-body transparency-an emulation of the habitat. Therefore, although transparency is rare on land or in coastal waters, it is extremely common in the open ocean and is closely tied to the pelagic lifestyle.
On the cover of this issue is a photograph of an octopus, Vitreledonella richardi, which spends its entire life swimming in the subtropical and tropical regions of the world's oceans at depths of 200-1000 meters. Not only are most of its tissues highly transparent, but it is further modified for camouflage; its non-transparent elongated gut and eyes are continually oriented vertically and thus cast a minimal shadow toward potential predators that may be cruising below. (Credits: photo by David Wrobel, Monteray Bay Aquarium).
For many years, the biology of animals like Vitreledonella was completely overlooked. Most transparent species are fragile, and they are therefore destroyed by the sampling nets deployed from oceanographic vessels. In a sense, therefore, these cryptic organisms were hidden from biologists, as well as from their predators. The development of manned and robotic submersibles, blue water diving techniques, and optical equipment that is both portable and reliable has greatly increased our knowledge of these animals and their transparency.
In this issue, Sonke Johnson reviews our current understanding of biological transparency. This field-still in its infancy-includes empirical studies by marine and fresh-water biologists, but especially work on such camouflage-breaking visual abilities as ultraviolet and polarization vision. Theoretical and empirical research into the physical basis of biological transarency is also being carried out, much of it driven by the need to prevent and treat lapses in ocular transparency, such as cataracts.
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Copyright © 2003 by the Marine Biological Laboratory.
