Reference: Bial. Bull. 201: 301-318. (December 2001) Hidden in Plain Sight: The Ecology and Physiology of Organismal Transparency SONKE JOHNSEN* Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts Abstract. Despite the prevalence and importance of trans-parency in organisms, particularly pelagic species, it is a poorly understood characteristic. This article reviews the current state of knowledge on the distribution, ecology, and physical basis of biological transparency. Particular atten-tion is paid to the distribution of transparent species relative to their optical environment, the relationship between trans-parency and visual predation, the physics of transparency, and what is known about the anatomical and ultrastructural modifications required to achieve this condition. Transpar-ency is shown to be primarily a pelagic trait, uncommon in other aquatic habitats and extremely rare on land. Experi-mental and theoretical studies in terrestrial, freshwater, and marine ecosystems have shown that transparency is a suc-cessful form of camouflage, and that several visual adapta-tions seem to counter it. The physical basis of transparency is still poorly understood, but anatomical observations and mathematical models show that there are various routes to transparency. Future avenues for research include examina-tion of the ultrastructure and optical properties of transpar-ent tissue, exploring the link between transparent species and special visual modifications in the species they interact with, and analysis of the evolution of transparency using comparative methods. ctenophores to transparent polychaetes, gastropods, and fish (Fig. 1). Transparency is one of the few forms of camou-flage possible in a habitat with no surfaces to match or hide behind. It is also the only form of camouflage, and one of the few adaptations, that involve the entire organism. Al-though the importance of transparency has been mentioned many times by pelagic ecologists, it is a relatively unstudied characteristic (Hardy, 1956; Fraser, 1962; McFall-Ngai, 1990; Meyer-Rochow, 1997). This review synthesizes the current knowledge on the distribution, ecology, and physical basis of biological trans-parency. It is divided into five sections. The first section reviews the phylogenetic distribution of transparent species. The second section reviews and attempts to explain the relationship between transparent species and their optical environment. The third section links transparency to visi-bility; reviews terrestrial, freshwater, and marine studies of transparency and visual predation, including the use of special visual adaptations; and lists known active uses of transparency. The fourth section presents the underlying optical principles of transparency and then applies these principles to the various anatomical and ultrastructural mod-ifications seen in transparent tissues. The final section sug-gests several avenues for future research. Introduction Transparency is a fascinating and surprisingly common characteristic that has received little attention because the majority of transparent species are found only in the pelagic regions of the open ocean. In these regions, however, the prevalence and diversity of transparent species is remark-able, ranging from the relatively well-known medusae and Received 30 May 2001; accepted 30 August 2001. * Current address: Biology Department. Box 90338, Duke University. Durham. NC 27708. E-mail:
[email protected] Phylogenetic Distribution The phylogenetic distribution of transparent animals is diverse, uneven, and strongly influenced by environment. Although significant levels of tissue transparency are found in a wide array of organisms (Figs. 1, 2), most transparent species are found in the following 10 groups, all of which are pelagic: cubozoans. hydromedusae. non-beroid ileno-phores. hyperiid amphipods, tomopterid polychaeles, ptero-tracheid and carinariid heteropods, pseudothec"somatous pteropods, cranchiid squid, thaliaceans, and chaetognaihs. 301
BioStor
