Transport of exogenous organic substances by invertebrate integuments: the field revisited

The notion that some marine invertebrates can use integumental uptake of organic compounds as a nutritional supplement dates back to the late 19th and early 20th centuries. This article provides a brief overview of more than a century's research, as it relates to significant stages in the development of experimental methods and concepts of general physiology. Emphasis is placed on changing paradigms and on the interplay between this specialized field of investigation and the mainstream of physiological thought. The present status of the field is summarized. The general consensus is challenged on the basis of previously published and new data from the author's laboratory. Particular emphasis is placed on data pointing toward an ultra-rapid turnover of amino acids in part of the epidermal space of the polychaete worm Nereis diversicolor. It is suggested that intra-epidermal L-alanine is compartmentalized metabolically or physically, and the consequences of this proposition are discussed in view of the general concepts of secondary active transport and intracellular isosmotic regulation. Future studies in this area of comparative physiology should concentrate not only on the molecular characteristics of the transporter proteins, but also on the way their function is integrated in the cellular physiology of the transporting cells. J. Exp. Zool. 289:254-265, 2001.