About the Cover
Cover
Many benthic marine invertebrates produce planktonic larvae that are dispersed by tides and currents. Even if, by chance, these larvae arrive timely at a location close to an appropriate substratum, specific behavioral responses must be evoked that will lead them to settle and to complete their metamorphosis to the adult form. These critical larval behaviors, and thus successful recruitment, are now known to be induced by chemical cues-some adsorbed to surfaces, others soluble-generated in the environment.
The illustration on the cover shows a pair of nudibranchs (Phestilla sibogae) crawling and eating on their characteristic habitat-and prey-Porites compressa, an abundant coral in Hawaiian reefs. Phestilla has been a superior model for studying larval responses to chemical cues that lead to successful settlement. This small nudibranch (<3 cm) can be conveniently, rapidly, and repeatedly reared in the laboratory throughout the year, and its metamorphosis is well understood. More important, its larvae are induced to settle by a small polar molecule released by Porites, its coral substrate and source of food. Furthermore, details of the signal-transduction pathway, including the location of the receptor that induces metamorphosis, are also known.
In nature, however, the interaction between larva and cue occurs in a real environment, and the reef-comprising coral and other types of attached plant and animal species-is a very complex physical habitat indeed. The wave-driven, turbulent flow of seawater over and through the reef disperses the chemical cue released from the coral in extremely fine filaments that swirl in essentially clean water. The Phestilla larvae, mixed with cue by the turbulence, enter and leave filaments at high frequency, so each encounter with the cue is very brief. In this issue of The Biological Bulletin (p. 28), M. G. Hadfield and M. A. R. Koehl consider for the first time, and in detail, these instantaneous responses of Phestilla larvae to encounters with a soluble settlement cue.
As shown in the inset on the cover, individual larvae (shell length, 210 um) were stuck to the tip of a fine insect pin and inserted into a mini-flume; the flow rate toward the larva was adjusted to equal that of a larva freely swimming in still water. Filaments of fluorescein-tagged seawater (green area in both panels), with or without cue, were produced and could be moved over and past a larva, simulating its passage into and out of a natural filament. In clean water (left panel), the larva quickly extends its foot and velum (the larval swimming organ) from the shell, beats its cilia, and "swims." In a filament containing seawater with settlement cue (right panel), it partially retracts into its shell and would sink if it weren't tethered. These rapid responses to a dissolved settlement cue can enhance the rapid transport of microscopic larvae to the substratum, even in the wave-driven, turbulent flow across a coral reef.
The image of Phestilla sibogae (with egg masses) on coral was taken by Robin B. Kinnel; the two images of a tethered larva in the mini-flume are from Figure 2 (p. 32) in the paper by Hadfield and Koehl and were taken by the authors. The cover design was drafted by M. Lynn Milstead (The Whitney Laboratory, University of Florida) and completed by Beth Liles (Marine Biological Laboratory).
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Copyright © 2004 by the Marine Biological Laboratory.
