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Oregon Institute of Marine Biology and Department of Biology, University of Oregon, P.O. Box 5389, Charleston, Oregon 97420
* To whom correspondence should be addressed. E-mail: remlet{at}uoregon.edu, svetlana{at}uoregon.edu, ashanks{at}uoregon.edu, cmyoung{at}uoregon.edu
Larval forms have been central to the study of evolutionary and developmental biology since the inception of these fields in the mid-19th century, yet they still offer contemporary biologists tremendous opportunities for deciphering fundamental biological principles. Larvae are found between the embryonic and juvenile stages of those metazoans with indirect development, which includes the vast majority of marine invertebrates and many marine fishes. As developmental stages that interact behaviorally with their environments, they play important roles in development, ecology, and evolution and are therefore of interest to a wide community of biologists. The recent rise of "Evo Devo" (Evolutionary Developmental Biology) has propelled the study of larval forms to center stage in organismal biology. However, many marine invertebrate larvae are still undescribed and the variety of developmental types is greatly underestimated, especially in phyla that historically received little attention. Increased application of confocal microscopy in combination with fluorescent dyes is now revealing a previously unsuspected diversity of development and facilitating comparative studies of larval neurogenesis, myogenesis, and anatomy.
Most larvae are small, on the order of 0.5 to a few millimeters in size, and they generally occupy different habitats from those of the juveniles into which they metamorphose. For planktonic larvae, small size and slow rates of movement with cilia or muscle-driven appendages require them to have shapes and functions different from those of juveniles and also leave them more or less at the mercy of the water currents that disperse and mix them. Because many marine invertebrates have planktonic larval stages and benthic adults, larvae link adult populations genetically and contribute to local population sizes and dynamics. A recent renewed interest in marine conservation focuses on larvae as the dispersal stage that provides the all-important connectivity among metapopulations, including those in marine reserves. Larval stages are also key to the understanding of recruitment, a critical issue in this age of declining fisheries resources.
As transitional stages between embryos and juveniles, larvae have structures, physiology, and behaviors that allow them to operate in their own environments while developing anatomical and physiological attributes that will be used by the juvenile stages in a new environment. In addition to similarities and differences of invertebrate larval forms among major clades such as phyla and classes, many lower level clades also show large variations in larval form, length of larval life, and timing of expression of traits, all of which provide opportunities to study how the differences arose.
In the context outlined above, larval biology is a very broad, very active, and very rich area for exploration of basic scientific questions, so it is fitting that the fourth virtual symposium in The Biological Bulletin be devoted to the Biology of Marine Invertebrate Larvae. This journal has a rich history of papers that examine reproduction, development, and larvae of marine organisms, especially those of invertebrates. This tradition began with two of the five papers published in Volume 1, Issue 2, in 1897. Papers in this field increased exponentially between the 1950s and the early 1980s (Young, 1990), and the tradition continues to this day. Between 1980 and 2008 the Biological Bulletin has published an average of 20 papers per year focused on some aspect of reproduction, development, or larvae, representing 29% of all papers the journal published during this interval.
As editors and larval biologists, each with our own view of the importance of invertebrate larvae, we have invited an eclectic group of scientists, from all career stages, to help portray the richness and diversity of current studies in invertebrate larval biology. The virtual symposium is composed of position papers, original research papers, syntheses, and reviews. Topics include the origins of larvae (Nielsen; Page), novel larval types (Maslakova and von Döhren), diversity of larval myogenesis (Wanninger), nervous system and sensory structure, development, and evolution (Wanninger; Elia et al.; Bishop and Hall), dispersal of larvae (Metaxas and Saunders; Shanks), changes in larvae through time (Burgess et al.) or across closely related species (Krug), quantification of egg size and content (Moran and McAlister), larval feeding (Pochelon et al.) and a review of the larval biology of Rithropanopeus harrisii, a crab that has yielded major insights in larval behavior, ecology, and physiology (Forward).
We emphasize that these papers are a small but representative sample of some areas of active research in larval biology. There are many other larval, developmental, and evolutionary biologists who work on some aspect of larval biology. It is our hope that this compilation will convey the enthusiasm and creativity that marine invertebrate larvae inspire in those who study them.
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  D. I. Williamson Caterpillars evolved from onychophorans by hybridogenesis PNAS, November 24, 2009; 106(47): 19901 - 19905. [Abstract] [Full Text] [PDF]
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