The Systematics of Vertebrate Larvae

THE interpretation of the larval characteristics of the fishes and amphibians in terms of systematics and evolution presents many challenging problems. For many years, studies on early developmental stages were guided by the idea that larval characters have an ancestral, recapitulative significance, or by the belief that they are secondary adaptations with little or no evolutionary importance. Neither theory is adequate from the standpoint of systematics, and together they have somewhat hampered effective use of larval data by taxonomists. A third and more significant concept, based on principles formulated by Von Baer in the 1800s, has gained prominence in recent years through the work of de Beer and others. It emphasizes comparative studies on equivalent growth stages of different species, and provides a much sounder basis for interpreting developmental characteristics in terms of systematics and evolution. The larval stage may serve different ecological functions in different animals. A suitably adapted larva may enable a species to succeed in a region that might otherwise be closed to it-a conspicuous factor in the distribution of many frogs. A pelagic larva may be the major dispersal stage in many marine fishes having sluggish or strongly territorial adults, paralleling a well-known life-history pattern in the invertebrates. It is often suggested that inadequate provision for embryonic nutrition is the primary factor governing the occurrence of a larval stage. Whatever the reason for its establishment in the ontogeny may be, the larva is subject to the operation of genetic evolutionary processes, like any other developmental stage, since all growth stages of the individual are under the control of a single genetic system. The same basic procedures of systematics are applied to both larval and adult stages. These procedures include the analysis of variation and the study of differences and of resemblances between species and groups; and the interpretation of the basic data in terms of the primary trends of phylogeny and of secondary phenomena of adaptive radiation, convergence, rectilinear trends, and other patterns of evolution, at all systematic levels.