Hypotheses on the origins of marine larvae

(i) Varied origins of larval forms and metamorphoses are indicated by comparisons among extant animals. (ii) Size-specific and stage-specific constraints on survival and growth may result in distinctive larval traits, but to explain larval origins, size- or stage-specific advantages must be extrapolated to ancient environments and ancestral traits. Adaptations for habitat selection or dispersal do not account for evolution of long precompetent periods of larval feeding and growth or for larvae in holoplanktonic life histories. (iii) Transverse bands of cilia on ridges or posterior edges of larvae meet a common functional requirement for propulsion, are expected to be convergently similar, and may have arisen numerous times. (iv) Biases imposed by ancestral larval or juvenile traits might result in convergent similarity of derived larval traits with a result resembling homology. (v) Some extant feeding larvae persist with unusually simple structures or low performance. These larvae suggest possible intermediate steps in the evolution of feeding larvae. (vi) Direction of evolutionary transitions of larval traits are indicated by distribution of traits within clades and by vestigial structures. Feeding larval stages have been lost often and gained rarely. (vii) Most types of feeding larvae evolved at most once, are in clades that diverged early, and are therefore of ancient origin. Other evidences of antiquity are fossil traces of larval molluscs and brachiopods from the Ordovician and fossils of crustacean nauplii from the Upper Cambrian. (viii) Inferred combinations of ancestral traits can differ from combinations known from extant descendants. Examples are external fertilization combined with a feeding larva or small adult size. (ix) In some clades, feeding larval forms that originated earlier may be associated with small parental investment per offspring. (x) Unreliable rules for evolutionary inferences include the assumptions that primitive traits are correlated in extant animals, that evolution proceeds from simple to complex structures, that earlier stages in development are more conservative, and that structures that develop earlier evolved earlier.