The ‘origin of larvae’ has been widely discussed over
the years, almost invariably with the tacit
understanding that larvae are secondary specializations of early stages
in a holobenthic life cycle.
Considerations of the origin and early radiation of the metazoan phyla
have led to the conclusion that the
ancestral animal (=metazoan) was a holopelagic organism, and that pelago-benthic
life cycles evolved
when adult stages of holopelagic ancestors became benthic, thereby changing
their life style, including their feeding biology.
The literature on the larval development and phylogeny of animal phyla
reviewed in an attempt to infer
the ancestral life cycles of the major animal groups. The quite detailed
understanding of larval evolution in
some echinoderms indicates that ciliary filter-feeding was ancestral within
the phylum, and that
planktotrophy has been lost in many clades. Similarly, recent studies of
the developmental biology of
ascidians have demonstrated that a larval structure, such as the tail of
the tadpole larva, can easily be lost,
viz. through a change in only one gene. Conversely, the evolution of
complex structures, such as the ciliary
bands of trochophore larvae, must involve numerous genes and numerous adaptations.
The following steps of early metazoan evolution have been inferred from
The holopelagic ancestor, blastaea, probably consisted mainly of choanocytes,
which were the feeding
organs of the organism. Sponges may have evolved when blastaea-like organisms
settled and became reorganized with the choanocytes in collar chambers.
The eumetazoan ancestor was probably the gastraea, as suggested
previously by Haeckel. It was
holopelagic and digestion of captured particles took place in the archenteron.
Cnidarians and ctenophores
are living representatives of this type of organization. The cnidarians
become pelago-benthic with the
addition of a sessile, adult polyp stage; the pelagic gastraea-like planula
larva is retained in almost all major
groups, but only anthozoans have feeding larvae.
Within the Bilateria, two major lines of evolution can be recognized:
Protostomia and Deuterostomia. In
protostomes, trochophores or similar types are found in most spiralian
phyla; trochophore-like ciliary bands
are found in some rotifers, whereas all other aschelminths lack ciliated
larvae. It seems probable that the
trochophore was the larval type of the ancestral, pelago-benthic spiralian
and possible that it was ancestral
in all protostomes. Most of the non-chordate deuterostome phyla have
ciliary filter-feeding larvae of the
dipleurula type, and this strongly indicates that the ancestral deuterostome
had this type of larva.