Phylogeny and Adaptive Radiation
Phylogeny and
Adaptive Radiation
Phylogeny
Hemichordate phylogeny has long been puzzling. Hemichordates share characters with both echinoderms and chordates. With chordates they share gill slits, which serve primarily for filter feeding and secondarily for breathing, as they do in some protochordates. In addition, a short dorsal, somewhat hollow nerve cord in the collar zone may be homologous to the nerve cord of chordates (Figure 24-7). The buccal diverticulum in the hemichordate mouth cavity, long thought homologous to the notochord of chordates, is now considered a synapomorphy of hemichordates themselves. Early embryogenesis of hemichordates is remarkably like that of echinoderms, and the early tornaria larva is almost identical to the bipinnaria larva of asteroids, suggesting that echinoderms form the sister group of hemichordates and chordates (Figure 24-7). However, Brusca and Brusca* placed lophophorates as the sister group of hemichordates and chordates, required by their proposed synapomorphy for all these groups of a crown of ciliated tentacles containing extensions of the mesocoel. Their hypothesis is not supported by analysis of the base sequence of the gene encoding the small-subunit of rRNA, which indicates a deuterostome clade (Echinodermata, Hemichordata, and Chordata) and places the lophophorate phyla in superphylum Lophotrochozoa of Protostomia.
Other than their shared deuterostome characters, the relationship of chaetognaths to deuterostome phyla is enigmatic. Sequence analysis of the gene encoding small-subunit rRNA supports placement of chaetognaths among protostomes. Some investigators suggest, however, that chaetognaths are neither protostomes nor deuterostomes but originated independently from an early coelomate lineage.
Adaptive Radiation
Because of their sessile lives and their habitat in secreted tubes in ocean bottoms, where conditions are fairly stable, pterobranchs have undergone little adaptive divergence. They have retained a tentacular type of ciliary feeding. Enteropneusts, on the other hand, although sluggish, are more active than pterobranchs. Having lost their tentaculated arms, they use a proboscis to trap small organisms in mucus, or they eat sand as they burrow and digest organic sediments from the sand. Their evolutionary divergence, although greater than that of pterobranchs, is still modest.
Phylogeny
Hemichordate phylogeny has long been puzzling. Hemichordates share characters with both echinoderms and chordates. With chordates they share gill slits, which serve primarily for filter feeding and secondarily for breathing, as they do in some protochordates. In addition, a short dorsal, somewhat hollow nerve cord in the collar zone may be homologous to the nerve cord of chordates (Figure 24-7). The buccal diverticulum in the hemichordate mouth cavity, long thought homologous to the notochord of chordates, is now considered a synapomorphy of hemichordates themselves. Early embryogenesis of hemichordates is remarkably like that of echinoderms, and the early tornaria larva is almost identical to the bipinnaria larva of asteroids, suggesting that echinoderms form the sister group of hemichordates and chordates (Figure 24-7). However, Brusca and Brusca* placed lophophorates as the sister group of hemichordates and chordates, required by their proposed synapomorphy for all these groups of a crown of ciliated tentacles containing extensions of the mesocoel. Their hypothesis is not supported by analysis of the base sequence of the gene encoding the small-subunit of rRNA, which indicates a deuterostome clade (Echinodermata, Hemichordata, and Chordata) and places the lophophorate phyla in superphylum Lophotrochozoa of Protostomia.
Other than their shared deuterostome characters, the relationship of chaetognaths to deuterostome phyla is enigmatic. Sequence analysis of the gene encoding small-subunit rRNA supports placement of chaetognaths among protostomes. Some investigators suggest, however, that chaetognaths are neither protostomes nor deuterostomes but originated independently from an early coelomate lineage.
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| Figure 24-7 Cladogram showing hypothetical relationships among deuterostome phyla. Brusca and Brusca considered the crown of ciliated tentacles (containing extensions of the mesocoel) a character borne by ancestors of lophophorates, hemichordates and chordates. The tentacular crown would have become the lophophore in lophophorate phyla and retained as a primitive character in pterobranchs. Because molecular evidence indicates that lophophorates are protostomes, we removed them from this cladogram; the ciliated tentacular crown in pterobranchs and lophophorates can be considered a convergent character. |
Adaptive Radiation
Because of their sessile lives and their habitat in secreted tubes in ocean bottoms, where conditions are fairly stable, pterobranchs have undergone little adaptive divergence. They have retained a tentacular type of ciliary feeding. Enteropneusts, on the other hand, although sluggish, are more active than pterobranchs. Having lost their tentaculated arms, they use a proboscis to trap small organisms in mucus, or they eat sand as they burrow and digest organic sediments from the sand. Their evolutionary divergence, although greater than that of pterobranchs, is still modest.
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