indicative of former differentiation in the process of being swamped by introgression, or of recent speciation.
Molecular work on the starfish A. planci, including specimens from Panama,
revealed that it is not in fact a single species, but a pan-Indo-Pacific species
complex consisting of four deeply diverged clades (Pacific, Red Sea, Northern
Indian Ocean, and Southern Indian Ocean) (Vogler et al. 2008). These clades
reportedly diverged between 1.95 and 3.65 mya (Pliocene to early Pleistocene)
and have genetic distances (8.8–10.6 %) equivalent to the distances between other
sibling species of starfish (Waters et al. 2004). Vogler et al. (2008) propose that
this speciation process was driven by sea level changes (Pillans et al. 1998) that
isolated populations.
Zulliger and Lessios (2010) published a global phylogeny of the genus
Astropecten based on 117 specimens belonging to 40 species. This genus is one of
the most species-rich genera among starfish containing over 150 described species
with six species recorded from Panama: A. armatus Gray, 1840, A. articulatus (Say,
1825), A. cingulatus Sladen, 1883, A. exiguus Ludwig, 1905, A. fragilis Verrill,
1870 and A. regalis Gray, 1840. Such diversity is remarkable, because most species
of Astropecten have a long-lived planktotrophic larval stage. Consequently one
would expect this to lead to a low speciation rate (Zulliger and Lessios 2010).
Zulliger and Lessios (2010) compared their molecular phylogeny to Döderlein’s
(1917) morphological phylogeny. They found high levels of congruence on the
whole, but many discrepancies on a local scale. Phylogentic inference from their
data reveals that that morphological and ecological convergence has taken place in
Astropecten, resulting in allopatric non-sister taxa with similar morphologies and
habitat preferences. Varieties of several species were also shown to exhibit genetic
distances large enough to justify recognizing them as separate species.
Reviews on speciation in organisms separated by the Isthmus of Panama have
been written by Lessios (1998, 2008). Lessios (1998) wrote a chapter in ‘‘Endless
Forms: Species and Speciation’’ on the first stage of speciation as seen in organisms separated by the Isthmus of Panama. Lessios points out that although geminate species have had a long history of contributing evidence relevant to
speciation, they remain an underutilised tool for understanding vicariant speciation. Lessios’s (2008) review showed that a total of 38 regions of DNA have been
sequenced in nine clades of echinoids, 38 clades of crustaceans, 42 clades of fishes,
and 26 clades of molluscs with amphi-isthmian subclades. Of these, 34 clades are
likely to have been separated at the final stages of Isthmus completion, 73 clades
split earlier and eight clades maintained post-closure genetic contact (e.g. via a
circumglobal route or through the Panama Canal). As no vicariant event is better
dated than that of the Isthmus of Panama, molecular divergence between species
pairs remains to be of great interest, particularly as adaptive divergence can be
seen in species life history parameters.
4 Echinoderm Diversity in Panama
127
Molecular work on the starfish A. planci, including specimens from Panama,
revealed that it is not in fact a single species, but a pan-Indo-Pacific species
complex consisting of four deeply diverged clades (Pacific, Red Sea, Northern
Indian Ocean, and Southern Indian Ocean) (Vogler et al. 2008). These clades
reportedly diverged between 1.95 and 3.65 mya (Pliocene to early Pleistocene)
and have genetic distances (8.8–10.6 %) equivalent to the distances between other
sibling species of starfish (Waters et al. 2004). Vogler et al. (2008) propose that
this speciation process was driven by sea level changes (Pillans et al. 1998) that
isolated populations.
Zulliger and Lessios (2010) published a global phylogeny of the genus
Astropecten based on 117 specimens belonging to 40 species. This genus is one of
the most species-rich genera among starfish containing over 150 described species
with six species recorded from Panama: A. armatus Gray, 1840, A. articulatus (Say,
1825), A. cingulatus Sladen, 1883, A. exiguus Ludwig, 1905, A. fragilis Verrill,
1870 and A. regalis Gray, 1840. Such diversity is remarkable, because most species
of Astropecten have a long-lived planktotrophic larval stage. Consequently one
would expect this to lead to a low speciation rate (Zulliger and Lessios 2010).
Zulliger and Lessios (2010) compared their molecular phylogeny to Döderlein’s
(1917) morphological phylogeny. They found high levels of congruence on the
whole, but many discrepancies on a local scale. Phylogentic inference from their
data reveals that that morphological and ecological convergence has taken place in
Astropecten, resulting in allopatric non-sister taxa with similar morphologies and
habitat preferences. Varieties of several species were also shown to exhibit genetic
distances large enough to justify recognizing them as separate species.
Reviews on speciation in organisms separated by the Isthmus of Panama have
been written by Lessios (1998, 2008). Lessios (1998) wrote a chapter in ‘‘Endless
Forms: Species and Speciation’’ on the first stage of speciation as seen in organisms separated by the Isthmus of Panama. Lessios points out that although geminate species have had a long history of contributing evidence relevant to
speciation, they remain an underutilised tool for understanding vicariant speciation. Lessios’s (2008) review showed that a total of 38 regions of DNA have been
sequenced in nine clades of echinoids, 38 clades of crustaceans, 42 clades of fishes,
and 26 clades of molluscs with amphi-isthmian subclades. Of these, 34 clades are
likely to have been separated at the final stages of Isthmus completion, 73 clades
split earlier and eight clades maintained post-closure genetic contact (e.g. via a
circumglobal route or through the Panama Canal). As no vicariant event is better
dated than that of the Isthmus of Panama, molecular divergence between species
pairs remains to be of great interest, particularly as adaptive divergence can be
seen in species life history parameters.
4 Echinoderm Diversity in Panama
127