In clypeasteroid sand dollars, the modified ambulacra, or petals, are among the
most conspicuous bilaterally paired structures. Lawrence et al. (1998) studied the
bilateral symmetry of the petals in the Ecuadorian sand dollar Encope micropora
L. Agassiz, 1841 versus others two species of sand dollars (considering large and
small specimens): (1) Mellita tenuis collected at Mullet Key, Florida, USA
(Scutellina, same sub-order of E. micropora); and (2) Arachnoides placenta collected at South Mission Beach, Queensland, Australia (Clypeasterina, different
sub-order of E. micropora). Analysis of the anterior and posterior petals of
E. micropora suggested they were asymmetrical. This fluctuating degree of
asymmetry for some petal pairs might be a means to evaluate environmental stress.
Knowing the type of asymmetry is necessary to understand its origin, because
certain environmental conditions and forces can affect test form in sea urchins
(Ellers and Telford 1992; Ellers 1993).
Other Sea Urchins
Distribution and Density
Pinela and Calvache (1997) studied the distribution (Morisita index), and density
of Echinometra vanbrunti in four different localities along the southwest coast of
Ecuador. They found that this sea urchin had an aggregated distribution at all sites,
and that its densities ranged from 0.87 to 8.80 ind m
-2 . The abundance of this
species along mainland Ecuador is remarkably higher than in the Galápagos.
Brandt and Guarderas (2002) reported that E. vanbrunti presented densities of
0.3 ind m
-2 in the Galápagos Archipelago. In addition, Wellington (1975)
reported that E. vanbrunti was abundant in the southern, central (including the
eastern side of Isabela Island) and northern regions (with 4.38 ind m
-2 ), but rare
in Fernandina and on the western side of Isabela Island where Caenocentrotus
gibbosus (Fig. 6.4h) was the dominant species. Three other sea urchins (D. mexicanum, see Fig. 6.3e; Toxopneustes roseus, see Fig. 6.3f; and Centrostephanus
coronatus) were also reported by Brandt and Guarderas (2002). However their
abundances were significantly much lower (\0.01 ind m
-2 ) than the other sea
urchin species found there. General findings of the Galápagos Marine Biodiversity
Baseline, including density patterns of sea urchins, were also published by Edgar
et al. (2004). Lawrence and Sonnenholzner (2004) documented the occurrence of
eight species of echinoids at seven sites with distinct habitats in central Galápagos.
The most diverse assemblage was at Itabaca Channel with E. galapagensis,
L. semituberculatus, T. depressus, T. roseus, C. coronatus, E. vanbrunti, C. gibbosus, and D. mexicanum. All had densities \2.3 ind m
-2 . The assemblages of
species are similar to those reported for the Florida Keys and the Hawaiian Islands
(Kier and Grant 1965; Ebert 1971).
206
J. Sonnenholzner et al.
most conspicuous bilaterally paired structures. Lawrence et al. (1998) studied the
bilateral symmetry of the petals in the Ecuadorian sand dollar Encope micropora
L. Agassiz, 1841 versus others two species of sand dollars (considering large and
small specimens): (1) Mellita tenuis collected at Mullet Key, Florida, USA
(Scutellina, same sub-order of E. micropora); and (2) Arachnoides placenta collected at South Mission Beach, Queensland, Australia (Clypeasterina, different
sub-order of E. micropora). Analysis of the anterior and posterior petals of
E. micropora suggested they were asymmetrical. This fluctuating degree of
asymmetry for some petal pairs might be a means to evaluate environmental stress.
Knowing the type of asymmetry is necessary to understand its origin, because
certain environmental conditions and forces can affect test form in sea urchins
(Ellers and Telford 1992; Ellers 1993).
Other Sea Urchins
Distribution and Density
Pinela and Calvache (1997) studied the distribution (Morisita index), and density
of Echinometra vanbrunti in four different localities along the southwest coast of
Ecuador. They found that this sea urchin had an aggregated distribution at all sites,
and that its densities ranged from 0.87 to 8.80 ind m
-2 . The abundance of this
species along mainland Ecuador is remarkably higher than in the Galápagos.
Brandt and Guarderas (2002) reported that E. vanbrunti presented densities of
0.3 ind m
-2 in the Galápagos Archipelago. In addition, Wellington (1975)
reported that E. vanbrunti was abundant in the southern, central (including the
eastern side of Isabela Island) and northern regions (with 4.38 ind m
-2 ), but rare
in Fernandina and on the western side of Isabela Island where Caenocentrotus
gibbosus (Fig. 6.4h) was the dominant species. Three other sea urchins (D. mexicanum, see Fig. 6.3e; Toxopneustes roseus, see Fig. 6.3f; and Centrostephanus
coronatus) were also reported by Brandt and Guarderas (2002). However their
abundances were significantly much lower (\0.01 ind m
-2 ) than the other sea
urchin species found there. General findings of the Galápagos Marine Biodiversity
Baseline, including density patterns of sea urchins, were also published by Edgar
et al. (2004). Lawrence and Sonnenholzner (2004) documented the occurrence of
eight species of echinoids at seven sites with distinct habitats in central Galápagos.
The most diverse assemblage was at Itabaca Channel with E. galapagensis,
L. semituberculatus, T. depressus, T. roseus, C. coronatus, E. vanbrunti, C. gibbosus, and D. mexicanum. All had densities \2.3 ind m
-2 . The assemblages of
species are similar to those reported for the Florida Keys and the Hawaiian Islands
(Kier and Grant 1965; Ebert 1971).
206
J. Sonnenholzner et al.