markedly, from homogeneous dark brown during the day (Fig. 6.3b, left) to banded
gray and black at night (Fig. 6.3b, right). Another conspicuous behavioral response
to light is negative phototaxis: O. wendtii is able to detect shadows and quickly
escape from predators into dark crevices, of which they are able to identify from
several centimeters away. The latter reaction is particularly unexpected in these
animals as the behavior is usually associated with the presence of discrete
photosensory organs. No specialized eyes have, however, been documented in
brittlestars and their reactions to light have been linked to diffuse dermal receptors.
The sensitivity to light seems to correlate with the specialized skeletal structure
of the dorsal arm plates (DAPs). These ossicles protect the upper part of each joint
in brittlestar arms (Fig. 6.3c). Skeletal elements of echinoderms are each composed
of a single crystal of oriented calcite shaped into a unique, three-dimensional mesh
(stereom). The diameter of the typical stereom in the DAPs of Ophiocoma is about
10–15 mm (Fig. 6.3d). In O. wendtii as well as in other photosensitive species, the
outer surface of the DAP stereom bears a characteristic array of enlarged spherical
structures 40–50 mm in diameter (Fig. 6.3d, f). In cross section, they have a
remarkably regular double-lens shape (Fig. 6.3g). The optical axis of the constituent
calcite is oriented parallel to the lens axis and perpendicular to the plate surface.
The mean geometry of the lenses was inferred from the measurements of lens
diameter (L) and thickness (t) in 20 random lenses sectioned through the center
(Fig. 6.3g).
The absence of such structure on the DAPs in various relatively light-insensitive
species, such as O. pumila (Fig. 6.3a, e) raises the possibility of the direct involvement of calcitic microlenses in photoreception. Calcitic microlenses were used by
the trilobites. The presence of transparent regions of compact stereom has been
reported also for sea stars and sea urchins. Aizenberg et al. (2001) proposed that
these calcitic microstructures might have a function in directing and focusing the
light on photosensitive tissues. On the basis of the results, they suggest that the
array of calcitic microlenses with their unique focusing effect and underlying neural
receptors may form a specialized photoreceptor system with a conceivable compound-eye capability.
The demonstrated use of calcite by brittlestars, both as an optical element and as
a mechanical support, illustrates the remarkable ability of organisms, through the
process of evolution, to optimize one material for several functions, and provides
new ideas for the fabrication of smart materials.
6.1.1.2 Vaterite
Calcium carbonate, in its three non-hydrated crystalline polymorphs calcite, aragonite and vaterite, represents one of the most important inorganic materials with
respect to the biomineralization processes in organisms. Of the three modifications,
calcite is thermodynamically the most stable one, followed by aragonite and then
vaterite, which is the least stable one. Vaterite is highly unstable, its crystal system
is hexagonal, P63/mmc. When exposed to water, it can recrystallize to calcite
148
Q. Feng
gray and black at night (Fig. 6.3b, right). Another conspicuous behavioral response
to light is negative phototaxis: O. wendtii is able to detect shadows and quickly
escape from predators into dark crevices, of which they are able to identify from
several centimeters away. The latter reaction is particularly unexpected in these
animals as the behavior is usually associated with the presence of discrete
photosensory organs. No specialized eyes have, however, been documented in
brittlestars and their reactions to light have been linked to diffuse dermal receptors.
The sensitivity to light seems to correlate with the specialized skeletal structure
of the dorsal arm plates (DAPs). These ossicles protect the upper part of each joint
in brittlestar arms (Fig. 6.3c). Skeletal elements of echinoderms are each composed
of a single crystal of oriented calcite shaped into a unique, three-dimensional mesh
(stereom). The diameter of the typical stereom in the DAPs of Ophiocoma is about
10–15 mm (Fig. 6.3d). In O. wendtii as well as in other photosensitive species, the
outer surface of the DAP stereom bears a characteristic array of enlarged spherical
structures 40–50 mm in diameter (Fig. 6.3d, f). In cross section, they have a
remarkably regular double-lens shape (Fig. 6.3g). The optical axis of the constituent
calcite is oriented parallel to the lens axis and perpendicular to the plate surface.
The mean geometry of the lenses was inferred from the measurements of lens
diameter (L) and thickness (t) in 20 random lenses sectioned through the center
(Fig. 6.3g).
The absence of such structure on the DAPs in various relatively light-insensitive
species, such as O. pumila (Fig. 6.3a, e) raises the possibility of the direct involvement of calcitic microlenses in photoreception. Calcitic microlenses were used by
the trilobites. The presence of transparent regions of compact stereom has been
reported also for sea stars and sea urchins. Aizenberg et al. (2001) proposed that
these calcitic microstructures might have a function in directing and focusing the
light on photosensitive tissues. On the basis of the results, they suggest that the
array of calcitic microlenses with their unique focusing effect and underlying neural
receptors may form a specialized photoreceptor system with a conceivable compound-eye capability.
The demonstrated use of calcite by brittlestars, both as an optical element and as
a mechanical support, illustrates the remarkable ability of organisms, through the
process of evolution, to optimize one material for several functions, and provides
new ideas for the fabrication of smart materials.
6.1.1.2 Vaterite
Calcium carbonate, in its three non-hydrated crystalline polymorphs calcite, aragonite and vaterite, represents one of the most important inorganic materials with
respect to the biomineralization processes in organisms. Of the three modifications,
calcite is thermodynamically the most stable one, followed by aragonite and then
vaterite, which is the least stable one. Vaterite is highly unstable, its crystal system
is hexagonal, P63/mmc. When exposed to water, it can recrystallize to calcite
148
Q. Feng