31
35 km per hour. Plankton feeding species like leatherback turtles ( D. coriacea ), spend
entire years in the ocean. Being the paradigmatic, skillful oceanic navigators, they
are known to return to specifi c places to breed every 2–3 years. Intriguingly, these
turtles, fi nding their way back home after long periods in the oceanic environment
(Sale and Luschi 2009 ).
Recently, it was proposed that “sea turtles imprint on the magnetic fi eld of their
natal areas and later use this information to direct natal homing,” (Lohmann et al. 2008 ).
The majority of marine species are predominantly carnivorous. However, for
example, the green sea turtle changes to a vegetarian diet at the end of the juvenile
stage. Some marine turtles are omnivorous. The hawksbill ( Eretmochelys imbricata ),
an endangered marine turtle distributed in the coral reef regions in Caribbean, feeds
almost exclusively on sponges, even though they contain a huge mass of very sharp
siliceous spicules (Meylan 1988 ). Since marine invertebrates and plants are generally
similar in salt content and contain three times as much salt/kg body water as sea
turtles (Holmes and McBean 1964 ), the salt burden for feeding turtles can be
considerable. Therefore, sea turtles use their tear ducts to excrete salts. We can observe
this “crying” when the animal is out of the water. The need for salt excretion in such
reptiles as sea turtles and marine iguanas is determined by the fact that their kidneys
are much less effi cient than those of mammals (see for review Ellis and Abel 1964 ;
Marshall 1989 ; Reina et al. 2002 ). Unlike the skin of amphibians, reptile skin is
impermeable to NaCl. The transition to a tougher skin meant a loss in salt-releasing
ability (see for details Peaker and Linzell 1975 ). Calculations suggest that the
sea turtle salt gland has a high volume-handling capacity, equivalent to that of the
mammalian kidney (Nicolson and Lutz 1989 ).
It is proposed that, the turtle “provides an ideal case study for understanding
changes in the developmental program associated with the morphological evolution
of vertebrates,” (Kuratani et al. 2011 ). In contrast to armored constructs observed in
other vertebrate groups, the turtle shell exhibits very special topography of musculoskeletal elements as well as involves the developmental re-patterning of the axial
skeleton (Gilbert et al. 2001 ). The carapace (dorsal part of shell) is joined at the sides
to the plastron (ventral part). This is in turn notched at the front and rear ends where
the limbs emerge from the shell. The scutes on a turtle’s carapace and plastron are the
equivalent of scales on other reptiles. However, the body plan of the turtle represents
“an example of evolutionary novelty in the acquisition of the shell,” (Kuratani et al.
2011 ). The epidermis that covers the body shell of turtles forms these structures.
When new layers of dermis are deposited over the bone, old epidermal cells are
pushed towards the outside. As these epidermal cells die, they are keratinized and
form the scutes. A depression forms between old, keratinized layers; and new layers
of epidermis are called scute annuli (Wilson et al. 2003 ). Counting the number of
scute annuli on the carapace or plastron of a turtle is a common way of determining
the age of turtles (see for review Germano and Bury 1998 ; Ergene et al. 2011 ).
Turtles lack teeth, and therefore their shearing surfaces are formed via a keratinized beak, the rhamphotheca. The rhamphotheci are the structural elements of both
the upper and lower jaws of cheloniids. They cover the premaxillary, maxillary, and
vomer bones of the upper jaw, and the dentary of the lower jaw. The upper and lower
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview
35 km per hour. Plankton feeding species like leatherback turtles ( D. coriacea ), spend
entire years in the ocean. Being the paradigmatic, skillful oceanic navigators, they
are known to return to specifi c places to breed every 2–3 years. Intriguingly, these
turtles, fi nding their way back home after long periods in the oceanic environment
(Sale and Luschi 2009 ).
Recently, it was proposed that “sea turtles imprint on the magnetic fi eld of their
natal areas and later use this information to direct natal homing,” (Lohmann et al. 2008 ).
The majority of marine species are predominantly carnivorous. However, for
example, the green sea turtle changes to a vegetarian diet at the end of the juvenile
stage. Some marine turtles are omnivorous. The hawksbill ( Eretmochelys imbricata ),
an endangered marine turtle distributed in the coral reef regions in Caribbean, feeds
almost exclusively on sponges, even though they contain a huge mass of very sharp
siliceous spicules (Meylan 1988 ). Since marine invertebrates and plants are generally
similar in salt content and contain three times as much salt/kg body water as sea
turtles (Holmes and McBean 1964 ), the salt burden for feeding turtles can be
considerable. Therefore, sea turtles use their tear ducts to excrete salts. We can observe
this “crying” when the animal is out of the water. The need for salt excretion in such
reptiles as sea turtles and marine iguanas is determined by the fact that their kidneys
are much less effi cient than those of mammals (see for review Ellis and Abel 1964 ;
Marshall 1989 ; Reina et al. 2002 ). Unlike the skin of amphibians, reptile skin is
impermeable to NaCl. The transition to a tougher skin meant a loss in salt-releasing
ability (see for details Peaker and Linzell 1975 ). Calculations suggest that the
sea turtle salt gland has a high volume-handling capacity, equivalent to that of the
mammalian kidney (Nicolson and Lutz 1989 ).
It is proposed that, the turtle “provides an ideal case study for understanding
changes in the developmental program associated with the morphological evolution
of vertebrates,” (Kuratani et al. 2011 ). In contrast to armored constructs observed in
other vertebrate groups, the turtle shell exhibits very special topography of musculoskeletal elements as well as involves the developmental re-patterning of the axial
skeleton (Gilbert et al. 2001 ). The carapace (dorsal part of shell) is joined at the sides
to the plastron (ventral part). This is in turn notched at the front and rear ends where
the limbs emerge from the shell. The scutes on a turtle’s carapace and plastron are the
equivalent of scales on other reptiles. However, the body plan of the turtle represents
“an example of evolutionary novelty in the acquisition of the shell,” (Kuratani et al.
2011 ). The epidermis that covers the body shell of turtles forms these structures.
When new layers of dermis are deposited over the bone, old epidermal cells are
pushed towards the outside. As these epidermal cells die, they are keratinized and
form the scutes. A depression forms between old, keratinized layers; and new layers
of epidermis are called scute annuli (Wilson et al. 2003 ). Counting the number of
scute annuli on the carapace or plastron of a turtle is a common way of determining
the age of turtles (see for review Germano and Bury 1998 ; Ergene et al. 2011 ).
Turtles lack teeth, and therefore their shearing surfaces are formed via a keratinized beak, the rhamphotheca. The rhamphotheci are the structural elements of both
the upper and lower jaws of cheloniids. They cover the premaxillary, maxillary, and
vomer bones of the upper jaw, and the dentary of the lower jaw. The upper and lower
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview