Roles of Sea Turtles in Marine Ecosystems: Reconstructing the Past
267
in the Caribbean is 13,500. Estimates of sex ratio have ranged from male biased to
female-biased (León and Diez, 1999), so if we assume a 1:1 sex ratio, the estimated
total number of adult hawksbills in the Caribbean today is 27,000.
Sponges are abundant on modern Caribbean coral reefs, where their biomass
and diversity often exceed that of corals (Goreau and Hartman, 1963; Rützler, 1978;
Suchanek et al., 1983; Targett and Schmahl, 1984). Hawksbills in the Caribbean
feed primarily on a relatively few species of sponges, although they also consume
other invertebrates (Bjorndal, 1997; León and Bjorndal, in press). As the largest
sponge predator, how much sponge biomass would an adult hawksbill consume
annually? Unfortunately, there are no data on intake or digestion of sponges in
hawksbills. We can derive a rough estimate, however, if we assume that the digestible
energy intake of hawksbills would lie between those of the green turtle (an herbivore)
and the loggerhead (a carnivore that feeds on invertebrates with fewer antiquality
components than sponges) (Bjorndal, 1997).
The energy intake of green turtles feeding on T. testudinum can be estimated
by multiplying the average annual intake from Table 10.2 (1.23 kg T. testudinum
dry mass per kg turtle) by the energy content of grazed T. testudinum blades (14,000
kJ/kg dry mass [Bjorndal, 1980]), which equals 17,220 kJ/kg turtle each year. To
estimate digestible energy intake, this value is multiplied by the energy digestibility
coefficient for a diet of T. testudinum (60% for adults [Bjorndal, 1980]), which
yields an estimate of 10,332 kJ/kg green turtle each year. For loggerheads, an annual
energy intake of a highly digestible, balanced diet was estimated to be 13,140 kJ/kg
turtle (Bjorndal, in press). With an estimate of 90% energy digestibility for the
high-quality diet, our estimate of annual digestible energy intake for loggerheads
is 11,826 kJ/kg turtle.
Therefore, a very rough estimate of annual digestible energy intake for a
hawksbill would be 11,000 kJ/kg. To convert this estimate to the biomass of sponges
consumed annually by an adult hawksbill, we will use the sponge Chondrilla nucula
as the prey species because it is the best studied of the sponges in terms of
composition and digestibility, and is a major prey species of hawksbills. Chondrilla
nucula was consumed by hawksbills in seven of the eight studies of hawksbill diet
in the Caribbean and, in most cases, made a major contribution to the diet (summarized in León and Bjorndal, in press). In the only study of selective feeding in
hawksbills, there was strong selection for C. nucula (León and Bjorndal, in press).
Because C. nucula has high energy, organic matter, and nitrogen content relative
to most sponge species consumed by hawksbills (León and Bjorndal, in press),
intake values for hawksbills estimated for a diet of C. nucula will be conservative.
The average mass of an adult hawksbill is 70 kg (Witzell, 1983), the energy content
for C. nucula is 15,900 kJ/kg dry mass (Bjorndal, 1990), and we will use a range
of energy digestibility coefficients of 43–90%. The low value in this range is based
on a value of 43.4% energy digestibility of C. nucula measured in green turtles
(Bjorndal, 1990). Digestibility should be higher in hawksbills because they feed
primarily on sponges. The upper estimate (90%) is near the upper limit of digestibilities of animal tissue measured in reptiles (Zimmerman and Tracy, 1989). The
resulting estimate of sponge consumed by an adult hawksbill each year is 54–113
kg dry mass [(11,000 ¥ 70)/(15,900 ¥ 0.90) or (11,000 ¥ 70)/(15,900 ¥ 0.43)].
1123 book.book Page 267 Monday, November 11, 2002 11:11 AM
267
in the Caribbean is 13,500. Estimates of sex ratio have ranged from male biased to
female-biased (León and Diez, 1999), so if we assume a 1:1 sex ratio, the estimated
total number of adult hawksbills in the Caribbean today is 27,000.
Sponges are abundant on modern Caribbean coral reefs, where their biomass
and diversity often exceed that of corals (Goreau and Hartman, 1963; Rützler, 1978;
Suchanek et al., 1983; Targett and Schmahl, 1984). Hawksbills in the Caribbean
feed primarily on a relatively few species of sponges, although they also consume
other invertebrates (Bjorndal, 1997; León and Bjorndal, in press). As the largest
sponge predator, how much sponge biomass would an adult hawksbill consume
annually? Unfortunately, there are no data on intake or digestion of sponges in
hawksbills. We can derive a rough estimate, however, if we assume that the digestible
energy intake of hawksbills would lie between those of the green turtle (an herbivore)
and the loggerhead (a carnivore that feeds on invertebrates with fewer antiquality
components than sponges) (Bjorndal, 1997).
The energy intake of green turtles feeding on T. testudinum can be estimated
by multiplying the average annual intake from Table 10.2 (1.23 kg T. testudinum
dry mass per kg turtle) by the energy content of grazed T. testudinum blades (14,000
kJ/kg dry mass [Bjorndal, 1980]), which equals 17,220 kJ/kg turtle each year. To
estimate digestible energy intake, this value is multiplied by the energy digestibility
coefficient for a diet of T. testudinum (60% for adults [Bjorndal, 1980]), which
yields an estimate of 10,332 kJ/kg green turtle each year. For loggerheads, an annual
energy intake of a highly digestible, balanced diet was estimated to be 13,140 kJ/kg
turtle (Bjorndal, in press). With an estimate of 90% energy digestibility for the
high-quality diet, our estimate of annual digestible energy intake for loggerheads
is 11,826 kJ/kg turtle.
Therefore, a very rough estimate of annual digestible energy intake for a
hawksbill would be 11,000 kJ/kg. To convert this estimate to the biomass of sponges
consumed annually by an adult hawksbill, we will use the sponge Chondrilla nucula
as the prey species because it is the best studied of the sponges in terms of
composition and digestibility, and is a major prey species of hawksbills. Chondrilla
nucula was consumed by hawksbills in seven of the eight studies of hawksbill diet
in the Caribbean and, in most cases, made a major contribution to the diet (summarized in León and Bjorndal, in press). In the only study of selective feeding in
hawksbills, there was strong selection for C. nucula (León and Bjorndal, in press).
Because C. nucula has high energy, organic matter, and nitrogen content relative
to most sponge species consumed by hawksbills (León and Bjorndal, in press),
intake values for hawksbills estimated for a diet of C. nucula will be conservative.
The average mass of an adult hawksbill is 70 kg (Witzell, 1983), the energy content
for C. nucula is 15,900 kJ/kg dry mass (Bjorndal, 1990), and we will use a range
of energy digestibility coefficients of 43–90%. The low value in this range is based
on a value of 43.4% energy digestibility of C. nucula measured in green turtles
(Bjorndal, 1990). Digestibility should be higher in hawksbills because they feed
primarily on sponges. The upper estimate (90%) is near the upper limit of digestibilities of animal tissue measured in reptiles (Zimmerman and Tracy, 1989). The
resulting estimate of sponge consumed by an adult hawksbill each year is 54–113
kg dry mass [(11,000 ¥ 70)/(15,900 ¥ 0.90) or (11,000 ¥ 70)/(15,900 ¥ 0.43)].
1123 book.book Page 267 Monday, November 11, 2002 11:11 AM