Bolten -- 1 Bolten, A.B. 2003. Variation in sea turtle life history patterns: neritic vs. oceanic developmental stages. Pages 243-257 in P.L. Lutz, J. Musick and J. Wyneken (editors), The Biology of Sea Turtles, volume II. CRC Press, Boca Raton, FL CHAPTER 9 Variation in Sea Turtle Life History Patterns: Neritic versus Oceanic Developmental Stages Alan B. Bolten Please address all correspondence to: Alan B. Bolten Archie Carr Center for Sea Turtle Research Department of Zoology PO Box 118525 University of Florida Gainesville, FL 32611 USA Tel: 352-392-5194 Fax: 352-392-9166 Email: [email protected]
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Bolten -- 1
Bolten, A.B. 2003. Variation in sea turtle life history patterns: neritic vs. oceanic developmental stages. Pages 243-257 in P.L. Lutz, J. Musick and J. Wyneken (editors), The Biology of Sea Turtles, volume II. CRC Press, Boca Raton, FL
CHAPTER 9
Variation in Sea Turtle Life History Patterns:
Neritic versus Oceanic Developmental Stages
Alan B. Bolten
Please address all correspondence to:
Alan B. Bolten Archie Carr Center for Sea Turtle Research Department of Zoology PO Box 118525 University of Florida Gainesville, FL 32611 USA Tel: 352-392-5194 Fax: 352-392-9166 Email: [email protected]
hatchling size, clutch frequency, and remigration interval) and
developed a dendrogram illustrating the relationships among the
species (Figure 9.3, right dendrogram). The Van Buskirk and
Crowder (1994) dendrogram is not congruent with the phylogeny of
Bowen and Karl (1997) but has a greater similarity to the 3 life
history patterns presented in Figure 9.1. This better fit would
be expected between life history patterns (Figure 9.3, center)
and reproductive behavior and demographic traits (Figure 9.3,
right dendrogram). The two species with the greatest difference
in reproductive traits (leatherbacks and flatbacks) are also the
two species with the greatest difference in life history
patterns.
9.5 A CLOSER LOOK AT THE TYPE 2 PATTERN – ONTOGENETIC HABITAT
SHIFTS
Species exhibiting either the Type 1 or Type 3 pattern
commit to either the neritic or oceanic zone, respectively, for
their entire developmental stages as well as for the adult
foraging stage. Only turtles with the Type 2 pattern have a
major habitat change during their development. The Type 2
Bolten -- 16
pattern is the most successful pattern if success is defined by
the number of species with this life history pattern (5 of the 7
extant species exhibit the Type 2 pattern). As presented above
in section 9.4.1, the Type 1 pattern is hypothesized to be the
ancestral pattern that still exists today (although presumably
secondarily derived) in the Australian flatback. Why post-
hatchling turtles leave the neritic zone for the oceanic, and
why, after an extended development period in the oceanic zone,
the turtles return to the neritic to complete their development,
are two intriguing questions.
The existence of an early developmental stage in the
oceanic habitat may be a result of higher predator pressure in
neritic habitats and/or intra- and interspecific competition for
food in neritic habitats. Such competition may not be apparent
now because of depleted sea turtle populations, but evidence for
density-dependent effects on growth rates has been reported for
a population of green turtles in a neritic foraging habitat
(Bjorndal et al. 2000b).
Even more puzzling is the shift from oceanic to neritic
habitats. Why do juvenile turtles leave the oceanic zone where
they have spent the first years of their lives successfully
finding food, growing, and surviving? When they leave the
oceanic zone for the neritic zone, they enter a new habitat with
which they are unfamiliar, and must learn to find new food
Bolten -- 17
sources and avoid a new suite of predators. A current
hypothesis to explain why ontogenetic habitat shifts occur is
that a species shifts habitats to maximize growth rates (Werner
and Gilliam 1984). Bolten (in press) presents evidence for the
Atlantic loggerhead population that supports the Werner and
Gilliam hypothesis. The extrapolation of the size-specific
growth function for the oceanic stage intersects the size-
specific growth function for the neritic stage (Figure 9.4).
Therefore, for a given carapace length greater than
approximately 64 cm (a size by which almost all of the
loggerheads have left the oceanic zone [Bjorndal et al. 2000a]),
growth rates will be greater in the neritic zone than in the
oceanic zone. Snover et al. (2000) present data from a
skeletochronology study that also demonstrate increased growth
rates of turtles that have left the oceanic zone and entered the
neritic zone.
There is substantial variation among the species that have
the Type 2 pattern, suggesting a fair amount of variation in
lifestyles. Morphological differences in oceanic-stage turtles
include variation in countershading (suggesting different
predator avoidance behaviors) and front flipper length relative
to body length (suggesting differences in swimming activity and
resulting feeding behavior). The swimming behavior of post-
hatchling green turtles appears to be different than that of
Bolten -- 18
loggerheads (Wyneken 1997). Another source of variation is the
duration of the oceanic developmental stage that may be
significantly different for the different species based on the
size at which they recruit to neritic habitats (see section
9.3.2). In addition, resource partitioning along temperature
gradients and among foraging strategies probably occurs among
Type 2 species, but data are lacking. For example, loggerheads
(Type 2 species) and olive ridleys (Type 3 species) apparently
partition resources in the Pacific by water temperature;
loggerheads inhabit cooler waters than do olive ridleys (Pitman
1993; Polovina et al., in review). Thus, as more is learned
about this life stage, further divisions in the Type 2 pattern
may be required.
9.6 ANTHROPOGENIC IMPACTS ON THE EARLY JUVENILE STAGE
The future is bleak. Early juvenile-stage sea turtles face
a myriad of obstacles irrespective of whether they are in the
neritic or oceanic zones. Directed take of very small turtles
for food is not common. However, directed take for the souvenir
trade in polished shells or whole stuffed turtles, such as the
once-popular but now illegal tourist trade in Madeira, Portugal
(Brongersma 1982), still exists in some regions.
Indirect take in fisheries, whether it is the high seas
drift nets, longlines, or coastal trawlers, is a very serious
Bolten -- 19
problem for juvenile turtles (National Research Council 1990;
Wetherall et al. 1993; Balazs and Pooley 1994; Witzell 1999;
Bolten et al. 2000). Throughout the world’s oceans, the size
distribution of loggerhead turtles caught in longline fisheries
is the largest size class for the oceanic development stage
(Bolten et al. 1994; Ferreira et al. 2001; Bolten, in press),
which has significant demographic consequences (Crouse et al.
1987; Heppell et al., in press).
The lethal and sub-lethal effects of debris ingestion and
entanglement are also major concerns (Balazs 1985; Carr 1987b;
McCauley and Bjorndal 1999; Witherington, in review b). Habitat
loss, particularly in coastal areas, has been documented;
habitat degradation in the oceanic zone is more difficult to
document but nonetheless acute when the effects of pollution are
considered (Lutcavage et al. 1997). Both oceanic and neritic
ecosystems are changing as a result of overfishing and
pollution. Changes to the suite of species interactions and
food webs in these ecosystems are undoubtedly having a major
negative impact on sea turtles.
9.7 CONCLUSIONS – RESEARCH DIRECTIONS
Our ability to solve the “mystery of the lost year” for
those species for which the early juvenile stages remain unknown
has been improved by the development of new research tools.
Bolten -- 20
Biotechnology is providing molecular tags to identify
populations and track movements; biotelemetry is allowing
researchers to evaluate movement and distribution patterns.
Stable isotopes may provide clues where to look for early
juvenile stages and also provide information on their trophic
relationships. For researchers to make rapid progress in the
study of early juvenile stages, multidisciplinary teams should
be developed with expertise in the fields of physical and
biological oceanography, population genetics, statistical
modeling, demography, nutrition, and ecosystem analyses.
9.8 ACKNOWLEDGEMENTS
My research on the oceanic juvenile stage has focused on
the loggerhead population in the Atlantic and has been conducted
in collaboration with Karen Bjorndal whose inspiration and
enthusiasm has made this project possible. Our research has
been funded by the US National Marine Fisheries Service and the
Disney Wildlife Conservation Fund. Colleagues at the Department
of Oceanography and Fisheries at the University of the Azores,
especially Helen Martins and “equipa tartaruga,” have not only
made our research possible but most enjoyable. I would also
like to thank Karen Bjorndal, Jeffrey Seminoff, and Jack Musick
for their comments on earlier drafts of this manuscript.
Bolten -- 21
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Bolten -- 27
Figure Legends
Figure 9.1: Three distinct sea turtle life history patterns
illustrating the sequence of ecosystems
inhabited. See text for a description of each
type.
Figure 9.2: Life history diagram (modified from Bolten, in
press) of the Atlantic loggerhead sea turtle.
Boxes represent life stages and the corresponding
ecosystems. Solid lines represent movements
between life stages and ecosystems; dotted lines
are speculative.
Figure 9.3: The relationships of the three types of life
history patterns (see Figure 9.1) with a
phylogeny based on mtDNA (left dendrogram of
figure; modified from Bowen and Karl 1997) and a
dendrogram of reproductive traits (right side of
figure, modified from Van Buskirk and Crowder
1994). The branching of the phylogenetic and
reproductive trait dendrograms are not to scale.
The triangle indicates the Type 1 life history
pattern, the circles indicate those species
Bolten -- 28
exhibiting the Type 2 pattern, and the squares
indicate those species exhibiting the Type 3
pattern. The olive ridley (Lo) is diagrammed
with both a circle (Type 2) to represent West
Atlantic and Australian populations and a square
(Type 3) to represent East Pacific populations
(see text for discussion). Cc = Caretta caretta,
Cm = Chelonia mydas, Dc = Dermochelys coriacea,
Ei = Eretmochelys imbricata, Lk = Lepidochelys
kempi, Lo = Lepidochelys olivacea, and Nd =
Natator depressus.
Figure 9.4: Size-specific growth functions of oceanic-stage