244
The Biology of Sea Turtles, Vol. II
Except for the loggerhead ( Caretta caretta ), little progress has been made in our
understanding of the early juvenile stage beyond what was summarized in Musick and
Limpus (1997) and Bjorndal (1997). Recent studies on the early life stages of the
loggerhead sea turtle have improved our knowledge of the biology of the oceanic
juvenile stage (for a review, see Bolten, in press; Bjorndal et al., 2000a, in review).
In this chapter, three generalized sea turtle life history patterns are identified
and evaluated with respect to phylogenetic relationships and reproductive traits.
Characteristics of the developmental stages (oceanic vs. neritic) and adult foraging
stage (oceanic vs. neritic) are the primary differences that distinguish the three
patterns. These variations are reviewed, the consequences of oceanic vs. neritic
developmental stages are discussed, and finally, speculation about how these differences may have evolved is presented. The dramatic decline in sea turtle populations
and the extensive degradation of their ecosystems make it difficult to determine the
functional roles of sea turtles in their ecosystems (see Chapter 10), and therefore,
it is difficult to evaluate the selective factors that led to the present-day sea turtle
life history patterns. It is particularly difficult, in the context of massive faunal
declines and food web alterations, to speculate on the relative importance of the
evolutionary pressures from competition for resources and predation that may have
resulted in these observed life history patterns.
9.2 TERMINOLOGY
The terminology used to describe the life histories of sea turtles has been inconsistent
for both the oceanographic terms and the developmental stages (Bolten, in press).
To be consistent with standard oceanographic terminology, the following terms
should be used (see Lalli and Parsons [1993] for review):
• The neritic zone describes the inshore marine environment (from the
surface to the sea floor) where water depths do not exceed 200 m. The
neritic zone generally includes the continental shelf, but in areas where
the continental shelf is very narrow or nonexistent, the neritic zone conventionally extends to areas where water depths are less than 200 m.
• The oceanic zone is the vast open ocean environment (from the surface
to the sea floor) where water depths are greater than 200 m.
• Organisms are pelagic if they occupy the water column, but not the sea
floor, in either the neritic zone or oceanic zone. Organisms are epipelagic
if they occupy the upper 200 m in the oceanic zone.
• Organisms on the sea floor in either the neritic zone or oceanic zone are
described as benthic or demersal.
Organisms can therefore be pelagic in shallow neritic waters or in the deep
oceanic waters. Similarly, organisms can be benthic in shallow neritic waters as well
as in the deep ocean. We should describe sea turtle life stages by the oceanic realm
that they inhabit. Therefore, the early juvenile stage found in the open ocean should
be described as the oceanic stage, not the pelagic stage, and the juvenile stage found
in coastal waters as the neritic stage, not the benthic stage.
1123 book.book Page 244 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
Except for the loggerhead ( Caretta caretta ), little progress has been made in our
understanding of the early juvenile stage beyond what was summarized in Musick and
Limpus (1997) and Bjorndal (1997). Recent studies on the early life stages of the
loggerhead sea turtle have improved our knowledge of the biology of the oceanic
juvenile stage (for a review, see Bolten, in press; Bjorndal et al., 2000a, in review).
In this chapter, three generalized sea turtle life history patterns are identified
and evaluated with respect to phylogenetic relationships and reproductive traits.
Characteristics of the developmental stages (oceanic vs. neritic) and adult foraging
stage (oceanic vs. neritic) are the primary differences that distinguish the three
patterns. These variations are reviewed, the consequences of oceanic vs. neritic
developmental stages are discussed, and finally, speculation about how these differences may have evolved is presented. The dramatic decline in sea turtle populations
and the extensive degradation of their ecosystems make it difficult to determine the
functional roles of sea turtles in their ecosystems (see Chapter 10), and therefore,
it is difficult to evaluate the selective factors that led to the present-day sea turtle
life history patterns. It is particularly difficult, in the context of massive faunal
declines and food web alterations, to speculate on the relative importance of the
evolutionary pressures from competition for resources and predation that may have
resulted in these observed life history patterns.
9.2 TERMINOLOGY
The terminology used to describe the life histories of sea turtles has been inconsistent
for both the oceanographic terms and the developmental stages (Bolten, in press).
To be consistent with standard oceanographic terminology, the following terms
should be used (see Lalli and Parsons [1993] for review):
• The neritic zone describes the inshore marine environment (from the
surface to the sea floor) where water depths do not exceed 200 m. The
neritic zone generally includes the continental shelf, but in areas where
the continental shelf is very narrow or nonexistent, the neritic zone conventionally extends to areas where water depths are less than 200 m.
• The oceanic zone is the vast open ocean environment (from the surface
to the sea floor) where water depths are greater than 200 m.
• Organisms are pelagic if they occupy the water column, but not the sea
floor, in either the neritic zone or oceanic zone. Organisms are epipelagic
if they occupy the upper 200 m in the oceanic zone.
• Organisms on the sea floor in either the neritic zone or oceanic zone are
described as benthic or demersal.
Organisms can therefore be pelagic in shallow neritic waters or in the deep
oceanic waters. Similarly, organisms can be benthic in shallow neritic waters as well
as in the deep ocean. We should describe sea turtle life stages by the oceanic realm
that they inhabit. Therefore, the early juvenile stage found in the open ocean should
be described as the oceanic stage, not the pelagic stage, and the juvenile stage found
in coastal waters as the neritic stage, not the benthic stage.
1123 book.book Page 244 Monday, November 11, 2002 11:11 AM