68
E. Jaramillo and M. Lastra
3.4 Tidal Movements and Burrowing Behavior
It has long been known that large suspension feeders of sandy beaches (e. g.
Emerita, Donax) move up and down the shore with tidal variation (Branch
and Branch 1993). However, not much is known about the effects of different
beach types upon that sort of movement. Jaramillo et al. (2000) compared
tidal variability of E. analoga between a dissipative and a reflective beach of
southern Chile (ca. 42° S) and found that the movement of this species was
greater at the dissipative site. Along a tidal cycle that included two lows and
one high tide {ca. 12h) the population mode of Emerita moved up and down
following the limits of the swash zone as it moved up and down the beach. In
contrast, in the reflective site, most of the animals remained below the lowest
swash level through the tidal cycle suggesting that the width of the swash zone
may affect the extent of tidal movement in this suspension feeder. In other
words, this supports the swash exclusion hypothesis (McLachlan et al. 1993)
in the sense that crabs remained below the effluent line on that reflective
beach.
Exposed sandy beaches are coastal habitats dominated by sediment instability and strong hydrodynamic forces. One of the main adaptative mechanisms to cope with these factors is burrowing behavior; i. e., in exposed
beaches organisms must burrow fast enough to avoid drifting in strong waves
and currents. Burrowing rates of sandy beach organisms are usually affected
by body size of organisms (see, e.g., McLachlan et al. 1995). Jaramillo et al.
(2000) measured burrowing rates of E. analoga on both a dissipative and a
reflective beach in southern Chile and found that crabs burrowed at similar
rates in sediments from both beaches, which suggests that this crab is a
sediment generalist (cf. Alexander et al. 1993). Laboratory experiments using
Donax trunculus from Spain (M. Lastra, unpubl. data) showed ontogenic
changes in clams burrowing in different sediment sizes. Those results show
that individuals measuring 5-25 mm in shell length burrow faster in medium
and fine sands, while bigger animals burrow faster in fine and very fine sands.
These results suggest that small (juvenile) clams are able to cope with more
reflective morpho dynamic conditions (e.g., coarser sands) than adults. To
determine whether this trend represents a plastic response of the individuals
that inhabit changing sedimentary environments, further studies are needed.
3.5 Across- and Along-Shore Zonation
The patchy distribution of sandy beach organisms has been mainly studied in
bivalves {e.g. Sastre 1985; Defeo et al. 1986; Jaramillo et al. 1994; Lastra and
E. Jaramillo and M. Lastra
3.4 Tidal Movements and Burrowing Behavior
It has long been known that large suspension feeders of sandy beaches (e. g.
Emerita, Donax) move up and down the shore with tidal variation (Branch
and Branch 1993). However, not much is known about the effects of different
beach types upon that sort of movement. Jaramillo et al. (2000) compared
tidal variability of E. analoga between a dissipative and a reflective beach of
southern Chile (ca. 42° S) and found that the movement of this species was
greater at the dissipative site. Along a tidal cycle that included two lows and
one high tide {ca. 12h) the population mode of Emerita moved up and down
following the limits of the swash zone as it moved up and down the beach. In
contrast, in the reflective site, most of the animals remained below the lowest
swash level through the tidal cycle suggesting that the width of the swash zone
may affect the extent of tidal movement in this suspension feeder. In other
words, this supports the swash exclusion hypothesis (McLachlan et al. 1993)
in the sense that crabs remained below the effluent line on that reflective
beach.
Exposed sandy beaches are coastal habitats dominated by sediment instability and strong hydrodynamic forces. One of the main adaptative mechanisms to cope with these factors is burrowing behavior; i. e., in exposed
beaches organisms must burrow fast enough to avoid drifting in strong waves
and currents. Burrowing rates of sandy beach organisms are usually affected
by body size of organisms (see, e.g., McLachlan et al. 1995). Jaramillo et al.
(2000) measured burrowing rates of E. analoga on both a dissipative and a
reflective beach in southern Chile and found that crabs burrowed at similar
rates in sediments from both beaches, which suggests that this crab is a
sediment generalist (cf. Alexander et al. 1993). Laboratory experiments using
Donax trunculus from Spain (M. Lastra, unpubl. data) showed ontogenic
changes in clams burrowing in different sediment sizes. Those results show
that individuals measuring 5-25 mm in shell length burrow faster in medium
and fine sands, while bigger animals burrow faster in fine and very fine sands.
These results suggest that small (juvenile) clams are able to cope with more
reflective morpho dynamic conditions (e.g., coarser sands) than adults. To
determine whether this trend represents a plastic response of the individuals
that inhabit changing sedimentary environments, further studies are needed.
3.5 Across- and Along-Shore Zonation
The patchy distribution of sandy beach organisms has been mainly studied in
bivalves {e.g. Sastre 1985; Defeo et al. 1986; Jaramillo et al. 1994; Lastra and