Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 516

rainfall and drainage from the continent affect both the salinity and the position
of the water table. As a result of the complex hydrological conditions of tidal
flats, a number of behavioral strategies are adopted for protection. For example,
the amphipod Corophium is a good osmoregulator that can tolerate salinities
between 2

. On the other hand, inhabiting a burrow or taking tem-
porary refuge in the sediment provides effective protection against strong salin-
ity variations ( Sanders et al., 1965 ). Also, horizontal migration represents a
strategy to minimize the dramatic salinity shifts in the upper intertidal zone.
In general, salinity tolerance controls the zonal distribution of intertidal ani-
mals, with euryhaline species being particularly abundant in the upper intertidal
zone ( Newell, 1979 ).
The hydrodynamic energy in intertidal zones, which results from the local
interplay of tides, waves, and currents, controls sediment mobility and bedform
development. High-energy zones of tidal flats are commonly characterized by
low-diversity assemblages of suspension feeders or passive predators repre-
sented, such as Diplocraterion or Skolithos (e.g., Cornish, 1986; M ´ ngano
and Buatois, 2004a; Simpson, 1991 ). In moderate- to low-energy coastal set-
tings, ichnodiversity increases and assemblages tend to be dominated by hori-
zontal traces of deposit feeders, detritus feeders, and grazers (e.g., M ´ ngano
et al., 2002a ). Scouring on the tidal flat, resulting from either tides or storms,
is conducive to erosional truncation of burrows, as illustrated by specimens
of Lockeia forming composite surfaces and revealing successive colonization
events by a bivalve fauna ( M´ngano et al., 1998 ).
The time of exposure is particularly significant on tidal flats, becoming a
limiting factor particularly in the upper intertidal zone ( M´ngano et al.,
2002a ). Many organisms have developed biological rhythms (e.g., quasi-tidal,
quasi-semilunar rhythms) of vertical or horizontal migration, controlled by
tidal cyclicity in order to cope with the stress associated with subaerial expo-
sure ( Palmer, 1995 ). Some species such as the crab Sesarma reticulatum hide
in their burrows during low tide and are active during high tide ( Palmer,
1967; Seiple, 1981 ). The isopod Eurydice pulchra lives buried in the sand
flat during emersion but rises into the water column with the flood tide to
swim at the water's edge in order to feed on epifauna, infauna, and detritus.
During the ebb tide, it retreats seaward and reburies itself for protection
( Warman et al., 1991 ). In general, marine invertebrate surface activity on
the tidal flat is typically more intense during high tide, whereas a relatively
large number of semiterrestrial and terrestrial animals (e.g., many terrestrial
crabs and the intertidal beetle Thalassotrechus barbarae ) may display an
activity peak during low-tide emersion ( Palmer, 1995; Pie ´ kowski, 1983;
Vader, 1964 ).
Tidal flats commonly experience rapid changes in temperature as a result of
periodical subaerial exposure, which represents a limiting factor particularly,
but not exclusively, in the upper intertidal zone ( M ´ ngano et al., 2002a ). In trop-
ical environments, the upper intertidal zone is extremely inhospitable for

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