PROTECTION FROM WHAT?
Life in False Bay presents a special
challenge to local organisms because, as water drains out, sediment and
the animals it contains are exposed. Protection must be thought of in two
different ways. Organisms must protect themselves from both changes
in the physical environment and other organisms. Although these are distinct
problems, the common solution to predation and physical pressures is living
within the sediment (infaunally). The problems of protection faced by the
organisms at False Bay provide an interesting contrast to those at Cattle
Point, which is a high-energy, rocky intertidal habitat.
PHYSICAL ENVIRONMENT
During the summer, False Bay usually
empties once during the day at low tide. Exposure to bright sun for several
hours can increase the surface temperatures
of standing water and sand
significantly. Many organisms survive these fluctuations by finding
a way to avoid them. Infaunal organisms take advantage of the insulation
provided by the sand. By living well below the
sediment surface, they are able to maintain a more constant
temperature regime, protected from the heat of the sun. One
can test this easily without a thermometer. Simply
reaching a finger into the burrow of a bivalve, mud shrimp, ghost shrimp,
or polychaete on a sunny day will reveal water that
is significantly cooler than the water standing in the tide pools.
A corollary of temperature stress is the problem of desiccation due to heat and exposure during low tide. Desiccation is also avoided by burrowing. One easy way to demonstrate this is to walk over and around many of the burrows in False Bay. Near the north end of the bay, bivalve burrows will spout water when you compress them. Chaetopterid tubes also spout water, presumably as they move due to the disturbance of footsteps. Since the bottom of False Bay is flat, the water table is extremely high. Even during low tide, the sand is saturated with water within a few centimeters of the surface, allowing burrowing animals to remain immersed.
Non-burrowing (epifaunal) organisms,
such as crabs and fishes, avoid desiccation in a different way. They
solve this problem by staying in tide pools and channels
during the low tide. Although this provides protection
from desiccation, it does not provide protection from the warm temperatures
achieved by tide pools exposed to continuous sunlight.
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Protection from predators is also necessary for many of the organisms in False Bay. One particularly striking example of the need for protection emerged from our fieldwork in the bay. Digging in the shrimp habitat, we disturbed several mud shrimp (Upogebia) and ghost shrimp (Neotrypaea) from their burrows. Although in pools of water, the animals had not all begun burrowing when we left the habitat. When we returned less than two hours later, several had been eaten, leaving only pieces of carapace behind. It is likely that they were eaten by local birds, such as crows or gulls. Although these shrimp live for as many as ten years in their burrows (Kozloff, 1996), once exposed at the surface they are extremely vulnerable to predators. This experience provides two messages. First, burrowing is an essential protective strategy for Upogebia and Neotrypaea. Second, care should be taken to disturb as few individuals as possible, because a trip to the surface may spell death for members of these species.
A few organisms even take advantage of the burrows and tubes of other organisms in order to find protection. Orobitella rugifera is a small bivalve that lives attached to the underside of the abdomen of mud shrimp (Upogebia). In this way it is able to take advantage of the burrowing power of the shrimp. It can live well below the surface of the sediment (and any surface predators) without exposing its siphon, but while obtaining sufficient water flow with which to feed. This bivalve is remarkable for the number of modes of protection it has. It lives in a burrow well below the sediment surface, it is protected by the shrimp by virtue of living on its abdomen, and it has the protection of its two calcified valves, even if the shrimp falls victim to a predator.
Pinnixa
tubicola is a small brachyuran crab found in the tubes of Terebellid
worms (spaghetti worms) in the Ulva area. Like Orobitella,
Pinnixa takes advantage of the camouflage provided by living within
the Terebellid tube, as well as
its own exoskeleton.
TRADEOFFS
It may seem that protection from predators
and protection from the physical environment go hand in hand at False Bay.
There may, however, exist tradeoffs within this life strategy. Some species,
such as Chaetopterus, live
in tubes formed of mucous, while others, such as Upogebia, live
in burrows whose sides are formed primarily of sediment. It is easy to
imagine that tube dwelling
organisms may have more control over their immediate
environment than mere burrowers, due to slightly greater isolation from
the surrounding conditions. Why donāt all infaunal animals live in tubes,
then? A partial answer may come from the observations of Woodin
(1972). She determined that the crab Cancer
magister selectively targets as prey tube-dwelling species with
tubes exposed at the surface in preference to burrowing species with only
burrow holes exposed at the surface. One might imagine that it is easier
to pull up a tube than to dig into a burrow. Thus, although tubes may provide
an organism with more active control over the conditions within which it
lives, mere burrowing may be a better deterrent to epifaunal predators.
When considering the protection provided by living within the sediment,
one must also consider the relative needs for protection from predators
and protection from fluctuations in the physical environment.
CONCLUSION
A walk around False Bay during low tide will provide ample evidence of the number of burrowing organisms. This infaunal lifestyle provides protection against both predators and fluctuations in the physical environment. Careful observation demonstrates that the burrows are not all the same, and that different types occur in different areas. The type of burrows present may reflect the relative pressures of predation and physical stresses. These burrows are not merely a convenient way to avoid the footsteps of human visitors; they are an essential part of survival in a bay subject to rapid and extreme changes in temperature, water level, and numerous predatory organisms.
The problems of protection at False
Bay provide an interesting contrast to those at Cattle
Point, Argyle Lagoon, on
the FHL dock floats,
and in the local plankton.