A Closer Look: In the Salt Marsh
A
Story and photographs by Jacob Zadik
salt marsh or saltmarsh, also known as a
coastal salt marsh or a tidal marsh, is a coastal
ecosystem in the upper coastal intertidal zone
between land and open salt water or brackish
water that is regularly flooded by the tides.
People who have visited Kiawah’s marsh for any length
of time know that the saltmarsh is never constant. In the
saltmarsh, the tides from the Atlantic Ocean ebb and flow
into a vast, open system dramatically changing the marsh’s
appearance from one hour to the next. Change is even more
apparent under the water. During high tide, the water floods
its grassy embankments and nutrients are raked out of the
water column. It is also in these grasses that small fish and
crustaceans take refuge from the marine predators that enter
the marsh from the ocean. The oyster beds are also open and
begin filtering out planktonic matter from the water at an
impressive rate. The flow of ocean water into the saltmarsh
on an incoming tide brings life and nourishment to this
incredibly rich and biodiverse ecosystem. As the tide ebbs,
everything changes and reveals what the ocean has nourished.
Imagine you are in the middle of the Kiawah River,
perhaps sitting in a kayak, feeling the full force of the tides. As
the water lowers, your view becomes constrained. The now-
towering grasses perched atop the sloping mud banks make
it challenging to see far out into the flat plains of the high
marsh. On average, Kiawah’s water level fluctuates six feet
between high and low tide. So now, during low tide, you are
required to focus on your immediate surroundings, looking
closer, and using other senses. It is during this ebbing tide
you are likely to hear snapping and popping noises emanating
from the mud beneath the grasses. This noise is created by
a small crustacean known locally as a snapping shrimp and
more scientifically known as the big-clawed snapping shrimp
(Alpheus heterochaelis). This one- to two-inch-long shrimp
is named for its one large claw that produces detectable
popping noises in the low tide marsh. The shrimp shuts its
big claw with such force that it creates a high-speed water jet,
a cavitation bubble, and sparks for a split second. The energy
released from this motion is comparable to the heat produced
on the surface of the sun. This amazing feat is used to deter
predators, stun prey, and as a display for potential mates—
everything necessary for the shrimp’s survival.
If the snapping noise enticed you to paddle over to the
banks of the river, you will likely encounter another large-
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clawed crustacean, the fiddler crab (Uca sp.). Three species
of this crustacean live in and around Kiawah’s marsh, and all
have a similar body structure. Males have one tremendously
large claw and one significantly smaller claw. Females, on the
other hand … or claw, are equipped with two proportionally
sized claws that match the size of the male’s smaller claw.
Each male’s large claw can constitute up to 60 percent of its
body mass, so there is no doubt this anatomical feature is
important to this group of crabs—otherwise, why spend
the time and energy to lug it around?
So what’s with the claw? Why do male fiddler crabs sport
such a massive appendage? Because they are detritivores—a
fancy way of saying they eat organic matter collected on
and in the mud—they do not need such a large claw to
gather food. The fiddler crabs do not need to hunt. Their
small claw is all they use to sift through the substrate of
our estuary. Their big claw is too large to be effective in
warding off predators. In truth, it is eye-catching, attracting
predators that will often take advantage of the male’s claw
size, snatching them from the ground by their large claw!
Fortunately, fiddler crabs will readily use their ability to
detach their claw and escape alive. When this happens,
fiddler crabs will switch from right- to left-clawed (or vice
versa). The small claw that is left behind during the predator
scuffle will begin to grow and replace the lost giant claw. And
where the original large claw once sat, the crab will begin to
regenerate a small claw.
All of this trouble caused by having a predator-attracting
claw and regrowing it when it is taken should demonstrate
that it is important to be sure you have a large claw in fiddler
crab world. As we know by now, it is not useful for eating
or self-protection, but it IS useful for getting oneself a mate.
Males can signal to potential mates or potential competitors,
providing necessary mating information to both, with
their large claw. The size of the claw certainly indicates the
possessor’s health. Some researchers suggest it also shows
how often he has survived a predator attack; other research
demonstrates that it indicates the size of the male’s burrow.
Both pieces of information would be important to a female
crab looking for a suitable mate. Ability to survive an attack
is pretty straightforward, but why would a female need to
know the size of the male’s burrow? Because this is where she
lays her eggs, and size is directly proportional to the burrow’s
incubation temperature.
Naturally Kiawah