III. Beach Zonation
See Figure 1
1. Environmental influences
The upper intertidal zone is the most terrestrial of the beach environments.
It exhibits the widest range of heat and cold. Thus, desiccation is a significant
problem for marine organisms in the summer, and freezing is a problem in
the winter. Fresh water, both surface water from rainfall and subsurface
flow, can impact populations. Because of the limited inundation time, marine
food resources are limited. The high intertidal fauna consists mostly of
scavengers, which rely on "islands" of wrack material for sustenance.
The wrack communities go through successional stages, being colonized first
by highly motile talitrids and flies, with later colonization by terrestrial
isopods and beetles.
2. Dominant species
Talitrid amphipods (see Bousfield 1957, Bowers 1964, Craig 1973)
In the high intertidal, the dominant marine organisms, i.e. organisms which
rely on the ocean for survival, are talitrid amphipod crustaceans. Commonly
known as 'beach hoppers' or 'sand fleas', they are highly motile animals
which can either crawl or hop along the sand surface. They are well modified
for the high intertidal zone, having gills that function almost as lungs.
They only enter the ocean occasionally to re-wet their gill cavities, and
indeed will drown if submerged in sea water.
Talitrids are generally nocturnal, both to reduce desiccation and limit
predation. They leave their shelter at night and migrate down the beach
searching for food. They are preyed upon by shorebirds and staphylinid beetles,
as well as unexpected predators such as foxes, raccoons, and burrowing owls.
While juveniles generally live beneath wrack, adults dig burrows to depths
of over 3 feet, which are guarded by males. Competition over females can
lead to territorial fights between males, after which amplexis takes place
in the winners burrow. Females brood their eggs in an internal brood pouch,
releasing fully active juveniles.
Much work has been done on orientation in talitrids, which seem to rely
on subtle beach gradients and landmarks to navigate on wide beaches. They
are non-specific scavengers, ingesting plant and animal material, as well
as human debris such as paper. The two most common talitrid species are Megalorchestia (formerly Orchestoidea) corniculata and
M. californiana. M. californiana
is generally found on exposed beaches of medium and fine sand backed
by dunes. M. corniculata
is generally found on more sheltered pocket beaches with coarser sand. Less commonly found is M.
benediction, a smaller species found on transient pocket beaches with
course sand, and several others species are occasionally seen (M. columbine,
M. pugetensis, Traskorchestia traskiana).
Because of their highly patchy distribution, it is difficult to get accurate
estimates of talitrid abundance, though they can be quite abundant (several
hundred/m²) in the early fall.
Insects and kin
Beach wrack is inhabited by a wide variety of insect and other arthropod
species. Coleopteran beetles and flies (Diptera) are the most abundant,
with 35 and 11 species respectively being found in one study. The larvae
of the flies feed on kelp and other beach wrack, while many of the beetles
are predaceous. Other groups include mites, spiders, pseudoscorpions, centipedes,
isopod crustaceans, hymenopterids (wasps), and orthopterids (Lavoie 1984).
Oligochaetes
The only strictly infaunal organisms to inhabit the upper intertidal are
several species of oligochaetes. They can be found in large numbers, and
are somewhat tidally influenced (Locy 1981).
B. Mid zone
1. Environmental influences
The mid-littoral zone is characterized by a moderate inundation time, but
is subject to many of the same rigors as the upper zone (temperature extremes,
fresh water). The mid zone is more subject to rapid sediment removal during
winter storms, requiring extreme mobility of its fauna.
2. Dominant species
Excirolana spp. (see Enright 1965, Klapow 1972, Johnson 1976, Oakden
and Nybakken 1977)
The dominant, defining species of the mid-littoral zone is the cirolanid
isopod, Excirolana. Excirolana is a carnivorous scavenger,
subsisting in Monterey Bay mostly on dead Emerita and Emerita
eggs, but vigorously attacking any animal material encountered. They
have been used by astute scientists to clean skeletons for laboratory use, and
will attack bathers' feet if left stationary for too long (pers. obs.).
Excirolana burrows into the top 1 cm of sediment at low tide, and
leaves the sand to swim rapidly about looking for food when inundated. This
rhythmic activity pattern persists for several days even after animals are
removed to laboratory aquaria. Females brood their young in a brood pouch.
The upper limit of their distribution is probably determined by having sufficient
inundation time for feeding, while the lower limit may be determined by
the mechanical stress of wave action. They are preyed upon by various shorebirds.
The species generally encountered on exposed sandy beaches is Excirolana
linguifrons. They grow to a length of about 1 cm, and reach abundances
of over 800/m2. A larger (to about 1.5 cm), less common species is Excirolana
chiltoni, generally found on more sheltered beaches with coarser sediment.
Another cirolanid, Cirolana hardordi is found in large numbers in
sand patches in rocky intertidal zones.
C. Swash zone
1. Environmental influences
The swash zone, where wave breaking and runup often occurs, is characterized
by the highest water movement, so consequently has a high degree of mechanical
stress and possibility of rapid sediment removal. It remains inundated over
half the time, and so is less subject to extremes of temperature and salinity.
2. Dominant species
Sand crab (Emerita analoga) (see Efford 1965, Efford 1966, Dugan et al. 1991,
Myers et al. 1980)
The dominant species in the swash zone is the hippid crab, Emerita analoga,
commonly known as the sand crab. Emerita is a filter feeder, facing
out to sea with only eyes and short first antennae exposed as a wave passes,
then filtering the receding waves through its feathery second antenna, which
it passes through its mouth to remove food particles. Sand crabs must be
highly active, moving and reburying themselves frequently, to maintain their
location on the beach.
Sand crabs often form dense aggregations on beaches. These aggregations
move up and down and along the beach in response to tidal changes, and may
help to increase feeding efficiency or reduce predation. Females are generally
larger than males, with a large (3.5 cm) female carrying in excess of 20,000
bright orange eggs. Larvae are broadcast into the water, where the planktonic
zoea stay for 3-6 months. Because of the long residence time in the plankton,
there is probably some intermingling of the Pacific coast populations of
Emerita.
Emerita, as the only widespread herbivore on the beach, serves as the
basis for much of the sandy intertidal food web. At high tide it is preyed
upon by several fishes, including surfperch (Embioticidae) and croakers (Scianidae). Numerous studies
have demonstrated its importance as a food source for shorebirds, including
plovers, sandpipers, sanderlings, and willets (e.g. see Pitelka 1978, Estelle
1991). When dead, it serves as the bulk of the diet of Excirolana and
the mole crab Blepharipoda. It is also is preyed upon by humans:
4,100 kg (roughly 2 million individuals) were taken for bait in southern
California in 1967 (Morris et al.1980).
Archaeomysis (see Oakden and Nybakken 1977, Ricketts et al.
1968)
The 2 cm glass shrimp, Archaeomysis grebnitzkii, is a mysid which
inhabits the swash zone and lower intertidal. They burrow into the top 1
cm of the sand, and emerge at high tide to feed on zooplankton.
Nephtys (see Clark 1962, Clark and Haderlie 1962)
The errant polychaete Nephtys californiensis is a ubiquitous but
sparse (4-8/m2) resident of the swash zone and lower intertidal. It is found
on moderately exposed beaches with clean, well-sorted sand. Nephtys can
get quite large, to 30 cm, but individuals over 15 cm are rare. Nephtys
is generally carnivorous on other infaunal animals, and is found in
the top 5 cm of sediment.
D. Low intertidal zone
See Figure 1
1. Environmental influences
The low intertidal zone, extending into the shallow subtidal, is almost
fully marine, being aerially exposed for short periods only on the lowest
tides. The low intertidal is subject to almost constant wave action, requiring
that the inhabitants be either rapid burrowers (e.g. Blepharipoda)
or protected against mechanical damage (e.g. Tivella). It houses
the most diverse faunal assemblage of the beach zones.
2. Dominant species
Pismo Clam (Tivela stultorum) (see Coe and Fitch 1950, Morris et al. 1980,
Stephenson 1977).
Until the early 1970's, the pismo clam, Tivella stultorum, was the
most characteristic species in the lower intertidal of the central California coast. There are records of
350,000 clams/year being harvested from central California in the 1930s;
plows were used to rake up clams at low tide. In the early 1970's, two factors
combined to eliminate the clam as a human food resource. First, overfishing
occurred, and was not solved by the institution of bag limits and a minimum
legal size of 4.5". However, legal-size clams were still occasionally
found on the beach in the early 70's, largely due to a huge breeding pool
of shallow-subtidal adults which were beyond the reach of clammers. However,
in 1972-73 the first wave of a resurgent sea otter population reached central
Monterey Bay, and around the same time reached Pismo Beach. Within a few
years the otters had consumed the subtidal clams. Now, while it is possible
to find occasional small pismo clams on the beach, generally the only sign
of adult clams are shells washed up on the beach exhibiting the characteristic
diagonal break of otter predation.
Pismo clams can reach a length of over 15 cm, and inhabit shallow burrows
with their hinge facing seaward. They are filter feeders, with a medium-sized
clam capable of filtering 60 liters of water/day. They have a very heavy
shell, both to provide protection from breaking waves, and to give them
sufficient weight to burrow rapidly.
Spiny Mole Crab (Blepharipoda)
The spiny mole crab, Blepharipoda occidentalis, inhabits the low
intertidal zone, generally on beaches with Emerita populations. The
mole crab reaches a length of 6 cm, and can burrow very rapidly. They are
scavengers, subsisting mostly on dead Emerita, and are preyed on
by surfperch (Embioticidae), and in deeper water, by bat rays (Myliobatis
californica). Because of their rapid burrowing and the difficulty of
working in the surf zone, they are difficult to accurately sample, but are
found in densities of several dozen/m².
Other Fauna
A variety of other fauna are found in the lower zone of sandy beaches. Table
2, from Moss Landing beach, is a representative list of the fauna found
on exposed beaches throughout the MBNMS. Other species occasionally found
on MBNMS beaches include the bean clam (Donax gouldii), the Northern
razor clam (Siliqua patula), moon snails (Polinices lewisi),
the polychaete Euzonus (3 spp.) on more sheltered beaches, 2 cumaceans
(Anchicolorous occidentalis, Diastylopsis tenuis), and several
other species of beach hoppers (Megalorchestes columbiana, M. puggetensis,
M. benedicti, Traskorchestia traskiana).
E. Transient organisms
Sand dollar (Dendraster excentricus) (see Birkeland and Chia 1971)
(also see Shallow Soft Bottom Habitats section)
Although not strictly an intertidal organism, the sand dollar Dendraster
excentricus is very commonly found washed up on MBNMS beaches, and in
sheltered beach environments outside the MBNMS does inhabit the intertidal
zone. Live sand dollars are gray, brown or purple, and covered with short
(2 mm) spines. The white tests of dead animals show the distinctive aboral
"petal" shapes, caused by the ambulacral (feeding) grooves.
Dendraster are found from the shallow subtidal to depths of 30 m,
but on open MBNMS coastlines generally have a center of distribution at
around 7-10m. They form dense beds (more than 625/m²), with animals on
edge protruding at an angle from the sand, aboral sides facing into the
current. The beds occur off sandy beaches along much of the MBNMS, and are
particularly abundant in Monterey Bay.
Dendraster are filter feeders, with food particles being trapped
by the spines on their surface and conveyed to their mouths. They spawn
into the water, with the eggs having a coating which seems to prevent them
from being eaten by the adults. Larvae are eaten, however, which means that
most recruitment takes place at the edge of beds, with individuals migrating
towards the center as they mature. They generally have successful recruitment
only every few years, leading to distinct size classes in beds. They are
fed on by bottom-feeding fish, particularly the starry flounder, and by
seastars such as Pisaster brevispinus and Pycnopodia healianthoides.
Velella (see Morris et al. 1980).
(also see Pelagic Zone section).
Velella velella is a pelagic Cnidarian (jellyfish) which generally
occurs floating on the surface in offshore waters worldwide. They have a
sail which is angled at 45° to their main body axis, either to the
right or left, so that animals drift to right or left of the true wind direction.
Right-handed Velellapredominate off California, meaning that the
prevailing northerly winds tend to hold the population offshore. However,
prolonged westerly or southerly winds, often found in the spring, can drive
them up on beaches in huge numbers. On beaches they don't seem to be fed
upon by the resident scavengers, but just dry up and blow away (pers. obs.).
Velella was once thought to be a siphonophore, related to the Portuguese
Man-o-war, but is now known to be a hydrozoan. They reach a length of 8
cm and are bright blue when alive, but turn transparent in death. They have
short tentacles, which hang into the water and capture plankton.
Next-Section IV. Selected Sandy Beach
Resources
Sandy Beaches Table of Contents
