COMPARISON OF THE COMMUNITY STRUCTURES IN
A PACIFIC AND AN ATLANTIC PANAMANIAN
SANDY BEACH
DEBORAH
M.
DEXTER
Department of Zoology, San Diego State College, San Diego, California 92115
ABSTRACT
The macroscopic infaunas of two panamanian sandy beaches, Shimmey
Beach on the Atlantic coast and Naos Island beach on the Pacific coast,
were examined during the summer of 1969. The Pacific beach, compared
with the Atlantic beach, had 6 times the density of individuals, 9 times
the biomass, and 3 times as many species. In majority, the faunas were
composed of closely related species, which showed similar patterns of
zonation. The most abundant organism, Cirolana mayana, occurred on
both beaches. Other abundant organisms included Ancinus spp., nerinid
polychaetes, Paraonides, and Cyclaspis.
INTRODUCTION
The purpose of this study was to make a preliminary description of two
Panamanian sandy-beach communities, one on the Pacific coast, and one
on the Atlantic coast. The objective was to establish species composition,
abundance, zonation, distributional patterns, and community structure.
Quantitative sampling of macroscopic invertebrates in sandy-beach communities in the Americas has been rather limited. The most comprehensive
study of any sandy-beach community is that by Pamatmat (1968) who investigated productivity and metabolism of the benthic community at San
Juan Island, Washington. Quantitative investigations have been made on
both epifaunal (Fager, 1968) and infaunal (Barnard, 1963; O'Gower &
Wacasey, 1967; Barnard, 1970) subtidal sand habitats. Recently several
intertidal sand-beach communities have been analyzed quantitatively along
the Atlantic coast in Massachusetts (Grant, 1965), North Carolina (Dexter, 1969), and Georgia (Croker, 1967).
Studies of tropical sandy beaches are even more limited. Dahl (1952)
described worldwide zonation patterns of boreal, temperate, and tropical
beaches, characterizing each by three or four species of larger invertebrates,
predominantly Crustacea. Previous studies on American tropical beaches
are those of Rodriguez (1959) in Venezuela and Wade (1967) at Jamaica.
No accounts of Pacific tropical sand fauna appear in the literature.
Physically, the Pacific coastline of Panama is more variable than the
Atlantic coast. On the Pacific side, an 18° temperature range (14°-32°C)
occurs as the result of upwelling of colder and more saline water between
January and April; the Atlantic fauna is exposed to only a 6° range in
450
Bulletin
of Marine
[22(2)
Science
ArraiJan
Flamenco I.
PI.
f)
PACIFIC
o
Scole
FIGURE
1.
I: 100,000
OCEAN
•
kilometers
Location of Pacific-coast
sampling area at Naos Island Beach.
temperature (24°-30°C) and to lower salinities as a result of heavier rainfall (Rubinofl', 1968). On the Pacific coast the maximum tidal range is
6 m compared with a maximum range of 0.5 m on the Atlantic Panamanian
coast. Martin et at. (1970) characterized the Pacific side as having 3-5
times the benthic biomass and 2-5 times the primary productivity of the
Atlantic side.
METHODS
The beaches chosen as study sites were located at Naos Island (Fig. 1)
near the Pacific marine laboratory of the Smithsonian Tropical Research
Institute, and at Shimmey Beach (Fig. 2) located in Fort Sherman on the
Atlantic coast. Both beaches were sampled during June, July, and August
1969.
A systematic sampling design was used. The beaches were divided into
strata of 10 m (Naos) and 2 m (Shimmey). Repetitive samples, 1 m apart,
were taken at each stratum; a total of 90 samples were collected at Naos
Dexter: Community Structures of Sandy Beaches
1972]
451
2
FIGURE
2.
Location
of Atlantic-coast
sampling
area
at
Shimmey
Beach.
and 80 samples at Shimmey Beach. Sampling procedure consisted of
placing a 0.I-m2 quadrat over the sand at low tide, removing sand to a
depth of 5 cm, and sieving it through a 500J.!.-meshnet. Organisms were
separated
from the sand residue
by careful
sorting
and by floatation
in a
dense sugar solution. Wet weights were obtained on formalin-preserved
specimens. Sediment particle-size was analyzed with the use of an Emery
settling tube (Emery, 1938).
RESULTS
The Pacific beach at Naos Island was composed of quartz sand; 80 per
cent of this substrate was fine sand (125J.!.-250J.!.), 19 per cent medium
sand (250J.!.-500/-L), and 1 per cent calcareous shell fragments. Eighty meters
of intertidal beach were exposed during a spring tide of -0.37 m. Shimmey
Beach, Atlantic coast, was composed of calcareous sand, of which 82 per
cent was fine grained and 18 per cent medium grained. Fourteen meters
of beach were exposed during a spring tide of -0.15 m.
452
Bulletin of Marine Science
[22(2)
The general structure of the sandy-beach communities at Naos Island
and Shimmey Beach is given in Table 1. The faunas of the two beaches
are similar and closely related. Two isopods, Cirolana mayana and
Exosphaeroma diminutum, are found on both beaches and comprise 31
per cent (by number) of the Atlantic fauna and 85 per cent of the Pacific
fauna. In addition, 59 per cent of the Atlantic fauna has cognate species
in the Pacific fauna belonging to the same genus. These cognates include
the following genera: Ancinus, Cyclaspis, Dispio, Donax, and Emerita.
Organisms belonging to genera not found on the other beach account for
only 10 per cent of the Atlantic fauna and 7 per cent of the Pacific fauna.
The dominant Pacific sandy-beach organism, Cirolana mayana, which
comprised 84 per cent of Naos fauna, was also abundant on Shimmey
Beach, comprising 30 per cent of its fauna. This isopod was previously
known from temperate regions of both Atlantic and Pacific oceans (Richardson, 1905) before the present canal was built. The dispersion of C. mayana
was clumped
on both beaches.
The distribution
of animals
occurring
in
0.I-m2 quadrats followed a negative binomial distribution. The maximum
likelihood estimate of k was 0.59 at Naos and 0.19 at Shimmey Beach,
indicating greater aggregation on the Atlantic coast. Although densities
were significantly higher on the Pacific beach (1200/m2 versus 70/m2)
there were similarities in intertidal zonation (Figs. 3, 4). Large and/or
gravid individuals of C. mayana were found at mean high tide of spring
tide on both beaches; a mixture of large and juvenile individuals of C.
mayana was found in the next lower stratum on both beaches. Below the
high-tide strata, juveniles comprised over 95 per cent of the population.
On each coast the highest densities were found in the high-tide zone, the
upper stratum of which remained exposed for week-long periods, twice
each month. C. mayana did not extend to mean low tide of spring tide on
either beach. It is a scavenger feeding mainly on dead and injured animals
washed up on the beach during tidal fluctuations, and this may partially explain its clumped distribution.
Ancinus sp. A and sp. B, deposit-feeding isopods, had similar zonation
patterns and similar densities on both beaches (Figs. 3, 4). These species
are undescribed; their closest relative is Ancinus brasiliensis (P. Glynn.
personal communication). Distribution of both species was clumped and
did not differ significantly from a negative binomial distribution. The
maximum likelihood estimate of k for Ancinus sp. A was 0.19 and for
Ancinus sp. B was 0.15, high values of aggregation. These species of
Ancinus are polymorphic in color and pattern. On the white calcareous
sands of Shimmey Beach, only white or pale brown individuals of Andnus
sp. B were collected. On the black volcanic sand at San Carlos, Pacific,
Panama, only black individuals of Andnus sp. A occurred. The color
of the sand at Naos Island was varied, as was the sand at Maria Chiquita,
Dexter: Community Structures of Sandy Beaches
1972]
453
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Bulletin of Marine Science
454
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Dexter: Community Structures of Sandy Beaches
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Bulletin of Marine Science
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[22 (2)
DENSITY/ M1
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t--l
>---1
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10
HT
20
30
20
t--l
MT
7
LT
80
3. The pattern of zonation of the beach fauna at Naos Island, Panama
(ordinate gives distance, in meters, from mean high tide of spring tide):
1, Ancinus sp. A; 2, Cirolana mayana; 3, Paraonides; 4, nerinid polychaete;
5, Donax panamensis; 6, phoxocephalid amphipods; 7, Emerita rathbunae.
FIGURE
Atlantic, Panama; on these beaches there were at least eight color patterns
of Ancinus in which red, white, brown, black, and yellow pigments were
found in differing proportions and locations. Ancinus sp. B was the
dominant organism at Shimmey Beach, while Ancinus sp. A was second
in abundance at Naos Island.
Undescribed nerinid polychaetes were also abundant on the beaches.
The Atlantic nerinid was primarily a low-tide species and more abundant
than the Pacific species (Figs. 3, 4). Abundant organisms at Shimmey
Beach also included the cumacean Cyclaspis sp. B, the bivalve Donax
denticulatus, and the isopod Exosphaeroma diminutum, all low-tide species
(Fig. 4). Emerita brasiliensis and Lepidopa richmondi, although rare, contributed significantly to the total biomass. The polychaetes Paraonides and
Glycera, the bivalve Donax panamensis, the anomuran Emerita rathbunae,
and phoxocephalid amphipods were additional common species found at
Naos Island; most of these were distributed at low tide levels (Fig. 3).
There were several major differences between the two sand-beach faunas.
The Pacific beach, compared with the Atlantic beach, had 6 times the density
and 9 times the biomass of infaunal invertebrates. The Pacific fauna was
almost 3 times as diverse in number of species (41) as the Atlantic
fauna (15 species). The taxonomic groups were also distributed differently.
At Shimmey Beach, 67 per cent of the species were Crustacea, 13 per cent
1972]
Dexter: Community Structures of Sandy Beaches
DENSITY
457
/ M2
o
HT
2
4
6
MT
8
10
LT
12
14
4. The pattern of zonation of the beach fauna at Shimmey Beach,
Panama (ordinate gives distance, in meters, from mean high tide of spring
tide): 1, nerinid polychaete; 2, Giro/ana mayana; 3, Ancinus sp. B; 4,
FIGURE
Exosplzaeroma diminutum; 5, Cyclaspis sp. B; 6, Donax denticu/atus.
molluscs, 13 per cent annelids, and 7 per cent other taxa. The Crustacea
and annelids each contributed 35 per cent of the species at Naos Island
and molluscs and other taxa contributed 15 per cent each.
The Shannon-Wiener diversity index takes into account both species
diversity (total number of species) and dominance diversity (numerical
equality in species representation) and is relatively independent of sample
size if the sample is larger than 200 individuals (Sanders, ] 968). A comparison of intertidal diversity on two temperate and two tropical
sandy beaches is given in Table 2. The Shannon-Wiener diversity
index of Shimmey Beach is twice that of Naos Island because dominance
is shared by more species. The diversity of the tropical Pacific beach is
similar to diversity on the temperate Atlantic beach, while higher and
similar values of diversity are found on the temperate Pacific and tropical
Atlantic beaches. From the data available on diversity of temperate and
tropical sandy beaches, it does not appear that species diversity, as expressed by the Shannon-Wiener formula, is higher in the tropics.
DISCUSSION
In majority, the faunas at Naos Island and Shimmey Beach are composed of closely related species belonging to the same genera and presumably occupying similar niches. The Naos fauna was composed of
many rare and a few very abundant species; 78 per cent of the species
occurred in less th'an 10 per cent of the samples. At Shimmey Beach 53
458
[22(2)
Bulletin of Marine Science
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1972]
Dexter: Community Structures oj Sandy Beaches
459
per cent of the species occurred in less than 10 per cent of the samples.
The faunas were dominated by the isopods Ciro/ana mayan a and Ancinus,
the cumacean Cyclaspis, the polychaete Paraonides, and nerinid polychaetes. The most obvious difference between the faunas is the marked
contrast in density of infaunal invertebrates.
Sandy beaches are usually subjected to widely fluctuating physical conditions, and this physical instability is often correlated with reduced diversity. Greater stability in physical factors presumably enhances increased
diversity (Dunbar, 1960), and thus one could predict greater faunal diversity on the Atlantic coast than on the Pacific coast of Panama. Striking
differences in density between the two sand faunas makes comparison of
diversity difficult. The number of species is significantly higher on the
Pacific coast; the Shannon-Wiener diversity index is higher on the Atlantic
coast. When the rarefaction method (Sanders, 1968) is used to estimate
the number of species for the Pacific beach, given the same number of
individuals collected as at Shimmey Beach (1824), the predicted number of
species is 24. This is still significantly greater than the 15 species found at
Shimmey Beach.
A comparative study of similar habitats along both sides of the Panamanian isthmus is essential for prediction of probable effects of alterations
imposed by man's intervention in the construction of a sea-level canal. The
construction of an interoceanic sea-level canal will provide a means of
dispersal for planktonic larval stages and free-swimming organisms between
the Pacific and Atlantic oceans. The general flow in the proposed interoceanic canal will be from the Pacific to the Atlantic, with a discharge
of about 50,000 cubic feet of water per tidal cycle (Sheffey, 1969) which
would probably block the dispersal of organisms from the Atlantic to the
Pacific and enhance the dispersal in the opposite direction. The effects of
such dispersal are not known.
Sandy beaches are less diverse than many other intertidal environments.
Presumably there is more inherent instability in simple communities than in
complex ones and simple communities are more vulnerable to invasions
(Elton, 1958). Most intertidal sandy-beach invertebrates have short life
cycles which result in wide population fluctuations. The Pacific fauna may
be better adapted to changing environmental features, since it is exposed to
greater variability. Together, the predominant direction of interoceanic
water flow, the short life cycle, and greater environmental adaptability
of the Pacific fauna could result in the establishment of Pacific forms along
the Atlantic beaches via a sea-level canal.
However, one can also argue that invasion of species may not occur.
First, sandy-beach organisms have limited migratory ability. The dominant
forms on many sandy beaches are peracaridans: isopods, amphipods,
mysids, and Cumacea (Table 2). All of these undergo direct development,
460
[22(2)
Bulletin of Marine Science
with juveniles released from the brood pouch. Thus many of the most
abundant sandy-beach organisms do not have planktonic larval stages and
probability of dispersion through the canal is reduced. Second, the most
abundant infaunal organism, Cirolana mayana, occurs on both coasts, and
interchange of populations is less likely to cause significant changes in community stability.
SUMMARY
1. Shimmey Beach, Atlantic coast of Panama, and Naos Island Beach,
Pacific coast of Panama, were sampled for sandy-beach community composition during June, July, and August 1969.
2. A systematic sampling procedure resulted in the collection of 41 species
and 12,905 individuals from the intertidal Pacific beach, and 15 species
and 1824 individuals from the Atlantic beach.
3. The quartz sand at Naos Island was dominated by the isopods Cirolana
mayana and Ancinus sp. A, the polychaetes
Paraonides, Glycera, and
a nerinid polychaete, phoxocephalid amphipods, and the decapod
Emerita rathbunae.
4. The calcareous sand at Shimmey Beach was dominated by the isopods
Ancinus sp. Band Cirolana mayana, the cumacean Cyclaspis sp. B, a
nerinid polychaete, and the bivalve Donax denticulatus.
5. The Pacific fauna was characterized by greater density and biomass,
while the Atlantic fauna was more diverse in terms of equitability of
species distribution.
ACKNOWLEDGMENTS
Financial support for this study was provided by the Smithsonian Tropical Research Institute. The author appreciates the determinations of the
species by the following specialists: Lawrence Abele (decapod Crustacea),
E. L. Bousfield (amphipods), Thomas E. Bowman (mysids), Robert
Bullock (molluscs), George W. Byers (Tipulidae), John H. Dearborn
(ophiuroids), Robert Given (Cumacea), Peter Glynn (isopods), Olga
Hartman (polychaetes), and Raymond B. Manning (stomatopods).
Thomas Ebert provided the program used in computer analysis of the data.
SUMARIO
COMPARACION
PLAYA
DE
LAS
ESTRUCTURAS
ARENOSA
UNA
PANAMENA
EN
EL
DE
EN
LA COMUNIDAD
EL
PAcIFICO
DE
UNA
CON
ATLANTICO
Shimmey Beach, en la costa del Atlantico de Panama, y Naos Island
Beach, en la costa del Pacifico en Panama, fueron muestreadas para ver
la composici6n de la comunidad de playa arenosa, durante junio, julio
y agosto, 1969.
Dexter: Community Structures of Sandy Beaches
1972]
461
Un procedimiento
sistematico de muestreo resu1t6 en la colecci6n de
41 especies y 12,905 individuos de la playa en el litoral del Pacifico, y
15 especies y 1,824 individuos de la playa en el Athlntico.
La arena de cuarzo en Naos Island estuvo dorninada por los is6podos
Cirolana mayana y Ancinus sp. A, los poliquetos Paraonides, Glycera y
un poliqueto nerlnido, anflpodos foxocefalidos
y el decapodo Emerita
rathbunae.
La arena calcarea
en Shimmey Beach estuvo dominada
por los is6podos
Ancinus sp. B y Cirolana mayana, el cumaceo Cyclaspis sp. B, un poliqueto
nerlnido y el bivalvo Donax denticulatus.
La fauna padfica se caracteriz6 por una mayor densidad y biomasa,
mientras que la fauna atlantica fue mas divers a en terminos de equitabilidad
en la distribuci6n de las especies.
LITERATURE CITED
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] 967. Niche diversity in five sympatric species of intertidal amphipods
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1952. Ecology and zonation of fauna of sand beaches. Oikos, 4: 1-23.
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ELTON,
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M. M.
1968.
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