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Chapter 31. A preliminary account of the distribution of Ostracoda in Recent reef and reef associated environments in the Pulau-Seribu or Thousand Island group, Java Sea
400 R. WHATLEY
AND K. WATSON
The Pulau Seribu (latitude 5” 35’S, longitude 106”35’E,are a group of small coral reefs which
commence 25 km from the north Java coast and extend in an approximate N-Schain for around
40 km. (Text-fig.1). A general description of the Pulau Seribu, also known as the Thousand Island
Group, is given by Cook and Saito (1982). The reefs have also been described by Scrutton (1976,
The reefs are situated on the NNE-SSW trending Seribu High, a structural high in the northwest Java Sea Basin which separates the Sunda sub-basin to the west and the Arjuna sub-basin to
the east. Although a deep channel to the south separates the Pulau Seribu from the Java mainland,
the reefs are situated on a shallow shelf ranging in depth between 3040 m. The reefs rise up steeply
from the sea floor; with slopes of the order 70-75”. At the surface, the reefs vary in size and shape
but many tend to be elongated in an approximately east-west direction. They range in length
from 50 m to 5 km (Text-fig. 2). Cook and Saito (1982) suggest that the reefs in the Pulau Seribu
can be categorized as “platform, platform lagoonal and elongate platform types” following Maxwell’s (1968) classification of reef types within the Great Barrier reef.
We are studying the Ostracoda of four of the reefs, which from north to south are: Pulau
Ringit, Pulau Petondang Kecil, Pulau Kottah Kecil and Pulau Pari (Text-fig. 2). The present study
is, however, concerned solely with Pulau Pari.
Text-figure 3 shows the reef complex associated with Pulau Pari. It comprises a complex of five
cays (low lying islands or emergent “reefs” of sand or coral) surrounded by a reef. Although Pulau Pari cay itself is only some 2.8 km by 0.5 km the entire complex is approximately 8 km long by
1-Regional setting isf the Pulau Seribu.
Ostracoda in Recent Reef of Java Sea 401
TEXFFIG.2-The reefs of the Pulau Seribu.
(at its widest) 4 km. The complex is aligned east-west. Along the north side of the reef the slope is
much shallower than on the south side and coral growth less vigorous. On the south side the reef
is more protected and coral growth is very vigorous, giving rise to a steep slope.
The lagoon associated with Pulau Pari is considered mature in that it contains well established
mangroves, particularly northeast of the island.
Although a total of 30 sediment samples are available to us from Pulau Pari, only 13 have been
processed in time for the study. Their positions are given in Text-fig. 3. The remaining samples from
Pulau Pari and the other islands will be studied in due course.
402 R. WHATLEY
AND K. WATSON
Outer rwf f l d
. _ _ - *'
TEXT-FIO.3-The reef complex of Pulau Pari indicating sample localities.
Text-figure 4 represents in diagrammatic form the various reef and reef associated environments
which have been sampled in this study of h l a u Pari. The seven different environments are described in turn:
Fore reef: Sample 1 (depth 20 m)
Sample 64 (depth 31 m)
This environment, on the seaward side of the reef slope provides the deepest water (20-40 m)
in the study. The substrate is of medium-coarse skeletal sands and gravels, and the sea-bottom
flat and featureless. Grain size increases towards the base of the reef slope where sea fans (Gorgonaria) begin to appear.
Reef slope: Sample 3 (depth 10 m)
Sample 63 (depth 10 m)
From the "growing edge" of the reef down to the sea-floor there is a steep slope of some 7075". Coral diversity declines downslope and from almost 100% cover there is a gradual decrease
as living coral gives way to areas of dead coral and pockets of sediment. The coral assemblages
change with depth to those more tolerant of reduced light intensity. At a depth of between 15-20 m
colonial corals give way to a fauna dominated by sea whips and sea fans. At around the same
level there is a general easing off in the reef slope and the slope is covered with more and more
sekeltal sand and gravel and larger fragments of dead coral from higher up the reef slope. This
trend continues to the base of the slope with all forms of tlora and fauna becoming rarer. The
sediments of the reef slope are largely coral, mollusc and algal in their derivation. They are poorly
sorted and unabraded. According to Cook and Saito (1982) the skeletal debris is usually composed of the following:
Ostracoda in Recent Reef of Java Sea 403
prolific living corol
and other branching corals.
mangrove growth may occur.
change in reef slo
and coral fauna
moderately sharp break at base of
reef slope to channel or shelf sediments:
depth varies 2 0 4 0 m.
YO1 TO SCALE
&Diagrammatic section through a typical reef of the Pulau Seribu (adapted from Scrutton, 1976).
Reefflat: Sample 26 (depth 1.5 m)
Sample 35 (depth 0.6,m)
This is an area of rather uniform shallows floored by skeletal sand. Maximum depth is some 3
metres and while parts of the reef may be emergent at low spring tides, it is usually covered by
about lm minimum of water. In the more sheltered areas the sediment is fine sand; elsewhere it
is medium to coarse-grained with large fragments of molluscan and coral debris. Patchy coral
growth occurs although coral diversity is low, reflecting diminution in oxygen and nutrient levels
relative to the “growing edge”. The individual grains are often abraded and range in shape from
angular to subrounded. According to Cook and Saito (1982) their composition is usually as follows:
2 M %
rarely exceeding 2% each
The xtef flat abounds in animal life. Apart from corals, holothurians, fish, molluscs, echinoderms, crustaceans and marine worms are all abundant and diverse.
Lagoon: Sample L3 (depth 1 m; some coral growth)
Sample L5 (depth 2 m; rich in algae)
There is a well developed lagoon on the northwest side of Pulau Pari cay. The sediment is
medium grained skeletal sand grading to fine grained sandy muds with carbonaceous material in
deeper parts. This supports some mangrove growth and also patches of “Stagshorn” and other
coral. The water depth is variable but probably does not exceed a few metres. The deepest sample
availabk to us was taken at 2 m. As there is no freshwater run off from the cay, the salinity of the
404 R. WHATLEY
AND K. WATSON
lagoon and the intertidal mangrove,.given high evaporation, is probably in excess of that of the
Intertidal Mangrove: Sample 37
Particularly on the northwest shore of Pulau Pari cay, well established mangroves occur in the
intertidal zone. The sediment is a fine grained dark organic-rich mud.
Intertidal: Samples 23, 32.
The environment is an intertidal sandy beach composed of medium to coarse skeletal sands, well
sorted and abraded.
Supratidal: Samples 30, 39.
These two samples were collected above maximum high water mark on sandy beaches. Sediment
type as for the intertidal zone.
The sediment samples were collected by hand, either by snorkel or scuba diving, during
the winter of 1983. Sediment was scooped from a shallow surface layer into a plastic bag (Turner,
pers comm., 1984). In the laboratory, samples were washed over a 230 mesh/inch sieve and the dried
samples split into 30, 60 and 100 mesh/inch fractions. A count of ostracods from a known weight
of each fra.ction of each sample was made. The count was then recalculated, dependent on the
amount of each fraction picked, to yield the total number of ostracods in the whole sample. Because grain size varied enormously in the samples, from coral rubble to find muddy sands, it was
impractical to pick equal volumes of sediment from each. For this reason the total number of individuals from each sample has been standardized to represent the number of ostracods recovered
from 100 g of sediment. The authors are fully aware, however, that this to a certain extent is dependent on sedimentation rate.
OF THE OSTRACODA
The 13 samples yielded a total of 49 genera and 141 species. The distribution of the taxa among
the 7 environments studied is given in Table 1. This also shows the number of species belonging to
each genus and the number of individual specimens of each genus (standardised to 100 g sediment)
recovered from each envrionment. The figures in brackets in Table 1 represent the precentage
abundance of each genus in each envrionment.
Table 1 illustrates that both diversity and incidence are very variable. For example, incidence of
individuals per 100 g sediment is higher on the outer reef flat and in the lagoon than elsewhere.
Generic and specific diversity is highest on the reef slope and in the lagoon.
Data from Table 1 is synthesized in Table 2. This clearly demonstrates the different diversity
patterns of the various Bnvironments. Since the faunas of the intertidal and supratidal environments
are probably and certainly derived respectively, they can largely be ignored. The diversity of
the fore-reef samples is surprisingly low but this is probably a reflection of the low incidence which
itself is probably due to the coarse nature of the sediment. The high diversity of the reef slope,
however, is apparent despite the relatively low incidence. Unfortunately, there has not been sufficient time to undertake a population age structure analysis (Whatley, 1983) of the faunas to determine which species are autochthonous to which environment. However, the faunas of the lagoon
and intertidal mangrove and to a larger extent the outer reef flat, seem to be composed in the main
of species whose age Structure is suggestive of a biocoenosis.
IN THE 7 ENVIRONMENTS
FROM 1ooO. SEDIMENT).
(No. of species)
Cullistocythere ( 5 )
Tanelfu ( 5 )
Cytherelloidea ( 5 )
Gen. Znd& (1)
Fore-reef Reef slope
Lagoon Intertidal Intertidal Supratidal
209(2.2) loo( 4.6)
406 R. WHATLEY
AND K. WATSON
STATISTICS ON THE DISTRIBUTION
OF OSTRACOD GENERA,
SPECIES AND INDIVIDUALS,h L A U PARI.
No. of individualst
No. of genera
No. of species
% made up by
% made up by 3
t Standardized to represent total no. of individuals recovered from 100 g sediment.
The relative abundance of the 31 most important genera is given in Text-fig. 5. The relative
abundances are expressed in percentages of the total population of each environment. Text-figure
5 illustrates that the faunas of certain environments are dominated by certain genera. For example,
although Neonesidea and particularly Paranesidea are widely distributed throughout the entire
area, they clearly dominate on the reef. The same is true of Triebelina, and to a lesser extent Mutilus.
Renaudcypris clearly exhibits its preference for environments of a restricted nature away from
the open sea. Hansacypris is similar but restricted to the intertidal mangrove. Both Xestoleberis
and Ornatoleberis are ubiquitously distributed as is Loxoconcha. The distribution of Caudites
and Ambostracon is somewhat similar.
The degree to which genera are restricted to environments is outlined below:
Only one species of the 141 is ubiquitous, that being a very abundant species of Arnbostracon.
No less than 48 species, however, are confined to a single environment as shown in Table 6.
CONFINED TO A SINGLE ENVIRONMENT.
Hansacypris 2 spp.
CytherelIa 1 sp.
Eucytherura 2 spp.
Hemicytherura 1 sp.
Gen. indet. 1 sp.
Trachyleberis 1 sp.
Semicytherura 1 sp.
Neobuntonia 1 sp.
Cytheropteron 2 spp.
Lagoon and intertidal mangrove only
Lagoon and reef slope (very rare in latter)
Reef slope and outer reef flat
Reef slope and fore reef
Environment Fore reef Reef slope Outer
Intertidal Intertidal Supratidal
Paranesidea ( 8 )
Callistocythere ( 5 )
Xestoleberis (1 6)
Cytherelloidea ( 5 )
TEXT-&. 5-The environmental distribution, in terms of relative abundance of selected genera, on Pulau Pati.
408 R. WHATLEY
AND K. WATSON
TABLESPECIES CONFINED TO A SINGLE ENVIRONMENT.
Outer reef flat
Semicytherura lsp. Bairdoppilata lsp. Paranesidea lsp
Intertidal mangrove Intertidal
Hansacypris 2 spp. Neonesidea
Propontocypris 1sp. Triebelina
Cytheropteron 2 spp. Triebelina 1sp.
Callistocythere 1sp. Loxoconcha lsp.
Tanella 2 spp.
Cytherelloidea lsp. Loxoconchella lsp. Loxoconcha lsp.
Neocyprideis lsp. Eucytherura 2 spp.
Paradoxostoma 1sp. Paradoxostoma 1sp.
Hemicytherura 1 sp.
Neomonoceratina 4 spp.
Gen. indet lsp.
Cytherelloidea 3 spp.
Propontocypris lsp. Tanella lsp.
Despite the preliminary nature of the study, a considerable degree of environmental endemism
is shown by the fauna of the Pulau Pari reef complex. The degree to which this is more widely
applicable to the complex as a whole will be tested when the study is complete.
COMPARISON WITH OTHER
Relatively few studies have been made of the Ostracoda of reef and reef-associated environments
and even fewer are directly applicable to the present study. This is either because the nature of the
reef body studied is not comparable with Pulau Pari or the descriptions of the environments sampled are such that comparisons between the results are difficult or impossible to make.
A considerable number of ostracod taxa have been described from reef environments but
relatively few comprehensive studies of the ecological distribution of Ostracoda in reefs have been
made. The majority of these have been carried out in the Caribbean/Gulf of Mexico region. These
include Tressler, 1949; Puri and Hulings, 1957; Kornicker, 1958, 1961, 1964; Puri, 1960; Pokorny,
1968; Van den Bold, 1974, 1977; Krutak, 1974, 1982; Krutak and Rickles, 1979; Maddocks,
1974; Teeter, 1975; Keij, 1974, 1975, 1976; Palacios-Fest and Gio-Argaez, 1979.
Neither the reefs nor the reef-associated environments studied in the Caribbean/Gulf of Mexico
are directly comparable to Pulau Pari and few are closely similar. Nonetheless, certain aspects of the
distribution of the Ostracoda are common between the two areas. For example, there are many
references to the enhanced diversity and incidence of Bairdiidae in immediate association with
coral bioherms and adjacent detrital carbonates (Puri, 1960; Puri and Hulings, 1957; Maddocks,
1974; Teeter, 1975; Keij, 1974). Other than the Bairdiidae, the reef and reef-associated faunas of
the Caribbean have many other genera which are also prominent in such environments in the Java
Sea. These incude Jugosocythereis, Caudites, Loxoconcha, Mutilus, Propontocypris, Paracytheridea,
Hemicytherura, Xestoleberis and Cytherelloidea.
Teeter (1975) in his study of the Holocene marine Ostracoda from offshore Belize encountered
Ostracoda in Recent Reef of Java Sea 409
many faunal assemblages which in their generic composition resemble those of the present study.
For example, the fauna inabiting the barrier rim, reef flat and apron of the reef flat was dominated
by Bairdoppilata, Triebelina, Loxoconcha and Xestoleberis and also included Quadracythere,
Cytherura and Paracytheridea. Referring to the fauna of these environments Teeter (p. 414) states
“The ostracod diversity attains a maximum and, inversely, the dominance shows a marked decrease in the shallow, stable, carbonate-platform environment. Especially striking is the very high
diversity of the barrier rim, attributable perhaps to the numerous niches available in complex
organic reef associations. The decreased dominance reflects the rising competition between members of the association”. Teeter could equally well have been referring to the faunas of the present
study from the reef slope (Tables 1, 2, Text-fig. 5).
The back reef lagoon described by Teeter, with water depths between 6 and 60 m cannot be
used as a standard of comparison with that of Pulau Pari with a depth of some 2m.
The most comprehensive study of a Pacific reef is by Allison and Holden (1971) who described
the distribution of the Recent Ostracoda of Clipperton Island in the eastern Pacific. The two
lagoons are not comparable since that of Clipperton Island is non-marine and inhabited by freshwater cyprids. It is instructive, however, to compare the distribution of the Ostracoda across the
Clipperton Island reef profile (Allison and Holden, 1971, p. 171, Table 1) with Text-fig. 5 and
Table 1 of the present study. Both incidence and diversity are higher in the present study but in both
studies the Bairdiidae are dominant on the seaward side of the reef. Mutilus is well represented in
both profiles as is Paracytheridea. Loxoconcha, a dominant at Pulau Pari is unaccountably absent at
Clipperton Island which also lacks Ornatoleberis and where Xestoleberis is much less diverse. The
low diversity of the Clipperton Island fauna is probably due to the isolation of the island from
adjacent bodies of shallow water.
The importance of the Bairdiidae in reef environments is not confined to Recent examples.
The association is of considerable antiquity extending back to at least the Carboniferous (Bless,
1983). It was certainly very well established in the Upper Triassic of the littoral of the Tethys
(Kristan-Tollmann et al., 1980) and has been subsequently documented by many authors in the
Cainozoic but particularly in the Upper Cretaceous (Pokornp, 1977; Babinot and Colin, 1983).
The strong environmental control imposed on the distribution of Ostracoda in reef and near
reef situations has considerable importance with respect to hydrocarbon exploration. Reefs and
other carbonate build-ups are, in many areas, important hydrocarbon reservoirs. Krutak (1 982)
has commented on the importance in this respect, of ostracods in the Gulf Coast Basin. Similarly,
Scrutton (1976, 1978) has referred to the importance of the study of the modern reefs in the Java
Sea with respect to achieving a better understanding of their oil bearing Miocene counterparts in
Indonesia. It is hoped that, when the present study is completed, it will provide data of considerable value in oilfield exploration.
Karen Watson wishes to acknowledge a NERC Studentship and financial and other assistance
from Robertson Research International and Robertson Research Singapore. We wish particularly
to record the good offices of Prof. Denis Wood of RRI and Dr. Colin Harris of RRS. Miss Penny
Turner, of Aston University collected the material on which this study is based and we thank her
for forwarding it to us together with much valuable documentation. Our friend and collegue Dr.
410 R. WHATLEY
AND K. WATSON
Timothy Palmer has offered much useful advice and comment. We thank him for that and for his
interest in the project. The diagrams were drawn by Arnold Thawley and the paper was typed
by Marian Mayes. We thank them both.
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