Environmental Micropaleontology, Microbiology and Meiobenthology, 2004, Vol.1, pp. 105-121 RECENT BENTHIC OSTRACODA FROM THE INNER SHELF OFF CHENNAI, SOUTHEAST COAST OF INDIA – IMPLICATION ON MICROENVIRONMENTS

*S. M. HUSSAIN1 , G. RAVI1, S. P. MOHAN1 AND N. RAJESHWARA RAO2

1. Department of Geology, School of Earth and Atmospheric Sciences, University of Madras,Guindy Campus, Chennai-600 025, India *E-mail:[email protected] 2. Department of Applied Geology, School of Earth and Atmospheric Sciences,University of Madras, Guindy Campus, Chennai-600 025, India E-mail:[email protected]

Fifty-six sediment samples collected from the inner-shelf off Karikkattukuppam (near Chennai), Bay of Bengal, southeast coast of India, yielded 51 ostracod taxa. Carapace-valve (C/V) ratio, predation, color pattern and ornamentation of the carapace were utilized to study the microenvironment of the area. Based on the C/V ratio, a moderately slow rate of sedimentation is inferred. Predation, a biological factor controlling the abundance and distribution of the community, supports the fact that the study area is a shallow, inner shelf environment. From the occurrence of a greater number of white and pale yellow colored specimens, it is concluded that sediments are deposited under normal oxygenated conditions. Based on direct observations from Scanning Electron Microscope photographs, the relationship between sculpture in ostracods and grain size of the substrate, is discussed. Among the 4 types of ornate forms recorded in the study area, smooth and fragile forms prefer fine-grained sediments such as sandysilt and silt, whereas highly calcified and ornamented forms prefer coarse-grained siltysand/sandy substrates. In summary, the entire ostracod faunal assemblage off Karikkattukuppam seems to be consistent with deposition under tropical, shallow water/inner neritic, ultrasaline to euhaline environmental conditions.

Key words: Recent Benthic Ostracoda, Bay of Bengal, Chennai, Micro-environment.

INTRODUCTION

Ostracods are one of the best documented groups within the whole of the animal kingdom, due to a wealth of characteristic features and a well calcified, tiny, bivalved carapace which fossilizes easily. They are known to inhabit a wide variety of aquatic environments such as marine, brackish, freshwater, even terrestrial, and also dwell as parasites in the intestines of fishes. Thus, ostracods have an edge over foraminifers in biostratigraphic and ecologic/paleoecologic studies of non-marine strata. Puri (1966) stated that ostracods live in an environment in which the controlling factors are temperature, bottom topography, depth, salinity, pH, alkalinity, dissolved oxygen, food supply, substrate and sediment organic matter content. But the major controlling factors

105 106 Hussain, Ravi, Mohan, and Rajeshwara Rao

governing the ostracod distribution in estuarine and continental environments are salinity, water temperature and substrate (Zhao and others, 1985; Bentley, 1988; Yassini and Jones, 1995). The area under investigation is off the coast of Karikkattukkupam, near Chennai (Lat. 12°50' N; Long. 80°16' to 80°24' E), in the Bay of Bengal, southeast coast of India. The climate is characteristically tropical, being hot and humid. During the beginning of the year, the monthly average temperature is generally low, whereas during summer (April to June), the temperatures are high and often soar to more than 40° C. The minimum temperatures are recorded during the end of December and beginning of January.

MATERIALS AND METHODS

The bottom sediment and water samples were collected from 15 stations, at depths ranging between 7 and 55 m, (Figure 1) every three months, for a period of one year, representing the four seasons - October 1995 (northeast monsoon), January 1996 (winter), April 1996 (summer) and July 1996 (southwest monsoon). The collections were made every kilometer in a single transect approximately perpendicular to the coastline off Karikkattukuppam. Petersen grab and Nansen reversible water samplers were used for collecting the sediment and water samples, respectively. Fifty-six bottom sediment and 60 bottom water (from the sediment-water interface) samples were obtained for the entire period. Sediment samples could not be collected at the fourth station because of underlying rocky exposures that rendered the grab ineffective.

Ostracod Faunal Composition

Fifty-one ostracod species belonging to 40 genera, 22 families, 3 superfamilies and 2 suborders of the order Podocopida were identified (Mohan and others, 2001). Of these, 49 species belong to the suborder Podocopa and the remaining 2 to Platycopa. While reporting the taxa from off Karikkattukuppam, Mohan and others (2001) established the following four new species: Hemitrachyleberis siddiquii, Puricythereis whatleyi, Neocytheromorpha reticulata and Pterygocythereis chennaiensis. A checklist of the taxa encountered in the study area is presented in Table 1. Hypotypes for all the species are reposited in the museum of the Department of Geology, University of Madras, Chennai, with register numbers GMOKR 1 – 51.

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Figure 1. Location map of the study area showing the sampling locations.

The sedimentological characteristics such as sand-silt-clay ratios, organic matter and CaCO3 content of the sediments and bottom water characteristics such as temperature, salinity and dissolved oxygen were determined by Mohan and others (2002). They correlated these parameters with the ostracod population recovered from each sample and observed that high calcium carbonate, low organic matter contents of the sediment and higher temperature, salinity and dissolved oxygen in the bottom waters favour larger population size and distribution. They further observed that siltysand appears to be the best substrate for thriving fauna in the study area.

Carapace-Valve Ratio

During the last two decades, the collection of statistical data on Ostracoda, such as juveniles and adults, closed and isolated valves, males and females, right and left valves, smooth and ornamented forms etc., besides color variation, pyritization and predation, has attained importance, especially with regard to

109 Ostracoda Off Chennai (India) interpretation of the environment of deposition, rate of deposition and assessment of potentiality of sediments as source rocks for hydrocarbons. Guha (1982), while dealing with the prospective of Bombay High and Cambay basins of India for hydrocarbon potential, stressed the need to make use of various types of ostracod data. Due to intense bacterial activity the carapaces in ostracods open up, the valves often getting separated. In an environment where deposition of the sediment is low, the carapaces are likely to open up by bacterial action. But, in an environment where deposition is very rapid, the carapaces sink into the soft bottom and are quickly covered by sediments. Thus, the carapaces have less chances of opening up, after the destruction of muscles and ligaments. Pokorny (1965) pioneered the usage of carapace-valve ratio to yield paleoecological information. Oertli (1971) reviewed Pokorny's work and related the carapace-valve ratio to potential for the information of hydrocarbons. He summarized that when the ratio is high, sedimentation is rapid, which minimizes disarticulation of carapaces into separate valves. With sufficiently rapid burial, organic matter is not absorbed by mineral particles and so retains potential for conversion into hydrocarbons. Honnappa and Venkatachalapathy (1978) studied the carapace-valve ratio to interpret the rate of deposition of sediments in the Mangalore harbor area, southwest coast of India. They found that the occurrence of open valves is much higher than the closed ones (ratio being 24:1). According to them, this is indicative of a slow rate of sedimentation in more agitating waters. While comparing Eocene/Oligocene ostracods from southeastern Australia and India for potential indicators of petroleum, McKenzie and Guha (1987) inferred a rapid rate of sedimentation by noticing high percentage of carapaces. Ahmad and others (1991), while studying Tertiary ostracods from Lindi area, Tanzania, observed higher rate of sedimentation in Upper Eocene and Lower Miocene than Oligocene, based on carapace and valve ratio. Sreenivas and others (1991) found rapid rate of sedimentation in Pulicat Lake estuary on the basis of occurrence of large number of closed carapaces. Hussain and Rajeshwara Rao (1996) observed a greater number of closed carapaces than open valves along the east coast of inner shelf sediments, while the number is much less from the sediments off the west coast of India. From this observation, they inferred that the rate of sedimentation is rapid on the east coast but slow on the west coast of India and attributed this to a larger number of streams/rivers flowing and delivering sediments into the Bay of Bengal. Hussain and others (2002) studied carapace and valve ratio and noticed faster rate of sedimentation in the inner shelf of Gulf of Mannar, off Tuticorin, southeast coast of India. They counted four-fold greater occurrence of carapaces to open valves, which was attributed to the inflow of sediments through Tamirabarani river. In the study area, the ratio between the carapaces and open valves has been taken into consideration for determining the rate of sedimentation. Out of the 56 sediment samples collected and studied, 7,896 ostracod shells, both adults and instars, were recovered (specimens of all the 51 species put together). Among these, 2,565 specimens were closed carapaces, while the remaining 5,331 specimens were open valves. The species-specific distribution of carapaces and valves, in raw numbers, during each season is presented in Table 2. Sample-

110 Hussain, Ravi, Mohan, and Rajeshwara Rao

specific and season-wise distributions of the carapaces and open valves are

111 Ostracoda Off Chennai (India) results are shown as histograms (Figure 2).

Carapaces Open valves

1600

1400

1200

1000

800

600

400

200 No. of carapaces and open valves 0 Oct.'95 Jan.'96 Apr.'96 July'96 SEASONS

Figure 2. Seasonal variations in carapaces and open valves.

The distribution of carapaces and open valves, either season-specific or for all the four seasons put together, reveals that open valves are always greater in number than carapaces. The ratios between the carapaces and open valves of the ostracod shells encountered in each season (14 samples put together) are given in Table 3. From the above observations, it may be concluded that a moderately slow rate of sedimentation prevails in the inner shelf region off Karikkattukuppam, as the carapace to valve ratio is 1: 2.

Predation

Predation can be stated as an interaction between two organisms which results in negative effects on the growth and survival of one of the populations (Odum, 1971). It can also be defined as a relationship between animals wherein one species eats another species. Predators include coelenterates and many

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gastropods (muricids and naticids). Muricids drill oblique holes in a deep-water environment, whereas naticids drill vertical holes in a shallow marine environment. There are active predators of many pelecypods, gastropods, scaphopods, ostracods and barnacles. Gastropod predators exhibit different patterns of predatory behaviour and drill holes of different shapes and sizes. Predators generally confine their activity to the upper sedimentary layers. Predation is a more common phenomenon in a community of organisms of benthic habit in shallow water environments. It is one of the limiting factors that affects the abundance, distribution and individuals of species (Clarke, 1954). According to Hecker (1965), predation may be considered as a “palaeopathological phenomenon” which occurs during the life of an organism. He also noticed that these holes are made on the shells of different organisms lying on the sedimentary layers. Such organisms are characteristic of certain facies, particularly those of shallow seas, not far from shorelines and, therefore, serve as good indicators of the regions. They can also be used successfully for determining the upper and lower surface of the beds. Sliter (1971) proposed that evidence of predation provides ecologic and paleoecologic information on living habits and habitats, and community structures of the organisms. This also gives an indirect clue in the interpretation of rate of sedimentation. He further found that predation is more common in benthic fauna of shallow water environment. Reyment (1971), while studying the predation on ostracods from Cretaceous, Tertiary and Recent sediments of Nigeria, stating that predation is one of the important biological factors which involve interaction between two or more species for limited food supply or, sometimes, one species as a part of the food for the other. From Texas region, Maddocks (1988) observed moderate predation in Cenomanian ostracods, noteworthy increase in predation of ostracod shells of Late Campanian, Cretaceous-Tertiary sediments and lower rates of predatory shells in Holocene

113 Ostracoda Off Chennai (India) assemblages. She also pointed out that ostracods are one of the food sources for juvenile naticids and smooth forms are more susceptible to predation. The large size of the carapace and dense populations also favor predation. The position, shape and dimension of the predatory drills found on ostracod carapaces can be utilized to interpret the environment of deposition and ecological implications. Murray (1973) found that the percentage of bored shells varies from 3% in shallow estuarine bodies to around 16% at a depth of 2000 m, in the Gulf of Mexico. Hussain and others (2002) while discussing the predatory aspects of ostracod assemblage of the Gulf of Mannar, noticed only 4.9% of shells were drilled. In the Bay, out of the 51 ostracod species encountered, the shells of 22 species exhibited predatory holes; 7 with smooth forms and 15 with moderately to highly ornate forms (Table 1). Out of the 7,896 ostracod specimens obtained from the 56 bottom sediment samples off Karikkattukuppam, only 289 specimens were found to have been predated. The predatory drill holes vary not only in size and shape but also with regard to their position. Station-specific and season-specific distributions of the shells exhibiting predation are shown in Table 4. These 289 drilled shells constitute 3.7% of the entire ostracod specimens recovered from the study area, a shallow inner shelf region with a maximum depth of 55 m. This is a similar incidence to that observed by Murray (1973) in the Gulf of Mexico.

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Color Diversity

Color diversity in ostracod carapaces has been utilized to interpret the ecology/paleoecology of sediments and their environment of deposition. Honnappa and Venkatachalapathy (1978) observed that white/milky white, pale yellow/light brown colored specimens were more numerous in sandy to sandy clay sediments enriched with heavy minerals such as hematite, limonite, magnetite, zircon, garnet and tourmaline. They inferred that these might have been deposited in normal oxygenated environment. A few black colored and partly pyritized specimens, however, were confined to clay sediments containing granules of pyrite and carbonaceous matter. In the study area also, almost all the open valves and carapaces are light yellow and white in color, supporting the fact that the sediments were deposited under normal oxygenated environment.

Surface Ornamentation and Sediment Texture

The carapaces of few species of ostracods have a smooth surface, devoid of any sculpture. However, in many species, the carapaces contain simple to complex surface ornamentation. Hence, surface ornamentation can serve as direct evidence for ecological interpretations. Though there are papers on taxonomy, systematics and studies on internal characters such as the normal pore system, muscle scar pattern and ocular sinuses of these microcrustaceans, papers pertaining to surface ornamentation of Ostracoda are relatively rare (except a few by Jones, 1956; Benson, 1961; Hulings and Puri, 1965; Puri, 1966; Krutak, 1972; Brasier, 1980; Annapurna and Rama Sarma, 1982; Vaidya and others, 1995; Sridhar and others, 1998; Hussain and others, 2002). Hence, an attempt has been made to discuss briefly the relationship between the sculpture in Ostracoda and grain size of the substrate based on direct observations with the help of Scanning Electron Microscope (SEM) photographs of the specimens. Environmental changes have been frequent in the geological past and are observed even today. New habitats have developed as a result of changes in the ecosystem due to these environmental changes. Although at times, the physical environment remains comparatively constant, the organic environment does not. Ostracods are very sensitive microorganisms and react rapidly to any change in the ecosystem. In order to changes in the environment, they have a tendency to adopt different morphological characters. A direct relationship between particle size of the sediment and surface ornamentation of the ostracod carapace can be observed. The substrate sediment texture has a control on the kind of ostracod fauna that can colonize a particular sediment type (Brasier, 1980). The texture of sediment comprising the substrate exerts a strong influence on marine ostracods. Smooth forms are predominant in fine-grained muds, whereas more ornamented forms are found in coarser or more calcareous sediment (Benson, 1961; Brasier, 1980).

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Presently, 4 types of ornate forms of ostracoda (including smooth ones) were noticed (species depicting different types of surface sculpture are shown in Plate 1 along with few predatory shells). They are categorized as follows:

Smooth and fragile forms: Cytheropteron sp., Macrocyprina decora, Paracytheroma ventrosinuosa, Paradoxostoma bhatiai, Phlyctenophora orientalis, Propontocypris (P.) crocata, P. (Schedopontocypris) bengalensis and Xestoleberis variegata.

Moderately calcified and pitted forms: Bairdoppilata (B.) alcyonicola, Basslerites liebaui, Bythoceratina mandviensis, Cushmanidea guhai, Cytherella hemipuncta, Hemikrithe orientalis, H. peterseni, Loxoconcha anomala, Microceratina punctata, Neomonoceratina jaini, Paracytheridea pseudoremanei, Semicytherura contraria and S. sp.

Fine to moderately reticulate and ridged forms: Cytherelloidea leroyi, Falsocythere maccagnoi, Hemitrachyleberis siddiquii, Keijella karwarensis, Loxoconcha cercinata, L. gruendeli, L. mandviensis, Neomonoceratina porocostata, Paranesidea fracticorallicola, Stigmatocythere indica, S. kingmai and Tanella gracilis.

Conspicuously ornate forms (typical box type, spinose and nodose etc.): Actinocythereis scutigera, Callistocythere flavidofusca intricatoides, Caudites javana, Chrysocythere keiji, Cytheretta trifurcata, Keijella reticulata, Keijia demissa, Lankacythere coralloides, Miocyprideis spinulosa, Mutilus pentoekensis, Neocytheretta murilineata, N. snellii, Neocytheromorpha reticulata, Neomonoceratina iniqua, N. spinosa, Paijenborchellina prona, Pterygocythereis chennaiensis and Puricythereis whatleyi.

The surface sculptures of ostracod carapaces have a direct relationship with the substrate type. Elofson (1941), Puri (1966) and Malz and Lord (1976) are of the opinion that more ornate and rather heavily calcified forms are present commonly in shallow water high-energy environments and inhabit sandy substrates in modern seas. In the Bimili backwaters and Balacheruvu tidal stream, near Visakhapatnam, southcentral coast of India, Annapurna and Rama Sarma (1982) noticed that the genus Phlyctenophora occurs in sand dominated areas and not in muddy areas. They further noticed that the moderately ornamented forms like Tanella, Loxoconcha, Paijenborchellina and Kalingella occur in considerable numbers in sandy areas. Vaidya and others (1995) observed relatively more ostracod faunal content in substrates consisting of medium- to fine-grained sand, whereas poor occurrence was noticed in clean, coarse-grained sand. Sridhar and others (1998) found more number of species in siltysand and sandy substrates and less in finer materials. They further observed that forms like Macrocyprina decora, Paracytheroma ventrosinuosa, Phlyctenophora orientalis, Keijella karwarensis and Paracypris sp. occur in considerable numbers in the samples with a mixture of clayeysand and clayeysilt,

116 Hussain, Ravi, Mohan, and Rajeshwara Rao

EXPLANATION OF PLATE 1 Bar scale = 100 µm

Fine to moderately reticulate, pitted and ridged forms:

Fig.1- Cytherelloidea leroyi Keij; ♀Right valve, External view Fig.2- Paranesidea fracticorallicola Maddocks; Right valve, External view

Conspicuously ornate forms:

Fig.3- Pterygocythereis chennaiensis Mohan et al.; Left valve, External view Fig.4- Callistocythere flavidofusca intricatoides (Ruggieri); Left valve, External view Figs.5-6 Miocyprideis spinulosa (Brady); 5, Left valve, External view; 6, Right valve, Internal view Fig.8- Actinocythereis scutigera (Brady); Right valve, External view Fig.9- Chrysocythere keiji Jain; Right valve, External view Fig.10- Stigmatocythere indica (Jain); Right valve, External view Fig.11- Lankacythere coralloides (Brady); ♂Right valve, External view Fig.12- Mutilus pentoekensis (Kingma); Dorsal view Fig.13- Cytheretta trifurcata (Lyubimova and Guha); Left valve, External view Fig.14- Neocytheretta snellii (Kingma); Right valve, External view

Moderately calcified and pitted forms:

Fig.7- Hemikrithe peterseni Jain; Right valve, External view Fig.15- Semicytherura contraria Zhao and Whatley; Right valve, External view

Smooth and fragile forms:

Fig.16- Xestoleberis variegata Brady; Right valve, External view Fig.17- Macrocyprina decora (Brady); Right valve, External view Fig.18- Propontocypris (Schedopontocypris) bengalensis Maddocks Left valve, External view

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118 Hussain, Ravi, Mohan, and Rajeshwara Rao

thus indicating that smooth forms occupy fine-grained sediments. Similar observations have also been made off Karikkattukuppam. Of the 51 species encountered in the study area, only 8 are smooth forms while the remaining are either moderately calcified, pitted or highly ornate forms. The present study area is considered to be shallow and the substrate is mainly sand, siltysand and sandysilt in nature. Certain forms are heavily calcified, whereas most of the other forms are ornamented with strong reticulations, spinosity and tubercles. These forms are Actinocythereis scutigera, Callistocythere flavidofusca intricatoides, Cytheretta trifurcata, Keijella reticulata, Lankacythere coralloides, Miocyprideis spinulosa, Mutilus pentoekensis, Neocytheretta murilineata, N. snellii, Spinoceratina spinosa, Paijenborchellina prona, Pterygocythereis chennaiensis, and Puricythereis whatleyi. Both the total numbers of specimens and species are found to be more in siltysand, followed by sand and sandysilt. The siltysand and sandysilt substrates containing some organic debris yielded good numbers of Ostracoda. These sediments also contain numerous other microorganisms such as foraminifers and microgastropods. The present material incorporates numerous carapaces and valves, but there are relatively fewer dimorphic forms. The number of carapaces, valves and sex ratio reflect ambient energy factors. Moreover, no pyritized carapaces were found in the study area. The following 8 species viz. Actinocythereis scutigera, Bairdoppilata (B.) alcyonicola, Callistocythere flavidofusca intricatoides, Cytherelloidea leroyi, Keijella reticulata, Loxoconcha gruendeli, L. mandviensis and Tanella gracilis occur in more than 75% of the samples with considerable population, suggesting that their carapaces are adapted to a wide range of tolerance to either active or passive parameters, such as surficial currents, salinity, temperature and high energy environment. Van Morkhoven (1962) and Reyment (1971) observed that shells with heavy ornamentation, amphidont hinge, much developed eye spots, sieve-type normal pores, surficial spines or ribs and subcentral nodes are characteristic of ultrasaline to euhaline condition. In the present euhaline environment, off Karikkattukuppam, species such as Actinocythereis scutigera, Callistocythere flavidofusca intricatoides, Chrysocythere keiji, Hemitrachyleberis siddiquii, Keijella reticulata, Lankacythere coralloides, Mutilus pentoekensis, Neocytheretta murilineata, N. snellii, Neocytheromorpha reticulata, Pterygocythereis chennaiensis, Puricythereis whatleyi, Stigmatocythere indica and S. kingmai occur with strong ornamentation, distinct hinge and eye tubercle. From the above observations, it is inferred that the faunal assemblage seems to be wholly consistent with deposition under tropical shallow water/inner neritic environmental conditions and on siltysand/sand or sandysilt substrates. Also, the smooth and finely pitted forms prefer finer substrate, while the highly calcified and ornamented forms prefer coarse-grained sediments.

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CONCLUSIONS

An analysis of the ratio between the carapaces and valves (ratio 1:2) indicates a moderately slow rate of sedimentation in the study area. A study on predation reveals that a few shells belonging to 22 species (seven smooth ones, 7 moderately ornamented and the remaining highly ornate forms) exhibit predatory holes. Out of the 7,896 ostracod specimens encountered, only 289 shells were drilled by predators (3.7% of the total population). A low percentage of predated shells is indicative of less depth (Murray, 1973). Out of the 4 types of ornate forms encountered in the study area, the smooth and finely pitted ones prefer finer substrate, while the highly calcified and ornamented forms prefer coarse- grained sediments. The presence of majority of pale yellow and white colored specimens suggests that the sediments were probably deposited under normal aerobic environmental conditions.

Acknowledgements

One of the authors (G.R.) is thankful to Prof. S.C.Khosla, M.L.Sukhadia University, Udaipur, and Prof. Q.A.Siddiqui, St. Mary’s University, Canada, for their valuable suggestions in this work. Authors are thankful to UGC-SAP and UGC-COSIST programs of the Department of Geology, University of Madras, for financial support. Thanks are due to Dr. S.G.D.Sridhar for his help during the field work. Authors also thank the Head, Metallurgy Department, Indian Institute of Technology (IIT), Chennai, for SEM photography and to Mrs. B.M.Indira Devi for cartography, respectively.

References

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