Introduction

Ostracods are one of the most triumphant inhabitants of almost every aquatic ecosystem, which includes lakes, brackish water lagoons, mangroves, estuaries, freshwater streams, etc. (Hussain et al. 2007). The carapace of the ostracods is defined by their nature of ornamentation and shape, which varies immensely within their families. The shape of their carapace can be bloated, orbicular, deflated either vertically or laterally and prolonged (Martens and Horne 2009). The distribution and abundance of ostracoda in the marine environment are dependent on temperature, type of sediment, depth, salinity, oxygen and the supply of food. (Neale 1964; Bonaduce et al. 1975; Pokorný 1998; Yassini 1979; Lachenal 1989; Cronin et al. 2002; Didié et al. 2002; Horne 2007; Boomer 2002; Ruiz et al. 2006; Martínez-García et al. 2013). The fossil ostracods are being extensively used in the reconstruction of paleo-environment and paleo-climate (Hussain and Cassey 2016). The bi-valved nature of the ostracods aids in easier preservation and are further analysed for various studies (Martens and Horne 2009). Many researchers have carried out studies on the ostracods in the Bay of Bengal sediments which includes: off Peninsular India (Mohapatra et al. 1992), East coast of India (Hussain and Rajeshwara Rao 1996), Gulf of Mannar (Hussain et al. 1996,1998; Hussain 1998), off Rameswaram (Sridhar 1996; Baskar et al. 2015), off Tuticorin (Hussain et al. 1996), off Pentakota and Kalingapatnam (Naidu et al. 1997), off Karikkattukuppam (Mohan et al. 2001, 2002; Hussain et al. 2009), the inner shelf of Chennai (Hussain et al. 2004, 2009), off Chennai (Hussain et al. 2004), East coast of south India (Mohammed 2006) southern east coast of India and Andaman Islands (Hussain et al. 2007), off Punnaikayal (Ganesan et al. 2008), Kameswaram coast (Elakkiya et al. 2013), Palk strait (Baskar et al. 2013), Northwestern part of Bay of Bengal (Nishath et al. 2015), North Chennai coast (Hussain and Cassey 2016), off central Bay of Bengal (Nishath et al., 2017). The Andaman Islands to have some records of ostracods presence. Some of the work done by the researches are; Madhupratap et al. 1981 found a fair abundance of ostracods presence while investigating for the zooplankton abundance at the Andaman sea. Hussain et al. (2006) have reported the occurrence of ostracoda to be in a minimal amount at the hut bay of Andaman also the reported form of species was found to thrive at Neritic zone.

The carapace-valve ratio accounts for the statistical studies in Ostracods such as closed or isolated valves, Adult or juveniles, left or right valves etc. (Guha 1982). According to Oertli (1971), the higher ratio of carapace-valve suggests rapid sedimentation occurring in that particular region; also, it disarticulates the carapace into separate valves. The sediments mainly droving the carapace - valve ratio at off Chennai region: smooth and finely pitted were preferred for finer substrate whereas the highly calcified and ornamented ones were preferred for the coarser substrate (Hussain et al. 2004); in the north-western part of Bay of Bengal most of the specimens were open valved and a minor amount of them had a complete carapace indicating a slower rate of sedimentation that took place under normal oxygenated environmental conditions (Nishath et al. 2015); off central Bay of Bengal the carapace-valve ratio suggests that there was a lesser amount of transportation of terrigenous material by the rivers from the shallower to deep marine regions which exhibits a gradual and slower rate of sedimentation from the shelf to slope region (Nishath et al. 2015). Most of the population abundance of Ostracoda at off Tuticorin and off Karaikkattukuppam is controlled by the relative increase of temperature, salinity and dissolved oxygen content in the bottom waters (Hussain et al. 1996, 2009). The Ostracoda population are also correlated with the sediment characteristics such as organic matter, calcium carbonate and sandy-silt-clay ratio. Wherein the south-east coast of India and the Andaman Islands, the accommodative substrate for the desirable thriving of the ostracods is found to be silty sand (Hussain et al. 2007). In Palk Bay, the favourable substrate is found to be of silty sand, followed by the sand (Sridhar et al. 1998). The seasonal observations on physicochemical parameters were correlated with the population of the Ostracoda, which exhibited a positive correlation for depth, pH, salinity, density, TDS, nitrite and silica. In contrast, a negative correlation was observed for turbidity, nitrate and phosphate (Baskar et al. 2015). Nishath (2017) has reported ostracods from the samples collected in two transects on the northern part of Nellore shelf. The main objective of the present study is to evaluate the abundance and diversity of ostracods and their relationship with environmental parameters like sediment types and physicochemical parameters of bottom water to know the factors influencing ostracods population. Hence, we have carried out a detailed investigation on ostracods in the entire shelf region off Nellore in the Bay of Bengal.

Material and Methods

Study Area

The area of investigation is located in Southwestern part of the Bay of Bengal, India (13.83706 N 80.30813 E to 14.91744 N 80.39536 E) covering an extent of 185 km (Fig. 1). This region experiences a tropical and humid climate conditions with an annual mean temperature of about 29 °C. The coastal brown sands are composed of sandy silt and clays, muddy and silty sands. These sediments, along with the estuarine and alluvial deposits present along the river banks, are transported through the rivers of Pennar, Swarnamukhi and Uppateru. The major contribution of sediment is found to be through the River Pennar, annual flow approximately 3228 million cubic seconds and discharges annually 6.9 × 106 tons of sediment loads to the Bay of Bengal (Rao et al. 1979). Residual weathering of the Archean rocks has led to the formation of capping’s, low-level laterite and sand mixed secondary laterite. These are the important geomorphic features in the coastal region. The other sources of sediment supply to these coasts are provided by the currents, wave action and geomorphic features (Anbuselvan and Senthil Nathan 2017, 2018).

Fig. 1
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Study area map with sample location

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Sample Collection (Surface Sediments and Bottom Water)

A total of 23 surface sediment samples were collected along five transects across the shelf region of Southeastern part of Bay of Bengal. Samples were obtained from sea bottom at different water depths ranging from 10 to 200 m using a Van Veen grab sampler (Table 1). The bottom water samples were collected using Niskin water sampler from all the sediment sampling locations, and their physicochemical parameters such as pH, salinity, temperature, conductivity and dissolved oxygen were measured in the field (Table 3). Both sediments and bottom water samples were collected in a cruise during September 2015. (Coastal Research Vessel Sagar Purvi, cruise IMO no. 9123829, National Institute of Ocean Technology, Ministry of Earth Science, India).

Table 1 Sampling geographical coordinates and depth
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Grain Size Analysis

Initially, the sediment samples were air-dried and then was held back in the oven for 50 °C for further drying. For carrying out the granulometric analysis, 100 g of dried sediment samples were taken which was then sieved with the supporter of stock of ASTM sieves 2.0 mm [−1 Φ (phi)], 1.0 mm (0 Φ), 0.5 mm (1Φ), 0.25 mm (2Φ), 0.125 mm (3Φ) and 0.63 mm (4 Φ). Weight per cent is used to express the amount of fraction for each sample. The sand and mud content for each of the sample is known with the guidance of Udden-Wentworth grain size scale (Folk 1954); whereas the mud content used for the assistance of textural class is known after Pejrup (1988); Reineck and Siefert (1980), modified after (Flemming 2000) (Table 2).

Table 2 Sample number, percentage of sand and mud, and textural class based on mud content after Reineck and Siefert (1980) and Pejrup (1988), modified Flemming (2000)
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CHNS Analysis

The sediment samples were dried in an oven to 50 °C and were then homogenised before the analysis was carried out. Using FRITSCH Pulverisette 7Agate Ball Mill, the samples that were homogenised were crushed to a finer powder. For decarbonisation, to be processed 1 N solution of Hydrochloric acid was taken for each 5 g of the samples; after this process, the samples were washed thrice by using deionised water from a centrifuge machine that removes the HCl absorbed from the sediment. The following samples were further dried, homogenised and then analysed with the aid of CHNS elemental analyser (Mode: Vario el cube Odu). The resulting values were taken as total organic matter content for each sample (Table 2).

Ostracod Analysis

The samples were washed by using a 0.063 μm sieve; later dried and were observed under a stereo binocular microscope and were arranged in the faunal slides for further analysis. The recent Ostracoda taxa were identified by following Moore (1961), Van Morkhoven (1963) and Hartmann and Puri (1974) classifications and along with other relevant literature has been followed in the present work. Then the identified specimens were cleaned with distilled water under the microscope and mounted on an aluminium stub fixed with double adhesive carbon tape. The specimens were then coated with carbon in the sputtering machine and made ready for Scanning Electron Microscope (Hitachi, Model: S-3400 N). The abundance and distribution of ostracod assemblages present at five transects are discussed further in detail.

Data Analysis

Multivariate statistical analyses such as Cluster (Q and R mode) analysis and diversity indices were carried out with the help of IBM SPSS (version 20) and Paleontological Statistics Software (PAST-Version 2.17). Shannon diversity index (H) was calculated using the following equation: H (S) = Σ [(ni/ n) × ln (ni/ n)], where “n” is the total count of individuals and “ni” is the number of individual present at each taxon (Shannon 1948; Murray 2006). Q-mode cluster analyses were performed to identify the similarity between sampling stations with respect to depth, grain size, organic matter and bottom water parameters. Abundant species (larger than 5%) in the analysed samples were taken for the R mode cluster analyses using the correlation coefficient method.

Results

In the study area, all the environmental parameters and ostracoda species are associated directly with depth changes, it can be eventually inferred that it is a significant ecological factor to be considered.

Bottom Water Characteristics

A similar trend is observable for both of the temperature and pH values; maximum temperature and pH are noticeable at transect 2 with a temperature of about 28.05 °C (35 m); pH of about 8.15 whereas the minimum temperature and pH values are observed at transect 3 with a temperature of 20.6 °C (105 m); pH of 7.61 (Table 3). It can be inferred that as the depth increases the temperature and pH decreases gradually. In the study area, the salinity ranges from 27.1 psu to 32.7 psu (Table 3) The maximum salinity is observed at station 17 (105 m) and the minimum at station 23 (16 m) which infers an increase in salinity with an increasing depth. The dissolved oxygen content in the study area has maximum value observed at station 23 (16 m) with 6.97 mg/l whereas the minimum value is observed at station 7 (200 m) with 3.02 mg/l (Table 3). The dissolved oxygen in bottom water is found to be high in the shallower region and low in the deeper part.

Table 3 Physicochemical parameters of the bottom water
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Sediment Characteristics

Textural analysis reveals that the sediment grain size distribution is more associated with depth. The present study area is covered by following type of sediments sand, slightly muddy sand, muddy sand, sandy mud. On considering the depth of about 10-200 m from the study area the following substrates were observed to be occurring: At 15 m depth, the sediment is characterized by sand to muddy sand in composition; in 30 m depth the sediment differs from slightly muddy sand to sand; at 80 m depth the range of sediments is found to be from muddy sand to sandy mud; at 100 m depth the variation of sediments is observed to be muddy sand to sandy mud and at a depth of 200 m it varies from sandy mud to slightly muddy sand in composition (Fig. 2).

Fig. 2
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Spatial distribution of map showing the Sand and Mud (%)

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Organic Matter Characteristics

The northern part of the study area is found to have higher TOC (Total organic carbon) and TN (Total Nitrogen) content when compared to the southern part. The TOC concentration value ranges between 0.10–1.7% with a mean value of 0.90%; whereas the TN concentration value ranges between 0.08–0.26% with a mean value of 0.18%. The TOC content is found to be increasing in all the four Transects (1, 2,4 and 5) except for Transect 3. The TN content is found to increase with water depth for Transects 3, 4 and 5 whereas decreasing for 1 & 2. The TOC concentration values are found to be highest in Transect 1 (1.79%) and lowest in Transect 2 (0.10%); the TN concentration values are highest in Transect 4 (0.26%) and lowest in Transect 2 (0.08%). The organic matter distribution is found to be more associated with mud compared to sand (Fig. 3).

Fig. 3
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Spatial distribution of map showing TN and TOC content

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Ostracod Characteristics

Twenty-two ostracod taxa belonging to 18 genera, 7 families, 5 superfamilies, 2 suborders and 2 orders has been identified. The ostracods identified (Table 4) from the study area are as follows: Cytherelloidea chapmani (Jones and Hinde 1849), Cytherelloidea maalaccaensis (Leroy, 1940), Hemikrithe orientalis (Van den Bold 1950), Actinocythereis scutigera (Brady, 1868), Alocopocythere kedengenesis (Kingma 1948), Echinocythereis sp (Puri 1954), Keijella reticulata (Whatley and Zhao 1988), Keijella karwarensis (Bhatia and Kumar 1979), Stigmatocythere indica (Jain 1978), Chrysocythere sp (Ruggieri 1962), Pterygocythereis chennaiensis (Mohan et al. 2001), Neocytheretta murilineata (Zhao and Whatley 1989), Loxoconcha sp. (Sars 1866), Propontocypris sp. (Sylvester-Bradley 1946), Cytherella sp (Jones, 1849), Lankacythere sp (Bhatia and Kumar 1979), Lankacythere multifora (Mostafawi,1992), Bairdoppilata paraalcyonicola (Titterton and Whatley 1988), Phlyctenophorea orientalis (Brady 1868a, b), Argilloecia sp (Sars 1866) and Pistocythere sp (Brandão and Karanovic 2020) (Fig. 6). At a depth of 15 m, Keijella reticulata is observed to be the dominant species occurring followed by Hemikrithe orientalis, at 30 m depth Hemikrithe orientalis predominates over Keijella reticulata and at 50 m depth again there is an abundance of Keijella reticulata observed followed by the abundance of Actinocythereis scutigera. Hence, the overall depth up to the range of 50 m is found to have species dominated by Keijella reticulata, Hemikrithe orientalis and Actinocythereis scutigera. At about 100 m depth Bairdoppilata paraalcyonicola predominates over Actioncythereis scutigera and at 200 m depth Keijella reticulata is found again to be the abundant form occurring followed by Bairdoppilata paraalcyonicola. So, on considering the overall depth of up to 200 m the study region is dominated by the species Bairdoppilata paraalcyonicola, Actinocythereis scutigera and Keijella reticulata. (Fig. 4). The carapace-valve ratio was observed to be at a ratio of 1:9, where the presence of open valves were higher in number than the carapace. The species diversity varies from 0 to 2.28. Highest is observed at station 18 and lowest at station 6. The dominant ones are found to be in the sub-orders of Cytherocopina and Bairdiocopina, Keijella reticulata is observed from the station 1 to 26 and most dominantly observed at station 21; Bairoppilata paraalcyonicola is observed from stations 11 to 26 and is most dominant at station 11. The remaining forms of taxa are represented by the sub-orders of Cypridocopina (Phlyctenophorea) which exhibits an occurrence at station 23.

Table 4 Abundance and occurrence of ostracod species at different depths of each sample
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Fig. 4
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Map showing the spatial distribution of the abundant ostracod species

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Diversity Indices and Cluster Analysis

The calculated values of Shannon diversity indices (H) varies from 0 to 2.28. Dominance was ranging from 0.12 to 1, Evenness from 0.45 to 1, Fisher alpha between 0 and 7.74 and Equitability from 0 to 1 (Fig. 7). The resulting dendrogram of Q-mode (Fig. 5) and R-mode cluster analysis (Fig. 6) represents the grouping of samples according to grain size, bottom water depth and abundance species (> 5%). Based on the Q and R mode cluster analysis, the study area can be divided into two segments and two species assemblages. Cluster analysis (R-mode) of abundant ostracod species (higher than 5%) distinguishes two assemblages; Assemblage I (Hemikrithe orientalis, Pterygocythereis chennaiensis and Lankacythere multifora) and Assemblage II (Actioncythereis scutigera, Bairdoppilata paraalcyonicola, Stigmatocythere indica and Keijella reticulata) (Figs. 6 and 7).

Fig. 5
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Dendrogram of Q-mode cluster analysis of the samples. Based on the total abundance of species higher than 5%. Two segments recognised

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Fig. 6
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Dendrogram of R-mode cluster analysis of the species based on the total abundance of the species higher than 5%. Two clusters recognised

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Fig. 7
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Plots of diversity indices of Ostracods

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Cluster I

The ostracod assemblage from the shallowest region ranges between 0 and 80 m water depth and is composed of the shallow ostracod species. The accommodative substrate is characterised by sandy to slightly muddy sand, but clay fractions are also found to be high at some of the locations sampled (Table 2). The ostracod assemblage is dominated by Hemikrithe orientalis, Pterygocythereis chennaiensis and Lankacythere multifora (Fig. 4).

Cluster II

This group of ostracod assemblage holds up the highest diversity of population from 80 to 200 m water depth. The accommodative substrates of sediment are slightly muddy sand to slightly sandy mud in composition (Table 2). Abundant species present over here are Actioncythereis scutigera, Bairdoppilata paraalcyonicola, Stigmatocythere indica and Keijella reticulata (Fig. 4).

Discussion

The study area is covered by four types of sediments: sand, slightly muddy sand, muddy sand and sandy mud. The shallower part of the shelf (up to 30 m depth) is dominated by sandy sediments (expect transect 4 and 5) that are deposited under wave action, indicating high energy conditions. When there is a reduction in wind and current speed, finer particles from the rivers settle to the bottom, which is responsible for the higher mud content in the shallower region (15 m depth). The particle size analysis reveals that the mud content increases with depth. The higher amount of mud content in the deeper part of the study area indicates low energy condition prevailing in this region (Anbuselvan and Senthil Nathan 2017, 2018) (Fig. 8).

Fig. 8
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(Plate): Lateral view of selected ostracod species. LV-Left Valve, RV- Right Valve: 1. Isolated valve of Loxoconcha sp.,RV; 2. Complete carapace of Chrysocythere sp. LV; 3. Isolated valve of Actinocythereis scutigera RV; 4. Isolated valve of Bairdoppilata paraalcyonicola LV; 5. Isolated valve of Keijella reticulata LV; 6. Isolated valve of Pterygocytheresis chennaiensis LV; 7. Isolated valve of Stigmatocythere indica LV; 8. Isolated valve of Alocopocythere Kedengenesis RV; 9.Complete carapace of Echinocythereis sp., LV; 10. Complete carapace of Keijella karwarensis RV; 11. Isolated valve of Lankacythere sp., RV; 12. Complete carapace of Neocytheretta murilineata RV; 13. Isolated valve of Phlyctenophorea orientalis LV;14. Complete carapace of Lankacythere multifora LV; 15. Isolated valve of Argilloecia sp., LV; 16 & 19. Isolated valve of Porpontocypris sp., LV;17. Isolated valve of Cytherelloidea sp., LV; 18. Isolated valve of Echinocytheresis sp., LV

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The Rivers Pennar and Uppateru strongly influence the bottom water characteristics through the input of water that controls the pH, temperature, salinity, oxygen and nutrient-rich layer. The bottom water parameters, such as temperature and pH gradually (10 to 200 m), decreases with depth. The observed salinity value fluctuates within a narrow range, and it remains around 29.5 o/oo at all depths except at 105 m the value is 32.7 o/oo. The submarine groundwater discharge and freshwater input are the causes for lower salinity at the shallower depth (Anbuselvan and Senthil Nathan 2017, 2018). The dissolved oxygen content of bottom water ranging from 3.02 to 5.95 mg/l, shows a decreasing trend with depth. The higher concentration of the dissolved oxygen content in the shallower part is due to the photosynthetic activities of phytoplankton. The bottom water below 100 m depth, is observed to be low-oxygenated due to prevailing low energy conditions.

The most abundant ostracods species present in the shallower region are Hemikrithe orientalis and Lankacythere multifora and are associated with sand to slightly muddy sand. This region exhibits a lower salinity level due to the freshwater input. With an increase in temperature, salinity and water depth, the abundance of ostracod population tends to increase away from the shore region (Zhao et al. 1985). The higher concentration of dissolved oxygen in the shallower part is due to the influence of turbidity action in the nearshore region and also due to photosynthetic activities of phytoplankton. All of these above reasons act as a favourable substrate for the thriving of the species mentioned above. Hemikrithe orientalis was first reported on the Indo-Pacific waters by Van den Bold (1950) in the grey mud at 78.6 m water depth; in Indian waters, they were first recorded by Hussain et al. (2009) and Mostafawi (1992) first reported Lankacythere multifora.

Keijella reticulata, Bairdoppilata paraalcyonicola, Actinocythereis scutigera and Stigmatocythere indica are the most abundant forms in the middle shelf region, where the salinity and dissolved oxygen content are found to be at a moderate level. The sediment in these regions is composed of muddy sand to sandy mud which is a favourable substrate for the thriving of these species. Whatley and Zhao (1988), first reported Keijella reticulata from Malacca strait. In Indian waters, this species was reported from the east coast of India off Cuddalore by Hameed and Achyuthan (2007). The east coast of India is dominantly found to have the occurrence of Keijella reticulata when compared to the west coast of India (Hussain 2017). Maddocks (1969) first reported Bairdoppilata paraalcyonicola from the Mozambique Channel. This species was reported at 73 m depth from off central Bay of Bengal by Nishath et al. (2017) which was one of the most abundant species. Similarly, in our study area, Bairdoppilata paraalcyonicola occur at the middle shelf region, below 100 m depth. Actinocythereis scutigera has been first reported by Brady (1868a, b). In Indian waters, Mohan et al. (2001) reported the dominance of both Actinocythereis scutigera and Keijella reticulata living species at off Karaikkattukuppam owning to their occurrences in tropical and shallow-water forms.

The deeper part consists of the Ostracod species: Pterygocythereis chennaiensis where the sediment is characterised by muddy sand to sandy mud in composition. The salinity in this region is observed to be increasing, whereas the dissolved oxygen content seems to be decreasing due to lower energy conditions prevailing in this region. The thriving environment of most of the living ostracods are not observed under lower oxygen content or at anoxic conditions; very few of the inhabitants over here (Elofson 1941). An earlier researcher reported that a few species of ostracod could survive in an anoxic environment (Elofson 1941). Pterygocythereis chennaiensis was initially reported at the south-east coast of Chennai at off Karaikkattukuppam by Mohan et al. (2001). The distribution and abundance of this species at off Karaikkattukuppam was controlled by water depth, salinity, and calcium carbonate content. Also, they noticed the living population was maximum during April than July, which is possibly due to higher temperature, salinity and dissolved oxygen of the bottom waters (Mohan et al. 2002). However, in the present area, Pterygocythereis chennaiensis is found to be more associated with high salinity and low temperature and dissolved oxygen of the bottom water conditions. Hence, we can infer that the overall thriving nature of Pterygocythereis chennaiensis in the study area is mainly influenced by salinity. The characterization of carapace was found to be thinner at the deeper region and thicker at the shallower region which reflects a gradual increase from coarse to fine sediments towards the deeper part; the overall carapace ornamentation is observed to be fresh and shiny. Similar observation was reported in the adjacent area (Nishath et al. 2017), the ostracod population contains only 10% of the carapace and the rest of 90% of open valves. The identified species were observed to be a mixture of adult and juvenile forms; also, they exhibited post-mortem transport referring to thanatocoenosis assemblage (Boomer et al. 2003).

Conclusion

From the 24 surface sediment samples, 21 species of Ostracoda were identified which belongs to 18 genera. The most favourable substrate for the thriving of these ostracods was found to be in mud dominated environment. The abundant forms of ostracod recorded are Keijella reticulata, Bairdoppilata paraalcyonicola and Hemikrithe orientalis. Among these Keijella reticulata is found to be the most abundant form distributed over the entire shelf region of the study area. The maximum population of ostracod observed at 100-200 m water depth, which is characterised by a moderate salinity and low dissolved oxygen content due to the lower energy condition induced in the environment. About 90% of valves and 10% of carapace where observed in the study area, which indicates a slower rate of sedimentation.