Assistant Professor

Department of Geology

Durgapur Government College, Durgapur

Paschim Bardhaman, West Bengal, India

Abstract

Intense predation pressure caused changes in behavioural pattern of bivalve families during the faunal turnover across Palaeozoic to Mesozoic era. There was peak rise in family level from epifaunal to infaunal life mode during that time interval. The present study was undertaken focusing on the infaunal bivalve Sanguinolaria acuminata and Sanguinolaria sanguinolaria of the Chandipur-on-sea, Odisha, in an effort to document the bivalve ecology and taphonomic constraints on the shell of the bivalve genus. The total population of both the species have been analysed in the three seasons namely winter, pre-monsoon and post-monsoon. The temperature, salinity, hydrogen ion concentration (pH), conductivity is also studied. Along with this the density of the individual valve are measured to understand taphonomic constraints. Thus, a comparative study was also undertaken between two species of a deep burrower bivalve genus, Sanguinolaria acuminata and Sanguinolaria sanguinolaria. Species abundance of both species have been analysed throughout a year, taking into consideration different physico-chemical parameters and biological interaction like predation vis-à-vis taphonomic constraint to infer the possible causes in disparity in occurrences of the species.

Key words: Bivalve, deep-burrower, Sanguinolaria, infauna, Chandipur.

Introduction

Chandipur, in Odisha, India (Fig. 1), is famous for vast tidal flat and occurrences of diverse marine fauna. An astonishing diversity of molluscs has been reported from Chandipur, Odisha. At Chandipur the shore slopes vary gradually and at the time of ebb-tides a 5km stretch of littoral area gets exposed (Mukherjee and Chakraborty, 1987). The molluscs found there include estuarine species as well as those which are truly marine (Subba Rao et.al, 1991). The bivalve community that was encountered during our study, consists of a total number of 22 species belonging to 16 genera; among which our prime focus was on the study of the two species of Sanguinolaria. It is observed that both the species Sanguinolaria acuminata and Sanguinolaria sanguinolaria are almost absent in the beach area of Chandipur, while about 3km away from the beach in the sand bar near their habitat in the intertidal zone they are present in huge number. Besides, Sanguinolaria sanguinolaria are much more abundant than Sanguinolaria acuminata. The total population of both the species have been assessed during the month of January-October 2019 and valves were collected by grid sampling. Ecology of the species of Sanguinolaria and taphonomic constraints applicable on its shell have been studied.

Sanguinolaria is a marine bivalve mollusc of the family Psammobiidae. They are suspension feeders feeding on the food particles that are suspended in water. The two species of Sanguinolaria was collected from the Chandipur sandflats from three seasons in winter, pre-monsoon and in post-monsoon. The systematic status, habitat and distribution of these species have been discussed here. The main objectives of the study are:

1.     To study the adaptive features of Sanguinolaria species.

2.     To study the role of siphon for their deep burrowing mode of living.

To study the role of taphonomic biasness for availability of shells of Sangunolaria during sampling.

Fig 1a. Geographic location of Chandipur, Odisha and major geomorphic units.

 

Fig. 1b. Sample collecting localities and different microenvironment in Chandipur, Odisha.

Materials and Methods

Both the species of Sanguinolaria, Sanguinolaria acuminata and Sanguinolaria sanguinolaria were studied in the field and individual measurements of each species were made by using vernier caliper. Sanguinolaria acuminata and Sanguinolaria sanguinolaria were collected from the Chandipur sand flats, lower littoral zone and from the mudflat areas during the period of January 2019 to October 2019. The observation and collection for this study were made on monthly basis during low tide when the lower littoral zone was accessible for proper investigation. Horizontal and vertical surveys were made in different time and tidal conditions (high-tide and ebb-tide). One thousand five hundred and fifty-seven specimens were examined during our field study in Chandipur area. The two species of the bivalve were carefully sorted out in two different sample bags and counted by hand picking methods. A grid of 10m by 10m was made by measuring tape and then the individual species were collected. During sample collection and observation, taphonomic grade, abundance, size variation of the individual species were noted. Sediment sampling from the habitat of the animal was made by inserting spade into the mud without disturbing the animals and then lifting the mud into a sample plastic bag.

On-site water sampling was made to measure the following parameters- temperature, salinity, conductivity and hydrogen ion concentration (pH). Global Positioning System (GPS) was used to find out the locations (latitude, longitude) of the sampling site. Parameters like temperature, salinity, pH, conductivity were recorded day to day by using Philip multimeter (OAKTON).  The sediment samples from the habitat location of Sanguinolaria are first dried in a hot air oven and after the complete drying of the sediment these are sieved in the laboratory by recording different fractions of sediment using sieving mesh to know about the nature of grain size of sediment where they live. For this study mesh size from 325 No. to 80 No. are taken and it has been seen that both species live in a region that consists of 9.8% of fine sand, 33.978% of very fine sand and 56.221% of silt. In the laboratory the density of both the right and the left valve of bored and unbored shell of different size are measured individually to know about the bored nature of the species. The articulate valves are also measured to know which valve is denser among right and left valve. For this study firstly we must measure the weight of the individual valve (both right and left valve) in a weighing scale then by measuring the volume (by taking water at 4°C temperature and put the valve into it and take the reading) and thus we get the density value of the individual valve (Density= Mass/Volume). The densities of the different sizes of the species are also determined.

Systematic description

Phylum- Mollusca

Class- Bivalvia

Subclass- Heterodonta

Order- Cardiida

Superfamily- Tellinoidea

Family- Psammobiidae

Subfamily - Sanguinolariinae

Genus- Sanguinolaria

Sanguinolaria acuminata Deshayes 1857

(Fig. 2a-b)

Type locality:  Chandipur sand flats and mouth of the river Subarnarekha.

Maximum size of collected specimens: Length: 9.042cm; Height:3.9cm

Minimum size of collected specimens: Length: 2.884cm; Height:1.246cm

Shell description:  Shell is moderately large, thin, flat valves almost oval in outline, two bifid teeth present in each valve and the pallial sinus equal on both the valves extending up to three-fourth length of the shell. The shell is bluish white in colour, covered with a blackish olive epidermis, muscle scar well developed, anterior end rounded whereas posterior end acuminates with obtuse angle; gap on posterior end is broad; posterior end narrower than anterior. The umbo is often a rounded knob-like protuberance, usually surrounded the beak, interior of the shell is purplish in colour and the hinge ligament is external and large. The mantle forms a thin membrane that covers the animal body and the mantle edges fuse at the posterior end of the shell to form two siphons (paired siphons), one is inhalant and the other is exhalant and these siphons are separated and quite long and are used in suspension feeding of the bivalve shell.

Distribution: India: Chandipur, Subarnarekha estuary, Mahanadi estuary, Puri coast, Gnjam coast, Talsari in Odisha; Hugli Matla estuary, Digha, Sagar Island, Medinipur, North and South 24 Parganas in West Bengal (Subba Rao et al. 1992); Astamudi near Travancore in Kerala, Godavari estuary in Andhra Pradesh, Maharashtra; Elsewhere- Karachi in Pakistan, Bangladesh, Sri Lanka, Myanmar, Malaysia, Malacca, Philippines, China and Japan (Subba Rao et al. 1991).

Remarks:  Animals lives inside burrow upto a depth of 2-3 ft within lower tidal zone (Mitra et.al 2010).

Sanguinolaria sanguinolaria (Lamarck, 1799)

(Fig. 2c-d)

Type locality:  Chandipur sand flats and mouth of the river Subarnarekha.

Maximum size of collected specimens: Length:9.2cm; Height:3.02cm

Minimum size of collected specimens: Length:3.84cm; Height:1.684cm

Shell description: The shell is moderately small, thin, elliptical in shape having a streamline body. The shell is bluish purplish in colour and no blackish olive epidermis is present, equivalve and inequilateral and the muscle scars are well developed. Pallial line is non-entire with deep pallial sinus and the bivalve shell has a desmodont dentition pattern. The anterior side of the shell is rounded, and the posterior side is slightly angular. The hinge line is straight, and an external ligament groove is present, and escutcheon are present on the posterior side. Interior of the shell is purplish in colour and the shell possesses a concentric ring like ornamentation. During our study season only small and medium sizes of this species are found.

Distribution: India: Chandipur, Subarnarekha estuary in Odisha (Mitra and Mishra, 2006; Mitra el al., 2010); Madras, Gulf of Mannar, Cuddalore, Annamalai, Nagapattinam in Tamil Nadu; Astamudi backwaters near Travancore in Kerala; Bombay in Maharashtra; Gulf of Kutch in Gujrat; Elsewhere: Sri Lanka; Malaysia; Malacca; Philippines and Japan (Subba Rao et al. 1991).

Remarks:  This species is found mainly towards the sea are differ from the preceding one by the presence of thin shell, absence of olive epidermis and by colour variation.

Size variation: The size variation of the two species of Sanguinolaria that is Sanguinolaria acuminata and Sanguinolaria sanguinolaria are studied in three seasons, Winter (January), Pre-monsoon (April) and Post-monsoon (October). The size variation of Sanguinolaria acuminata in winter season ranges from length 8.952 cm to 2.884 cm, height 3.8 cm to 1.246 cm whereas for Sanguinolaria sanguinolaria, length ranges from 9.2 cm to 3.02 cm and height from 4.16 cm to 1.02 cm respectively. In pre-monsoon the size of Sanguinolaria acuminata length ranges from 8.808 cm to 4.044 cm and height from 3.726 cm to 1.648 cm and in Sanguinolaria sanguinolaria length ranges from 8.3 cm to 4.52 cm and height from 3.84 cm to 1.684 cm. Similarly in the post-monsoon season the size of Sanguinolaria acuminata ranges length from 9.042 cm to 3.808 cm and height from 4.068 cm to 1.482 cm and Sanguinolaria sanguinolaria ranges length from 8.914 cm to 3.352 cm and height from 4.088 cm to 1.45 cm. This size variation indicates that the largest size is mainly found in post-monsson season October and the smallest size found during pre-monsoon season April.

Habitat preference: The species of Sanguinolaria are mainly found in the sheltered bays, slough, and estuaries in loose sand, mud or mixed coarse and fine sand. In our study area Chandipur, an estuary these species are found at the entrance channels washed by fast moving tides. By the action of these high tides some are found in the fine sand but most they prefer sticky mud, actually both the species are mainly found in silty sand.

Data analysis

The present study was carried out mainly on the two species of Sanguinolaria: Sanguinolaria acuminata and Sanguinolaria sanguinolaria that were collected and counted from the sandflats and lower littoral zone of Chandipur.

Monthly variation in species distribution:  Both the size of the species is individually measured by vernier calipers and then plotted on an excel sheet. The variation in species distribution for three seasons (winter, pre-monsoon and post-monsoon) during the study is shown in the Fig. 3.

Physical variables: The temperature of the surface air and tide water during the investigation period showed a more or less parallel trend of change through the seasons and thus the temperature ranges from maximum 30.9°C, 28°C and minimum temperature of 24.3°C, 22.8°C respectively. The tide water temperature was high and remains stable throughout the period which indicates that this temperature has no effect on the ecology of this species. The salinity of the tide water is the highest 9.72 ppt in the month of April and lowest in the 4.46 ppt in month of October. The pH varies between 9.81 to 8.84 during the period of observation and the conductivity varies between 16.71 to 8.11 during the period of observation.

Fig 2. A disarticulated specimen of Sanguinolaria acuminata preserving the original shell material of the right valve with its external view (a), concentric growth lines and bored shell; and its internal view (b), deep pallial sinus. A disarticulated specimen of Sanguinolaria sanguinolaria preserving the original shell material of the left valve with its external view (c), concentric growth lines and bored shell and its internal view (d). Scale bar = 2cm.

Results and discussion

During the study from January to October 2019, and subsequent distribution when plotted in a histogram, it has been noted that the maximum and minimum number of species are found in January and October respectively with regard to S. acuminata and S. sanguinolaria. Among both the species, Sanguinolaria sanguinolaria were found in lesser number than Sanguinolaria acuminata. The monthly variation of species distribution in the sampling area suggested that the minimum number of specimens were recorded during winter (January) and the maximum number of specimens were recorded during post-monsoon (October). By studying the species in these three seasons it has been noted that Sanguinolaria acuminata is dominated than that of Sanguinolaria sanguinolaria.  The nutrient rich estuarine waters and its bottom sediments formed by sand and clay mixture with a high organic matter, provides an ideal habitat for a large number of this species (Mitra et al., 2010). Further the mangrove vegetation along the criss-cross channels of the river mouth before joining the sea also facilitates this species. The variety and abundance of both the species can often be used as an indication of the status of the environment in which they live. According to the habitat most of the species are found in the mud and some of them are found in the sandy beach. The density of the species during each season has been measured which show that the right valve is denser than the left valve, while it has also been observed that the right valves are less bored by predatory gastropods than the left valves.

Fig 3. The season-wise distribution of Sanguinolaria acuminata and Sanguinolaria sanguinolaria during pre-monsoon, post-monsoon and winter.

The temperature was recorded high in the pre-monsoon and low during the winter and have in annual bimodal temperature oscillation seemed to be a characteristic phenomenon in this ecosystem. In the estuarine environments the salinity is affected tremendously by the additional freshwater freshets from the upstream river and surface runoff during monsoon every year. The average salinity of the tide water is 36.2ppt. The post-monsoon periods may be considered as a mixed period because the water is diluted with the just over seasonal rains, followed by the winter months with minimum record of temperature and precipitation. The maximum and minimum value of conductivity was 16.71ms and 8.11ms in the month of April and October respectively. The pH range varies from 9.81 to 8.84 during the period of observation. Sanguinolaria acuminata and Sanguinolaria sanguinolaria exhibits suspension feeding behaviour with exhalant siphon that collect food particles from sea water and are buried at a considerable depth of 10-12.5 cm beneath the sediment. Their siphons are long and are tube like in shape by which water flows (or more rarely on which air flows). In Sanguinolaria paired siphons are present which are situated at the posterior edge of the mantle cavity, and they are mainly attached with the substrate. Throughout the geological time of bivalve evolution, it has been seen that long siphons are present in burrowing bivalve which allow them to hide, buried below the surface of the sea floor. From the fossil as well as extant records it has been noticed that long siphons are inferred by the presence of pallial sinus observed interior of the shell, this is the chamber into which clam retracts, its siphons, so any bivalve that has pallial sinus marked on its shell must have had siphons. The main function of this siphon is to reach up to the surface of the sediment, so that the animal can respire, feed, and excrete and also to reproduce.

The density differences between the valves of Sanguinolaria acuminata and Sanguinolaria sanguinolaria are also examined. The left and the right valve of both the species, from the three seasons have been measured for the weight and volume to know the density values of individual specimens. It has been noted that the right valve of both the species are denser than that of their left valve, and for the S. sanguinolaria the density of shell varies isometrically with length of the shell (Fig. 4). On the other hand, in case of S. acuminata density variation remain less common with variation in size and it is also poorly correlated (Fig. 4).  Thus, the overall results indicates that the density of shells of S. acuminata is greater than that of S. sanguinolaria. As the shells of S. acuminata are more abundant in this area which indicates a taphonomic biasness towards availability of dead shells S. acuminata in comparison to S. sanguinolaria.

Fig 4. Scattered diagram showing body size vs. shell density of Sanguinolaria acuminata and Sanguinolaria sanguinolaria.

a) Disarticulation: The process of disarticulation is especially frequent among bivalves, both in recent and fossil shells, after decaying of soft parts and the organic ligament that join the valves together and here in our study area mostly the right valves as well as the disarticulated shells are found.

b) Fragmentation: Breakage of skeletons is usually an indication of high energy resulting from wave actions, currents, tides or winds (Parsons and Brett, 1991). Bivalve shells are tending to break along weakness lines or areas of the external ornamentations such as ribs, ridges and the shell of S. sanguinolaria are very thin and breaks easily so the bored valves cannot readily sustain mechanical breakage, and this also indicates a taphonomic biasness regarding availability of S. acuminata in dead shells.

Sanguinolaria prefers to live in calmer environment, within the silty mud substrate as deep infauna. Sanguinolaria has long siphon which create pressure within the shell to generate high fluid movement within the pedal part to protrude within the substrate and it does not need rocking movement of shell for emplacement it’s body within the sediment. Hence compactness of shell has also played a major role in postmortem-rare-survival of the shell after decomposition of visceral mass in case of S. sanguinolaria, as it has relatively less robust shell than S. acuminata.

Acknowledgement

PR gratefully acknowledge the infrastructural facilities provided by the Principal, Durgapur Government College for carrying out the research work.

References

1.     Mitra, S., Misra, A. and Pattnayak, J.G., 2010. Intertidal Macrofauna of Subarnarekha Estuary, Balasore, Orissa. Occasional Paper No. 313, Record of the Zoological Survey of India, Kolkata, 97 pp.

2.     Parsons, K. and Brett, C., 1991. Taphonomic processes and biases in modern marine environments: An actualistic perspective on fossil assemblage preservation. In: S. Devonian (Ed.), The Process of Fossilization. Belhaven Press, p. 303-323.

3.     Mukherjee, K.K., Das, S. and Chakraborty, A. 1987. Common physical sedimentary structures in a beach related open-sea siliciclastic tropical tidal flat at Chandipur, Orissa, India and evaluation of the weather conditions through discriminant analysis. Senckenbergiana Marit., 19: 261-293.

4.     Subba Rao, N.V., Rao, S. and Maitra, S., 1991. Marine Molluscs in Fauna of Orissa, (Part 3), State Fauna Series 1: Zoological Survey of India, p. 1-176.

5.     Subba Rao, N.V., Dey, A. and Barua, S., 1992. Estuarine and Marine Molluscs in Fauna of West Bengal, (Part 9), State Fauna Series 3: Zoological Survey of India, p. 129-268.