Benthic organisms are easily affected by anthropogenic aquatic discharges due to the particle reactive character of many metallic or organic pollutants. These organisms reflect exposure to toxic substances showing a broad range of responses, such as reduced locomotive ability, growth inhibition, morphological changes, alterations of populations and associations or biochemical reactions
Furthermore, benthic organisms, such as molluscs, capable of biomineralization may (i) incorporate pollutants in skeletal parts or shells; or (ii) they show growth characteristics indicative of adverse environmental conditions. If biomineral parts are preserved in sediments, they can therefore serve as environmental archives. This is particularly important in riverine systems, which are mainly dominated by coarse clastic sediments and thus show a lack of environmental archives compared to other aquatic systems (e.g. warves, peat bogs in limnic systems or carbonate reefs in marine systems). Riverine ecosystems are either dominated by freshwater or characterized by brackish conditions in coastal transition zones. In such systems aquatic gastropods represent the most abundant zoological order among the shell forming organisms. The group of grazing or carrion feeding benthic gastropods, which constitutes an important link between aquatic and terrestrial food chains, shows a range of habitat preferences and ecological tolerance (e.g. with respect to desiccation, salinity, food availability
The presence of shells, associations of species as well as shell growth characteristics thus yield information on the ecological conditions of the depositional environment and/or of the sedimentary provenance area. Therefore, the characterization and evaluation of gastropod shells in riverine sediments is an important research target, particularly in systems that are highly affected by anthropogenic discharges.
However, the preservation of gastropod shells can strongly be affected by mechanical, chemical or biological processes post-dating life of the individual organism. Such post-mortem effects, which are not indicative of the gastropod’s living conditions, must therefore thoroughly be described, characterized and evaluated in a taphonomic study if shells, their species associations or growth characteristics are intended to be used as environmental indicators.
Highly accelerated and uncontrolled urban development that can be observed on several continents is mainly focused in coastal areas around riverine or estuarine settings and places these ecosystems into the focal point of anthropogenic environmental impact. The megacity of Chennai (Tamil Nadu), one of India’s largest urban agglomerations with more than 10 million inhabitants, is a marked example of this development in a tropical region. With the two rivers Adyar and Cooum as well as coastal backwaters there are major freshwater and brackish aquatic settings showing significant environmental impact [4 and references therein]. The Muttukadu backwaters in the South of Chennai are representative of the large brackish water systems that occur close to the coast of this region (
While the surface distribution of recent gastropods was studied in lakes or ponds and ditches as well as in coastal settings of SE India
It is the aim of this study to deliver an overview of the presence and distribution of gastropod shells in sediments of the Adyar and Cooum rivers as well as of the Muttukadu Backwaters representing coastal brackish systems. Furthermore, the study aims at providing a thorough taphonomic characterization of shells allowing to evaluate their potential use as environmental indicator.
Chennai is the largest city in South India and characterized by a strongly enhanced economic and industrial growth in the last decades. This is in line with a tremendous increase of population reaching 11.12 x 106 inhabitants in the Chennai Metropolitan Area, covering 1189 km2
The investigation was carried out at sampling sites covering the Adyar and Cooum rivers from the rural suburban areas with an elevated proportion of cropland (
After air drying and sieving of sediments gastropod shells were sampled in fractions > 2 mm and optically inspected using a binocular lens with 10x magnification. Shells were carefully washed in water and cleaned with a soft brush. Some shells showed a sediment filling that could not be removed. Intensive cleaning with a harder brush or ultrasonic bath or mechanical attempts to remove sediments from the surface or internal volumes of the shell were not performed to avoid shell damage. Photographic documentation was carried out with a digital camera (Pentax K50) equipped with a bellow and a macro lens (Pentax DFA, 50 mm, F2.8).
Selected samples were carbon coated and investigated by scanning electron microscopy (SEM). Secondary and backscatter electron imaging was performed with a Quanta 650 F SEM (FEI/Thermo Fischer) operated with an acceleration voltage of 15 kV and a beam current of 10.00 +/- 0.05 nA.
Taxonomic identification was performed to the lowest level possible. In some cases, species level was achieved using descriptions given in
Incomplete shells and fragments were characterized. For indication of the shell area represented by a fragment, it was denoted according to the standard shell areas: 1 = apex, 2 = spire, 3= body whorl, 4 = outer lip, 5 = operculum, 6 = unknown, which is a slight modification of the key of
The preservation of shells or shell fragments was expressed using three ranks (i.e. good, fair, poor), which separately measure different taphonomic features
A total of 171 specimens of gastropod shells or shell fragments, which belong to nine different families, were found in the sediments of Adyar and Cooum rivers as well as of the Muttukadu backwaters (
|
|
|
|
|
|
|
|
A3 |
urban |
25 - 35 |
|
A5 |
rural |
0 - 5 |
|
A6 |
urban |
5 - 30 |
|
AD6 |
urban |
48 |
|
A7 |
rural |
0 - 15 |
|
AD7 |
urban |
- |
n.m.s.2 |
AD8 |
rural |
7 - 29 |
|
AD9 |
rural |
2 - 36 |
|
AD10 |
urban |
12 - 20 |
|
AD11 |
estuarine |
12 - 60 |
|
AD12 |
estuarine |
1 – 70 |
|
AD15 |
estuarine |
8 - 15 |
|
|
|
|
|
C1 |
urban |
5 - 30 |
|
CO1 |
estuarine |
5 - 30 |
|
C2 |
urban |
5 - 10 |
|
CO2 |
estuarine |
8 - 15 |
|
C3 |
rural |
0 - 10 |
|
CO4 |
urban |
- |
n.m.s. |
CO5 |
urban |
- |
n.m.s. |
CO6 |
rural |
- |
n.m.s. |
|
|
|
|
Mu2 |
backwater |
5 - 9 |
|
MuRef2 |
backwater |
0 |
|
Mu3 |
backwater |
1 - 25 |
|
Mu4 |
backwater |
1 - 81 |
|
Mu6 |
backwater |
1 - 16 |
|
Mu7 |
backwater |
15 - 35 |
|
Mu9 |
backwater |
17 - 35 |
|
1Upper and lower limit of shell containing sediment in cm
2n.m.s. = no molluscan shell
Shells of the genus
Gastropod shells are found in sediments of the entire study area from rural zones to the urban estuarine and backwater settings, although not in any sampling site. Two major groups of taxa can be distinguished, which do not overlap. These are
Several destructive post-mortem processes can be distinguished. They altered the gastropod shells to different degrees, which is expressed by individual rankings (good, fair, poor) for the different taphonomic features exfoliation, external luster, cracking, fragmentation, edge preservation, bioerosion and encrustation (
Almost all specimens are affected by exfoliation and have lost most or all of the periostracum. Only rare exceptions can be observed (
|
|
|
|
Bioerosion |
65.5 |
33.3 |
1.2 |
Fragmentation |
6.6 |
18.2 |
75.2 |
Cracking |
11.0 |
28.9 |
60.1 |
Exfoliation |
0.0 |
15.2 |
84.8 |
Edge preservation |
7.2 |
11.5 |
81.3 |
External luster |
2.4 |
17.0 |
80.6 |
Two specimens of
Exceeding the effect of biogenic activity mechanical processes, such as cracking or abrasion, also caused fragmentation or loss of shell matter. More than 90% of all shell specimens are incomplete, with fair or poor ranks of the taphonomic feature fragmentation. Almost the same percentage shows fair or poor ranks for the feature cracking. Fragmentation mainly affected apices as well as apertures. Cracking, which dominantly happened along whorls, most often left spires. Highly advanced fragmentation only left internal sections of the columella or parts of walls, for which assignment to either spire or body whorl is impossible.
Due to these post-mortem destructive processes most specimens are fragmented and represent different parts of the formerly complete shell. Their characterization using standard shell areas and combinations of them as shown in
|
|
|
1 |
apex |
0.6 |
2 |
spire |
45.6 |
12 |
apex+spire |
16.4 |
23 |
spire+body whorl |
7.0 |
123 |
apex+spire+body whorl |
6.4 |
234 |
spire+bodywhorl+outer lip |
5.3 |
1234 |
apex+spire+bodywhorl+outer lip |
8.2 |
2345 |
spire+bodywhorl+outerlip+operculum |
2.3 |
P2 |
planorbid-spire |
2.9 |
P23 |
planorbid-spire+body whorl |
2.3 |
5 |
operculum |
1.2 |
6 |
unspecified |
1.8 |
1 Standard shell areas, modified after [23], 1 = apex, 2 = spire, 3 = body whorl, 4 = outer lip, 5 = operculum, 6 = unknown, p = planorbid shell without apex
The composite taphonomic grade, averaging the individually ranked taphonomic features of each shell, yields a general characterization of the preservation of each specimen. Gastropod families represented by several specimens
The high number of shell specimens obtained in the limited volume of geoslicer samples proves the applicability of this tool for reconnaissance sampling and attests to the relatively high abundance of gastropod shells and shell fragments in the sediments. As this study aims at providing a first overview of the gastropod shell inventory in river and backwater sediments characteristic of the larger Chennai area, sampling was not carried out to distinguish or delineate gastropod habitats. Nevertheless, the suite of specimens allows to distinguish two different groups of gastropods. The species
Within the group of the freshwater gastropods, either rural or urban character of the environment are not reflected by the occurrence of individual species in the sediment, since
All of the freshwater gastropods represented in the sediments are at least in part reported from lakes, ponds and paddy fields of SE India (i.e. Porur Lake, Chennai,
The dominance of
Sediments of the Adyar and Cooum rivers as well as the Muttukadu backwaters show a rich inventory of gastropod shells. The specimens represent the following families and species with markedly contrasting abundance:
This study was funded by DFG grants SI810/2-1, TA 1071/2-1 to LT and SS, and it is an outcome of collaboration between the Institute of Applied Mineralogy and Economic Geology (RWTH Aachen University, Germany) and Annamalai University (India). Funding for field work by PB, NE and LB was provided by the Indo-German Centre for Sustainability. Support of Professor Srinivasan and his colleagues at Annamalai University and the Centre of Advanced Studies in Marine Biology (Parangipettai, India) is gratefully acknowledged. For the support during the field work, we want to thank the Public Works Department Chennai and the Chennai Rivers Restoration Trust. The authors thank R. Klinghardt for assistance during scanning electron microscopy, M. Dohms for help during sample selection and C. Lutter for literature retrieval.