Keywords

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The most commonly encountered trace-making endobenthic invertebrates, their general habits and over all geographic distribution patterns are enlisted in Table 2.1. These organisms are overwhelmingly dominated by decapod crustaceans (mainly crabs). Some gastropods, bivalves, polychaetes and anomurans also produce distinctive traces and are prime associates of the decapods.

Table 2.1 Taxonomic classification, general characteristics and distribution of the common endobenthic trace-producing invertebrates of the study area
Full size table

2.1 The Trace-Producers

The trace-producing decapods belong to 6 families, 13 genera and 19 species (Table 2.1). Of these, four species of Ocypode, three species of Metaplax and two species of Uca are best represented in the study area as prolific trace-producers. Xiphosura Limulas sp. (horseshoe crab ), Mollusca Turritella spp. , Telescopium telescopium , Macoma birmanica and Tagelus plebeius , and Polychaeta Diopatra cupria are also common trace-producers. Certain lebensspuren of unknown producers were also observed associated with the traces produced by the above endobenthic organic groups . Pisces Periophthalmus koelrenteri is most common vertebrate trace-making associate. The collection of biological samples contains several species identified for the first time from the study area (Table 2.1). These common trace producers are closely associated with a large variety of invertebrate and vertebrate organisms (Fig. 1.10). Although taxonomically many of the trace producers have been renamed currently, the older biological names are retained here in accordance with the formal identification reports prepared by the Zoologists of the Zoological Survey of India, Kolkata, in 1970s. The taxonomic description of the trace producers is beyond the purview of the present work.

2.2 General Distribution Patterns

The distribution of the common trace-producers as well as their ichnological activities (Fig. 2.1), as mapped in the study area, defines two broadly discernable faunal zones: a narrower coastal faunal zone and a wider lower estuarine faunal zone (south to north). The zonal boundary is somewhat overlapping faunistically. Further towards north with decrease in salinity and tidal influence, the lower estuarine faunal zone transforms successively into upper estuarine and freshwater faunal zones (Fig. 1.5). The coastal faunal zone is composed of several coast-parallel faunal subzones. The lower estuarine faunal zone, in contrast, contains faunal subzones (Fig. 2.1) that follow essentially the river courses or banks. The subzonal boundaries are flexible towards environmental dynamism in space and time. The faunal zones and subzones define, in turn, the respective neoichnological zones and subzones (Fig. 2.1).

Fig. 2.1
figure 1

Reproduced from De (2009) published in Marine Georesources and Geotechnology, 2009, 7:2, 89–113, https://doi.org/10.1080/10641190802625601

Neoichnological (=faunal) zones and subzones in the study area.

Full size image

A comparative picture of geographic distributions, substrate affinities, general trace making habits and habitats of most common trace-making invertebrates as observed in the study area is presented in Fig. 2.2. The coastal zone trace makers include all the enlisted species of the crabs Ocypode and Metaplax, and Ilyoplax pusillus , Heteropilumnus ciliatus, Diogenes spp. and Eurycarcinus grandidieri besides polychaete Diopatra cupria , bivalves Macoma birmanica and Tagelus plebeius (Table 2.1; Figs. 2.1 and 2.2). The trace-making decapods in the lower estuarine zone include the crabs Dotilla brevitarsis , Varuna literata, Scylla serrata and Alpheus sp. (Table 2.1; Figs. 2.1 and 2.2). However, some of them often transgress into coastal faunal zone (e.g. Dotilla sp.). The crabs Charybdis rostrata, Macrophthalmus tomentosus and two species of Uca and gastropod Turritella spp. and Telescopium telescopium cover both the zones (Fig. 2.2).

Fig. 2.2
figure 2

General geographic distribution, substrate affinity, trace-making habits and natural habitats of the common endobenthic invertebrates and their associates in the Sundarban Delta complex, the Bay of Bengal coast, eastern India. 1–2: coastal faunal zone, 3: lower estuarine faunal zone

Full size image

The bivlalves Macoma birmanica , crabs Ocypode spp., Ilyoplax pusillus , Metaplax spp., Diogenes sp. and Xiphosura Limulas are typically sand dwellers (Fig. 2.2). Polychaete Diopatra cupria , bivalve Tagelus plebeius, gastropods Turritella and Telescopium and decapods Alpheus, Heteropilumnus ciliatus, Scylla serrata , Dotilla brevitarsis and Varuna literata predominate in the clayey (soft to firm) substrates (Fig. 2.2). Limulus is restricted to sandy foreshore beaches and islands. Uca in association with the mudskipper fish Periophthalmus koelrenteri predominates in the clay-rich mangrove creeks , backswamps and mudflats along the coast and in the clayey riverbanks in the lower estuarine zone.

The Bay of Bengal trace-making organisms have global distribution in lower to middle latitudes covering the coastal tracts of Japan, Formosa, China, Hawaii, New South Wales, Red Sea, Africa, Sind and Baluchistan, South Korea, Australia and Indian Ocean (Deb 1998). Ocypode, Ilyoplax, Uca, Diopatra, Turritella, Telescopium and Macoma are common in shoreline environments in most of the above areas. Xiphosura Limulus is known from northwest Atlantic coast, Gulf of Mexico, Asian coasts of Japan, Korea and Philippines (Ruppert and Barnes 2001). All the enlisted trace-making brachyuran crab species of the Sundarban area (India) are common in the check listed mangrove brachyuran crabs reported from Malaysia and Singapore (Cheryl and Peter 1994).

2.3 General Trace-Making Habits and Habitats

Studying the habits and habitats of modern endobenthic invertebrates, especially of decapods, had remained one of the major activities of Zoologists for several decades (Ruppert and Barnes 2001; Pechenik 2002). Although their life processes in response to variable temperature, water pressure (i.e. depth), salinity and atmospheric pressure have been documented in great details in the literature on zoology and animal physiology, the details of their trace-making activities and their governing factors, trace morphologies and preservation potential of traces have remained largely unexplored in the field of neoichnology . These data are important for palaeontological, sedimentological and environmental interpretations in ancient depositional basins.

Taxonomically the modern crabs form two groups: Brachyura, the true crabs and Anomura, the hermit crabs. The true crabs show wide range of habitat adaptations in marine, brackish and terrestrial freshwater environments (Williams 1965). The strictly freshwater crabs inhabiting in brackish to freshwater usually return to salt waters for breeding. All land crabs construct burrows to protect themselves under sediment cover and to retain required body water content. They obtain respiratory water by burrowing down to the local water table (e.g. Uca, Frey and Mayou 1971). The brachyurans form five nonexclusive life-style categories (Warner 1977): running , swimming, burrowing, camouflaging and commensal or parasitic, each having characteristic morphological adaptations and behavioral patterns .

Burrowing, feeding and locomotion of benthic crabs produce various lebensspuren preservable in sediments. Locomotion typically results in various surficial grazing marks , trail marks, tracks and trackways , besides others. Feeding traces are conspicuously produced by amphibious crabs (e.g., Uca, Ocypode and sand-bubbler crabs ) in the form of tiny sand balls or feeding pellets often arranged into spectacular designs. The fiddler (or calling) crabs usually produce radiating scratch marks , feeding and excavation pellets around burrow apertures (Curran and Martin 2003). Microbial mats are productive grazing grounds for the fiddler crabs. Swimmer crabs usually prey on plankton, but often seek temporary refuge in shallow burrows . There are two distinct behavioral patterns discernible among the burrowing crabs. The side burrowers construct more regular domiciles than the back burrowers. Most of the side-burrowers are terrestrial or amphibious. The burrows of the ghost crab Ocypode are typically zoned across a beach profile (Frey and Mayou 1971). The distribution and population density of burrows of the fiddler crab Uca in clayey sediments are known to be influenced by grass root mats (Ringold 1979). Some sub-littoral crabs construct burrows underwater (Rice and Chapman 1971). The burrows serve various purposes including dwelling, brooding, protection against predators , storing of food, reproduction (e.g., Uca) and copulation (e.g. some male individuals of Ocypode; Hughes 1973). The larval and juvenile development of many endobenthic invertebrates including decapods occurs in saline water column to ensure a large geographic distribution and to avoid competition with adults (Verde and Martinez 2004). Two mandatory functions of decapods are egg incubation and juvenile recruitment from a planktonic phase (e.g. Upogebia affinis , Frey and Howard 1975; Atkinson and Taylor 1988). These often result in the formation of burrow base brood chamber and a number of narrow juvenile shafts emanating from the basal brood chamber or the adult burrow tubes .

In the Sundarban area, the majority of the crabs belonging to Ocypodidae (Table 2.1) are usually cubical, thick, sediment eaters and amphibious in nature. Typically, they are beach-dweller, sand-loving, environment sensitive and prolific burrowers. They require saline water to moisten their gills at regular time intervals to survive and hence thrive in areas close to seawater or else they extend their burrows to local groundwater level. They feed nocturnally on discarded fish. The members of Ocypode are well known as sand crabs in Aldabra Coast where their males produce strikingly dimorphic (clockwise and anticlockwise spirals) burrows (Farrow 1971). As in the Aldabra Atoll, in the study area Uca prefers mudflats and mangrove creeks to sandy beach.

The genera belonging to the Family Grapsidae have adapted to saline, brackish and freshwater (Frey and Mayou 1971). In the study area, they are represented by three species of Metaplax in the sandy coastal faunal zone and Varuna literata in the muddy lower estuarine environments. Both the genera are sediment eaters and burrowers.

Genera belonging to the Family Portunidae are generally flattish, swimmer, plankton-eater and adapted to saline and brackish water. In the study area Scylla serrata is adapted to lower estuarine zone, while Charybdis rostrata thrive both in the foreshore sandy beaches and muddy lower estuarine river banks and bars. Although they are basically swimmers, both can construct burrows especially in firm to rigid clayey substrates traversed by mangrove roots. Representatives of the Family Xanthidae usually prefer rocky beach, firm mudground and woodground to soft clayey or sandy substrates. They are burrowers and borers. In the study area Eurycarcinus grandidieri bores into exposed palaeoforest (woodground). However, Heteropilumnus ciliatus produces both burrows and boring cavities in soft to firm clays and woodgrounds respectively exposed in foreshore beaches.

Alpheus sp. (Family Alpheidae) is habitually swimmer and adapted to saline and brackish water. It selects muddy substrates to burrow. Alpheus crassimanus and Macrophthalmus depressus association is known to be typical of waterlogged mudflats in Aldabra Atoll (Farrow 1971).

The Diogenes sp. is crawler hermit crab in tropical land and lives in association with polychaetes , gastropods and corals. They have vulnerable abdomen, which they protect by living within empty organic shells, especially gastropods that they carry with them as they walk. They can tolerate brackish to freshwater and normally breed in saline water. They are commonly seen in the Indo-Pacific regions living close to shore.

Turritella attenuata and Telescopium telescopium (gastropods) thrives in astronomical number preferably in swampy and muddy substrates of the backshore beaches, creeks and inland clayey riverbanks. Both the genera can be traced from saline backswamps and saltmarshes through estuarine zone to even freshwater river banks located far beyond the northern limit of the study area (De 1997).

The trackways and narrow burrow systems of mud skipper fish Periophthalmus koelrenteri , typical of mangrove backswamps and estuarine river banks , are produced over the entire study area excepting the sandy foreshore beaches. They are capable of moving fast on moist muddy substrates and climbing mangrove trees because of development of fused pelvic fins as very special anatomical specialization. As in Taiwan coast (Dörjes 1978), their ubiquitous association with muddy substrates is also evident in the study area.

Sparse resting traces of the horseshoe crab Limulas sp. in the sandy substrates are usually restricted to the foreshore region. Their resting traces in live condition are normally obliterated by body movement and ultimately destroyed by the wave and current actions. Dead organisms leave deep and preservable resting traces on sandy beaches. They live in shallow-water, soft-bottom plowing through upper sandy surface. Both the larva and adult make impersistent shallow burrows .

Mollusca Macoma birmanica is habitually a shallow burrower in sandy to clayey substrates of coastal islands and lower foreshore beaches, while Tagelus plebeius (razor calm) selects firm muddy substrates to form shallow burrows in large numbers. Tagelus plebeius is known from southeastern coast of USA (Ruppert and Barnes 2001).

Polychaeta Diopatra cupria is essentially a tube dweller and forms characteristic shaggy and membranous tube in intertidal anoxic mudflats.

Besides the above mentioned trace-making organisms, many integruing traces of unknown (or unclassified) organisms are also encountered in this region.