Molluscs in Mangroves

Molluscs in Mangroves : A Case Study

Prof. A. Shanmugam and Mrs. S. Vairamani

Centre of Advanced Study in Marine Biology Annamalai University

n the geological time scale Molluscs evolved about 600 million years ago during the Cambrian period. The name ‘Mollusca” was first used

by Linnaeus in the year 1757. Structurally Molluscs are a heterogeneous group of animals with different structural form such as slugs, mussels, octopuses and snails. Majority of the mollusks are known by their shell; but in some forms the shell is absent. Molluscs have been classified based on their morphological, anatomical and biological features.

Molluscs are second only to Arthropoda in numerical abundance. The number of species identified under Phylum Mollusca vary between 80,000 to 1,00,000. Molluscs have colonized all possible habitats fro deep sea to high mountains. They are more abundant in the littoral zones of tropical seas. Gastropods and Bivalves constitute 98% of the total population of mollusca and they inhabit land, freshwater and marine environments. The other classes of Mollusca are exclusively marine.

Gastropods in Mangrove Ecosystem

The gastropod molluscs, represented by snails, whelks, cowries, limpets, sea hares and their allies, are among the commonest epifaunal species that exist in the mangrove ecosystems. The gastropods are suitably adapted to various macrohabitats of the mangrove ecosystems. Marine species are found in the bottoms as well as in water bodies, the pulmonate snail and several other groups have conquered mangrove lands with the elimination of the gills and conversion of the mantle cavity into lungs. The mangroves provide ideal conditions for higher productivity of gastropods which in turn, serve as food, particularly the veliger larvae for numerous other animals. Because of their predatory nature, the gastropods occupy a central role in maintaining the functioning and productivity of mangroves through “cleaning” root systems from the encrusting fauna like barnacles. The snails also serve

I

372 Molluscs in Mangroves : A Case Study

as intermediate host for many trematode parasities. Based on the structure of the molluscan assemblages, the pollution damage in mangrove forests can be assessed.

Thus gastropods have a significant ecological role to play in the mangrove ecosystems. However very little information is available on the gastropod biodiversity of mangroves. Hence it is necessary to document the biodiversity of the group of threatened ecosystems. Bivalves in Mangrove Ecosystem

Mangroves are highly zoned, typically occupying the upper half of the eulittoral and dominating the supra littoral fringe. They grow best in the soft mud and these two aspects alone partially explain the lack of the data on mangrove bivalves; the bivalves are in general best adopted to lower tidal levels and to firmer deposits. Lasaea rubra, for example, is one of the few bivalves capable of colonizing the high inter tidal almost world wide (Morton, 1960), although, as well as seen, the Spartina marsh associate Geukensia demissa has similar adaptations to a high zoned life (Lent, 1969), as do deposits by a filter feeder enhance the very real problem of the gill clogging and sediment removal.

Thus, little is known of mangrove bivalves, especially those few species which appear to be endemic components of the mangrove forests. As will be seen, large number of bivalves have been recorded from the seaward fringe of the mangroves, and their status as true mangrove associates is dependable apart from the obvious difficulties of working in a mangrove forest, numerous authors (Warner, 1969; Sasekumar, 1974; Murty and Balaparameswara Rao, 1977) ignored the bivalves in favor of the more active and therefore more conspicuous mangrove associates (the gastropod) (Robertson, 1960; Brown, 1971; Vermeiji, 1974).

Coomans (1969) has drawn attention to the inherent interest in mangrove molluscs and Bouchet (1977) has provided data on West Africa mangrove molluscs, drawing on the data by Binder (1968) on Ivory coast mangrove and by Coomans (1969) on the Caribbean fauna to compare the molluscan fauna of various mangrove regions. However, even these authors emphasize the mangrove gastropods, although there are mangrove bivalves of some interest and occasionally, such as the mangrove oysters (especially Crassostrea rhizophorae in the Caribbean) of much wider economic potential.

A. Shanmugam and S. Vairamani 373

Method of Collection

For the quantitative analysis, the mangrove molluscs were collected by hand picking in a transect of known area or using a quadrate of known size. At the same time the foulers like mussels and oysters were collected by scrapping those using knives or spatula from a known unit area either using a quadrate. Further the infaunal bivalves were collected by hand digging the substratum (Alfred et al., 1997). The arboreal forms were collected from the stems, roots and other parts of the mangrove trees vertically at every 25 cm height by hand picking (Sasekumar, 1974).

The diagrammatic profile of a mangrove swamp showing a scheme of the essential features of animal zonation is depicted below.

Identification of Gastropods

The shell characters such as shape, spire length and shape, mouth opening, opercular shape, umbilicus shape and size, colour and ornamentation of the shell are used mainly for the identification of gastropods apart from the internal characters of which the important one is radula. Identification of Bivalves

The bivalves are identified mainly based on the shell morphology. The shell comprises of two valves. If the valves are similar, the shell is said to


be equivalve (clams, mussels); if dissimilar, ineqivalve (scallops). The outer surface is usually covered with a periostracum. The outer surface may be striated or ribbed. The two valves are held together by an elastic ligament, which leaves a scar on the hinge. The hinge may in addition have interlocking ridges called the dentition. The individual ridges (or teeth) may be similar. The two valves are attracted to the soft body by adductor muscles that produce scar on the interior surface. If each valves has a single such scar, the shell is said to be monomyarian. If there are two scars on each valve, the shell is dimyarian. At hinge, the shell has a projection called the umbo; this always points towards the anterior end of the animal (i.e., the end where the mouth is). Thus we can distinguish an anterior adductor scar and a posterior adductor scar in dimyarian shells. A slender scar often touches these two that marks the attachment of the mantle folded into a posterior siphon for conveying water away from the body when the animal is feeding by converted ciliary currents such shells show a pallial sinus in the pallial line. Preservation of the sample

Preservation of sample is carried out in three stages namely, narcotization or anaesthetization, killing and fixation and permanent preservation. The process of narcotization ensures that organisms are expanded fully displaying their characteristic features.

Menthol / Magnesium chloride: The animals are kept in clean water in an enamel tray / Petri dish / bowl depending on the size of the sample. Powdered menthol or magnesium chloride is sprinkled over the water and covered with a lid. The sample is left undisturbed for at least 12 hours.

Alcohol or Chloral hydrate: 70% Ethyl alcohol or 1% Chloral hydrate is added drop by drop are frequent intervals to water in which animals are kept and ensuring that the sample is covered with a lid.

The next step is fixation. After ensuring that the animals are narcotized they are transferred to containers with fixatives. The common chemical used for fixation of animals in the field is 4 to 10% neutral formalin solution. For molluscs, ethyl alcohol is the best known killing and preserving medium.

The animals are finally preserved either in 4% formalin or 90% alcohol or rectified spirit.


Molluscan species in India

We made a molluscan survey along the east coast of India, which occupies 57% of total Indian mangroves. This coast is endowed with the world’s largest single block of mangrove forest, the Gangetic Sunderbans in West Bengal with an area of 2109 km2 in India. The coast is also colonized with luxuriant mangrove forests in the States of Orissa, Andra Pradesh and Tamil Nadu.

Table 1. Distribution of molluscan species on east coast of India

Name of mangrove areas in east coast of India

No. Name of species Su

ndarbans

Bhita

rkan

ika

Mah

anad

i

Muriagang

a

Krishna

God

avari

Pichavaram

Muthu

pet

1 Amphineura sp. ● 2 Assiminea beddomeana ● ● 3 A. brevicula ● ● ● ● ● 4 Auricula gangetica ● 5 Bactronophorus sp. ● 6 Bankia spp. ● 7 Cassidula nucleus ● ● ● 8 Cerithidea cingulata ● ● ● ● ● ● 9 C. obtusa ● ● ● ● ● 10 C. alata ● ● ● 11 Crassostrea gryphoides ● 12 C. madrassensis ● ● ● ● 13 C. cuttackensis ● 14 Dicyathifer sp. ● 15 Dostia crepidularia ● ● 16 Ellobium aurisjudae ● ● ● 17 Littorina melanstoma ● ● ● 18 L. scabra ● ● ● ● 19 Lyrodus sp. ● 20 Martesia sp. ● 21 Melampus ceylonicus ● ● 22 M. pulchella ● ● ● ● 23 Meretrix meretrix ● ● 24 Modiolus metcalfei ● 25 Nassa sp. ●


26 Nausitora sp. ● 27 Nerita articulata ● ● ● 28 N. violacea ● 29 Pseudonerita sulculosa ● 30 Neritina (Dostia) violacea ● ● ● 31 Nodilittorina ● 32 Nototeredo sp. ● 33 Onchidium sp. ● 34 O. tenerum ● 35 O. tigrinum ● 36 O. typhae ● 37 Perna viridis ●

38 Pila globusa (fresh water phyla)

●

39 Polymesoda bengalensis ● 40 Potamacmaea fluviatilis ● 41 Pythia plicata ● ● ● ● ● ● 42 Saccostrea cucculata ● 43 Stenothyra deltae ● 44 Tanysiphon rivalis ● 45 Telescopium telescopium ● ● ● ● ● ● 46 Teredora sp. ● 47 Thais sp. ● Biodiversity Measurement

1. Species Diversity (Shannon and Weaver, 1949)

2. Species Richness (Gleason, 1922)

3. Dominance Index (Mc Naughton 1967)

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4. Evenness (Pielou, 1966)

Factors affecting biodiversity and conservation, in general

The following are some of the important factors that are affecting

molluscan diversity in general.

1. Irrational and Over‐exploitation

In some locations, Parangipettai coast, Kakinada Bay and canal, the molluscs are exploited mainly for lime production and ornamental purposes and now to certain extent animal feed in aqua‐industry. For example nearly 22 bags of T. telescopium and 2‐3 bags of other gastropods were fished and sent to the lime industry every week during the six month long period of observation during 1986 from Pichavaram mangroves and Vellar estuary areas along the east coast of India (Kasinathan and Shanmugam, 1988).

2. Human activity stress in the natural bed or habitat (mangrove vegetation)

3. Human Exploding population:

The population increase has led to the increase in the production of wastes both industrial and domestic. The dumping of wastes in the breeding grounds of mollusks led to the mass mortality of molluscan forms (for example oyster beds) apart from making those sites unfit for any molluscan culture. Further for the recolonisation of the molluscan fauna in such areas shall take longer period.

4. Natural Calamities:

The sudden heavy downpour, floods, cyclones, draught and Tsunamies cause mass mortality of the burrowing mollusks. Further the mollusks in the rivers and estuaries are being carried by the flood water into coastal water where these molluscan forms die.

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5. Expansion of Agricultural diversity in India

The usage of fertilizers, pesticides and insecticides to increase the agriculture production to meet out the demand of the exploding human population lead to the arrival of excess / unutilized fertilizers, pesticides and insecticides into the coastal ecosystem via the rivers not only spoils the natural habitats of the mollusks but also pave the way for the bioaccumulation of the pollutants in many species of mollusks making them unfit for human consumption.

6. Habitat Destruction due to reclamation activities such as dam construction, factories, highways, mining operations (sub‐fossil collection) etc.

7. Illegal trade and poaching of the species under danger

8. To a lesser extent the professional shell collection is also affecting the molluscan resources.

Conservation Strategies

Biodiversity is the hot topic like hole in the ozone layer and global warming all over the world. There are biodiversity conventions signed by many countries to save their biological diversity. In the past one decade it has taken its momentum and its protection is the talk and need of the hour. The studies on biodiversity will not only give a clear idea about the resource assessment of any biotope / ecosystem but also ensure the sustainable use of coastal habitats which include biodiversity. The resource assessment is the basic pre‐requisite hot only for any type of biodiversity or faunal assemblage studies but also form the basis for IUCN categorization of the same in any biotope / ecosystem.

a. Regulation of mesh size of the hand dredge and dredging mainly in estuarine and shallow coastal habitat – Kalanadi estuary & Vembanad Lake – P. malabarica, M. casta & Villorita sp.

b. Strict vigilance in exploitation of undersized animals – Ashtamudi estuary – P. malabarica

c. Issuing license for fishing (like chank) & Lime shell fishing rights issue of license as practiced by the State Dept. of Mining & eeology (Kerala Miner Minerals Concession Rules, 1967)

d. Imposing ban on fishing in the spawning season (“Fishing Holidays”) – undersized chanks and Egg masses in Trawl catches along TN coast


e. Educating the fisherfolk about the need for conservation

f. Commercial leasing out of estuaries and natural beds – in consultation with experts from National Committee on Marine Parks

g. Demarcation of the area for dredging of the subfossil resources after thorough and detailed Geological investigations. For example, the mining of subfossil deposite in the estuaries and river beds in Kalanadi and Vembanad lake damages the natural habitat and adversely affect traditional occupation of fisherman (Nayar et al., 1984; Achari, 1988).

h. To replenish the clam resources, “Clam sanctuaries” or” Clam Park” in known clam fishing estuaries.

i. Clam farming by semi – culture (transplanting the seed clams from dense beds to other suitable places in the estuary) is suggested to augument production.

j. Identification of the species under threat and ranching (Giant clam in Lakshadweep and Andaman & Nicobar Islands).

k. If possible, paying some compensation or providing basic amenities or arranging some alternating job / income source during the fishing closing season

l. Since the pearl oysters in the natural beds are influenced by many biotic and abiotic factors in terms of their recruitment, growth, health etc. a “Pearl Breeding Reserve” in the same habitat (GOM) may be maintained; Likewise the “Chank Breeding Reserve”

m. Declaration as Prohibited areas for fishing ‐ In Andaman Nicobar islands – Trochus & Turbo – overexploitation by Divers lead to a decrease in the landings in recent years. So areas around Little Andamans, Nicobar, Katchai and Comorta islands upto 500 m from shore line

n. Resource survey should be intensified to collect more information on distribution of species / population / communities and their economic potentials and IUCN categorisation

A Case Study

Until 1994, there was no licensing system in Ashtamudi Lake. In 1994, State Government after having discussions with clam fishermen,


scientists and government officials laid down the following conservation measures.

i. Imposing ban on clam fishing (Fishing Holidays) from October to January when spawning and spat settlement occur

ii. Mesh size regulation in the dredge ‐ Paphia : more than 30 mm & other clams : more than 20 mm

iii. Meat count of clams exported ‐ always less than 1400 nos/kg

iv. Strict vigilance in exploitation of undersized clams References

Alfred, J.B., R.K. Varshney and A.K. Ghosh (Eds.) (1997). An assessment manual for faunal biodiversity in South Asia. SACEP/NORAD publication series on Biodiversity in South Asia No. 1: 181pp.

Binder, E. (1968). Repartition des Mollusques dans lalaguna Ebrie (Cote d’ Ivoire). Cah. ORSTOM, Ser. Hydrobiol, 2 : 2‐24.

Bouchet, P. (1977). Distribution des mollusques dans les mangroves du Senegal. Malacologia, 16 : 67‐74.

Brown, D. A. (1983). Gonad development and spawning in Anadara granosa (L.) (Bivalvia: Arcidae). Aquaculture, 30 : 211‐219.

Coomans, H.E. (1969). Biological aspects of mangrove molluscs in the West Indies. Malacologia, 9 : 79‐84.

Kasinathan, R. and Shanmugam, A. (1988). Overexploitation of molluscan fauna in the Vellar estuary and Pitchavaram mangroves. Galaxea, 7 : 303 – 306.

Lent, C.M. (1969). Adaptation of the ribbed mussel, Modiolus demissa (Dillwyn), to the intertidal habitat. Am. Zool., 9 : 283‐292.

Morton, J.E. (1960). The functional morphology of the British Ellobiidae (Gastropoda: Pulmonata) with special reference to the digestive and reproductive systems. Phil. Trans. R. Soc. (B), 239 : 89‐160.

Murty, A.S. and Balaparameswara Rao, M. (1977). Studies on the ecology of mollusks in a South Indian Mangrove Swamp. J. Moll. Stud., 43 : 223 – 229.

Robertson, R. (1960). The mollusk fauna of Bahamian mangroves. Bull. Am. Malacol. Union, Inc., Annu. Rep., 26 : 22‐23.

Sasekumar, A. (1974). Distribution of the macro fauna on a Malayan mangrove shore. J. Anim. Ecol., 43 : 51‐69.

Vermeij, G.J. (1980). Drilling predation in a population of the edible bivalve Anadara granosa (Arcidae). Nautilus, 94 : 123‐125.

Warner, G.F. (1969). The occurrence and distribution of crabs in a Jamaican mangrove swamp. J. Anim. Ecol., 38 : 379‐389.


Animals in the field

Melampus – crawling on the mud Pythia plicata – found on the mangrove leaf of Sonneratia sp.

C. obtusa found on the stem of Avicennia sp.

Littorina melanostoma found on the mangrove Leaf of

Rhizophora sp.

Onchidium crawling on the mud Pythia plicata crawling on the mangrove root system


Animals in the field

Ellobium found on wooden crevices Faecal matter of Ellobium found inside the damaged stem

C. cingulata crawling on the mud along with Assiminia

N. articulata found attached to the dykes

Assiminea crawling on the muddy substratum

Quantitative assessment of T. telescopium using quadrate

Molluscs in Mangroves

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