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Mollusca brings together a great deal of information about animals that at first glance appear to be radically different from one another—snails, slugs, mussels, clams, oysters, octopuses, squids, and others. About 600 million years ago, molluscs evolved in the Cambrian period. At length, the aplacophorans have been viewed as the most primitive molluscs and hence been considered of key significance to help us captivate the development of molluscs. Cutting edge aplacophorans are vermiform as in they all have an adjusted blueprint, no shells and a mantle with miniscule imbricating sclerites. Given their unusual appearance contrasted with alternate molluscs and their basic, diseased shape they have hence been thought to be primitive. On the other hand, the fossil record of aplacophorans are constrained and the main fossil up to this point that has been credited to the aplacophorans are the bisarre Acaenoplax from the Silurian of the UK. This fossil displays an inquisitive blend of aplacophoran and chiton like characters.
Advanced aplacophorans are vermiform as in they all have an adjusted diagram, no shells and a mantle with miniscule imbricating A developing thought has been that aplacophorans may be determined and that they are sister gatherings to chitons. Fossils, as Acaenoplax, propose that their precursor may even have had shells. This is reverberated in surviving aplacophoran embryology, which demonstrate that both real gatherings of aplacophorans shows a 7 fold cycle on the dorsal surface in late phases of trochophore hatchlings, which is lost amid transformation.
Molluscs are most diverse in the tropical environment and a vast majority are found intertidal, in estuaries and coastal lagoons, and in shallow areas on the continental shelf (Jara, 2009). Marine molluscs is a diverse group including Class Cephalopoda, Bivalvia, Gastropoda and Polyplacopora. The vast majority of these are marine and live on the ocean bed or tunnel in sand (Anandaraj, 2012). Molluscs are second only to Arthropod in numerical abundance. The number of species identified under phylum mollusca vary from 80,000 to 100, 000. They are more abundant in the littoral zones of tropical seas.
There are around 20,000 living types of bivalves. The greater part of these are marine and live on the ocean bed or tunnel in sand. There are about almost 50, 000 living species (Tangley 1997, Hickman et al. 2004) through 52 525 marine species (Bouchet 2006), 270 000 (Hawksworth and Kalin-Arroyo 1995), 70 000–75 000 (with perhaps more than 100 000) (Groombridge and Jenkins 2002), 81 000 (IUCN 2009b), 93 195 (Brusca and Brusca 2003), 110 000 (Hallan 2003) to 120 000 (Ponder et al. 2002). Hawksworth and Kalin-Arroyo (1995), Groombridge and Jenkins (2002) and Rosenberg (pers. comm.80) assessed a conceivable aggregate of around 200 000 species, and May (2000) if an appraisal of around 120 000. I have acknowledged a figure for the universe of c. 85 000 depicted species in light of 52 525 marine (Bouchet 2006), and 24 000 physical molluscs and 7 000 freshwater molluscs (Lydeard et al. 2004), with an aggregate world assessment of 200 000 species (Rosenberg pers. comm.80). Gauges for Australia are pretty nearly 8 700 portrayed species out of an aggregate of around 12 250 (DEH 2007).
Endemism of around 90% is accounted for in the 2001 Australian State of the Environment Report (DEH 2001), however Ponder et al. (2002) reported that just around 10% of tropical species (i.e. about â…” of all Australian species) and 95% of calm species are endemic, making an aggregate of around 38% endemism. There are 14 recorded undermined mollusc species in Australia (one undescribed) and one undescribed subspecies (DEWHA 2009a). Ten (or more the subspecies) are recorded as Critically Endangered and four as Endangered.
Gastropods and bivalves constitute 98% of the total population of mollusca. Gastropods is one of the largest and most important group within molluscs, which is more concentrated from mid to low water mark in the shallow intertidal and sub-tidal zones (Esqueda et al., 2000; Rochette & Dill, 2000; Mohamed, 2008, Ríos-Jara et al., 2009). They are the most fascinating group that hobbyist, businessmen, scientist and ecologist among others from around the world were concerned. There about 80,000 to 100,000 identified species under phylum mollusca and about 98 percent of the total population are the gastoropods and bivalves. 20,000 juvenile gastropods (Trochus niloticus) ranging from 4 – 7mm can consume an area of diatoms about 6.5 square meter within a week (Picardal et al, 2014). Terebra maculata (Linnaeus, 1758) ordinarily called Marlinspike, which is a vast, sand-staying, extended, thick and overwhelming shelled marine gastropod of the family Terebridae. Its length can achieve a greatest of 275 mm (Carpenter & Niem 1998) making it the biggest among six known species under the family Terebra (Ansari et al. 2006). Living molluscs range in size from microscopic snails to about 18 meters in squids. Most molluscs are free living multicellular animals and they have multi-layered calcareous conch on their back. They are supported by the exoskeleton to protect the soft organs including the muscular foot and other organs e.g. digestive system, reproductive system, excretion, respiratory and others.
Mollusca are extremely inexhaustible and structure a vital connection in the natural ways of life. Amongst marine items, molluscs constitute a palatable gathering by fish and scavangers. The gastropods and bivalves have a noteworthy natural part to play in the marine eco framework. Shellfish, Mussles, Clans, Pearl clams and Chank are the essential mollusks, misused in some other nation like India from time immemorial. The gastropods and bivalve fisheries are of sustenance nature and utilized for eatable reason, wellspring of lime, as embellishing shells (or) for modern reason. Mollusca are exceptionally rich and structure an essential connection in the natural pecking orders. Amongst marine items, molluscs constitute an eatable gathering by fish and crustanceans. Boring predation is the one of the survival instruments of molluscs particularly in the most caenogastropods, the Muricidea and Naticidea (Carriker, 1981) and some in the Marginellidae, Ranellidae, Fasciolariidae, Buccinidae and Tonnidae (Taylor et al, 1977).
Gastropods and bivalves with high monetary significance are broadly developed. Pearl shellfish culture and pearl cultivating is a multi-million dollar industry. A few animal categories (e.g. Tectus niloticus) utilized as a part of the generation of pearl catches had been transplanted outside their characteristic scope of circulation [5, 6], while endeavors to restore the populaces of overharvested species are broadly embraced [7-10] to fulfill the quickly expanding requests in the worldwide business. Molluscs, in general, had a gigantic effect on Indian convention and economy and were prevalent among regular individuals as decorations, money and doodad materials. Besides, Molluscs (shell) have been discovered to be critical crude materials for poultry encourages, concrete maker, manures. India exports sea shells primarily bivalves and substantial gastropods to different nations. Gastropods and bivalves have a noteworthy biological part to play in the marine environment. The gastropods and bivalve fisheries are of sustenance nature and utilized for consumable reason, wellspring of lime, as ornamental shells (or) for mechanical reason. In Sulu, the locals have used it as sustenance and the shells respected be of monetary significance. Despite the fact that, the current security risk ended up being valuable in liberating the territory from overexploitation of assets, the same issue had kept the locals unconscious and oblivious concerning redesigned data as to protection and society of the species which could conceivably be a wellspring of better occupation if given the correct exploratory learning. Shell assembling and exchange are vital fishery commercial ventures in an archipelagic nation like the Philippines. Shelled molluscs serve as wellsprings of protein and wage among seaside occupants (Schoppe et al. 1998, Del Norte-Campos et al. 2003, Floren 2003) and a noteworthy fare item (Hahn 2000; BFAR 2008). Monster mollusks (Cardiidae: Tridacninae), with just 11 surviving species (Othman et al., 2010; Su et al., 2014) around the world, are the biggest living bivalves that for quite a long time have been used as nourishment and their shells as decorations.
There are under 3000 types of molluscs are endemic in the Philippines Studies on molluscan species extravagance in Palawan are divided while studies managing all the more particularly with mangrove related molluscs in the territory appear to be non-existent. To date, molluscan differing qualities contemplates in Palawan are constrained toward the northern parts of the region where 716 species have so far been recorded and more than 200 species have been recorded in Tubbataha Reefs Natural Park. In any case, inordinate collecting of these mollusks for outer exchange has prompted neighborhood eradications of a few animal varieties in the Philippines, Micronesia, Singapore and Indonesia (see Tan and Zulfigar, 2003; Gomez and Mingoa-Licuanan, 2006; Othman et al., 2010; Neo and Todd, 2013). Every titan mollusk are recorded in Appendix II of the Convention on International Trade in Enganged Species of Wild Fauna and Flora (CITES, 2014). Hippopus porcellanus and the newfound three goliath mollusk species Tridacna costata, Tridacna rosewateri and Tridacna teveroa have the most confined topographical reaches (Othman et al., 2010), making them more helpless against gathering. In the Philippines, H. porcellanus has been accounted for just in the southern parts of Mindanao (Othman et al., 2010, Poutiers 1998b) and the Palawan gathering of islands (Villanoy et al., 1988). In 1993, just exhaust shells of H. porcellanus, H. hippopus and Tridacna derasa were experienced by Estacion et al. (1993) in Tubbataha Reefs Natural Park (TRNP). Around the same time, Calumpong and Cadiz (1993) discovered a couple of people of H. porcellanus, proposing that the species is seldom display or has been overexploited in the reefs of Tubbataha.
Uncontrolled fishing activities could have heavily impacted its molluscan fauna, a similar case for many paper marine sanctuaries in the country. With the gradual increase in the number of residents at the lower reaches of the river, species diversity may be affected with unregulated harvesting and mangrove cutting activities. The ecology of these organisms is considered to be affected by environmental factors like physico-chemical parameters (Garg et al., 2009), availability of food, competition, predator-prey interactions (Williams, 1970; Harman, 1972; McMahon et al., 1974; Lassen, 1975; Ofoezie, 1999), substrate architecture (Kershner and Lodge, 1990) and macrophytes (Bronmark, 1985; Costil and Clement, 1996; Ofoezie, 1999). Abiotic factors identified are substrate grain size, wave action, water content, and temperature and salinity of interstitial water (water contained in crevices and within the particles of the substrate). Unregulated exploitation and thereby population decline, however, have caused the volume of exported shells to dwindle over the years (Floren 2003; BFAR-NFRDI-PAWB 2005) resulting in the declaration of some species as threatened and endangered; specifically Fisheries Administrative Order 208, series of 2001, provides a list of mollusc species for which collection and trade is prohibited (DA 2001). Usually they are found in the water where calcium concentration is more (Tonapi, 1980). These bottom dwelling organisms play an important role in an aquatic community.