Bulk of the harvest in India is only from the natural seaweed beds of Gulf of Mannar Islands. The data on seaweed landings in Tamil Nadu reveal that the quantity (dry weight) exploited in a year is in the range of 102-541 tonnes for Gelidiella acerosa, 108-982 tonnes for Grcilaria edulis, 2-96 tonnes from G. crassa, 3-110 tonnes for G. foliifera and 129-830 tonnes for G. verrucosa (Silas and Kalimuthu, 1997; Ramalingam, 2000). Euchema cottonii has been introduced in the Gulf of Mannar for commercial farming. This particular species is from Thailand and has been introduced to provide a livelihood for locals. Since it is commercially very important for the production of agar. However, its deleterious effect on reef associated native species remains a matter of great concern (Arjun Rajasurya et al., 2000).
Seaweed cultivation in India is still at a neonatal stage and field cultivation of only some economically important seaweeds has been attempted. Over-utilisation coupled with short supply, and loss due to natural calamities like cyclones has prompted the cultivation of seaweeds, since cultivation helps conserve natural resources and improve elite germplasm. This enables farmers to get better and better varieties of sea weeds.
Sea grass: Sea grasses form 0.1 to 0.2 per cent of aquatic flora and provide a major habitat for the Dugong dugon, the only herbivorous marine mammal. Sea grasses also provide a habitat for a wide variety of meiofauna and flora, benthic flora and fauna, epiphytic organisms, plankton and fish.
Sea grass meadows account for 15 per cent of the ocean’s total carbon storage, with the oceans absorbing 25 per cent of all global carbon emissions. It is estimated that sea grasses/square metre are capable of binding about 1000 gm of carbon/ year.
Of the 60 species of sea grasses in the world’s oceans, 14 species are found in the Indian seas. Eleven species of sea grass are found in the Palk Bay, 13 in the Gulf of Mannar, nine in the Andaman and Nicobar Islands and seven in Lakshadweep (Kannan et al., 1999).
Often found in association with coral reefs, the sea grass ecosystem is one of the most widespread coastal vegetations. Indian sea grass habitats are mainly limited to mud flats and sandy regions, extending from the lower inter tidal zone to a depth of 10-15 m along the open shores and 5-6 m in lagoons around islands. Unfortunately, sea grasses are significantly declining everywhere in their vastness and density.
Mangroves: Occurring in 112 tropical countries of the world, mangroves cover an area of 1,89,399 square km, encompassing a hundred different species. In India, mangroves cover about 4,740 square km, with about 57 per cent of them along the east coast, 23 per cent along the west coast, with the remaining 20 per cent in the Andaman and Nicobar Islands. Mangrove formations can be deltaic, backwater-estuarine and insular.
Deltaic mangroves occur mainly along the east coast, the backwater-estuarine type along the west coast and insular in the Andaman and Nicobar Islands.
India accounts for 69 species of mangroves. While many species are cosmopolitan in distribution, five species, Aegialitis rotundifolia, Heritiera fomes, H. kanikensis, Rhizophora annamalayana and R. stylosa, are restricted to the east coast and one, Lumnitzera littorea, is present only in the Andaman and Nicobar Islands (Kathiresan, 1999). Mangroves serve as home to several faunal communities and in India, harbour over 2,359 faunal species, of which 20 are rare, endemic
Mangrove ecosystems are globally threatened by anthropogenic factors and climate change. In the Andaman and Nicobar, geomorphological changes following the 2004 tsunami and earthquake have resulted in massive mangrove loss. Adverse effects on mangroves could lead to serious consequences for the adjoining delicate and important ecosystems, such as coral reefs and sea grass beds. Mangrove is the only fringe ecosystem sharing resources with adjoining coral reefs and sea grass. The ecological and socioeconomic advantages offered by the mangroves are innumerable, immeasurable, and incomparable. Hence, conserving mangroves needs to be a priority in India’s conservation programmes.
Microscopic organisms: It is well known that marine ecosystems consist of a myriad life forms. Each millilitre of seawater is estimated to contain 106-9 microbes. Besides, a variety of very minute phyto- and zooplankton also exist in the seas. Diatoms are the dominant phytoplankton in estuaries and coastal waters. Diatom diversity along the west coast is relatively higher, with 148 species, while dinoflagellate species diversity in east coast estuaries is relatively small (15 species) compared to the west coast (76 species).
India is home to 2577 protozoan species, amounting to about 8 per cent of the total world protozoan fauna. Of these, 52 per cent are free-living while the remaining are parasitic.
About 32 marine ciliates of the order Tinitinnida, an important microzooplankton, occur in India. These are a major source of food for bivalves and gastropod veligers. But there is little understanding on their diversity and ecology as yet.
Marine Invertebrates: Sponges are the most primitive multicellular animals and cover a wide swathe from the intertidal region to the deepest ocean. Sponges play an important role in coral ecosystems and facilitate regeneration of broken reefs, and help bind live coral to the reef frame.
Sponges are of three types, Demospongiae, Calcarea and Hexactinellidae. Of these, Demospongiae are the most diverse, accounting for more than 85 per cent of the sponges identified till date.
Most studies on marine sponges in India have been carried out in the Gulf of Mannar, Kerala coast, Lakshadweep, Gulf of Kachchh and Gulf of Cambay. So far, 486 species have been reported in India (Thomas, 1998). India has only nine calcarean species—six occur on the Gujarat and Maharashtra coasts, two in the Andamans, and one in the Gulf of Mannar.
Coelenterates or cnidarians: Corals, sea anemones, jellyfish and hydra comprise cnidarians. The global estimates of their diversity vary between 9000 and 12,553 species. But, since all groups of cnidarians have not received the consistent attention of Indian researchers, the numbers remain unclear.
Studies on corals by Pillai (1967) and ZSI in 2003 together list 218 species. Successive surveys made between 2008 and 2013 have reported the occurrence of 519 hard corals in India (ZSI, 2015b). Among the four major reefs in India, Andaman and Nicobar Islands are rich in corals and their associated species diversity whereas, those on the Gulf of Kachchh are poorer. Lakshadweep Islands have more species than the Gulf of Mannar. Among the deepwater (ahermatypic) corals, 227 species have been reported from the Indian Ocean region (ZSI, 2015b) However, deepwater corals have received little attention, and hence, we know of only 44 species in the Indian Seas (ZSI, 2007).
Gorgonians are marine coelenterates that include sea fans, sea whips, and others. A total about 135 species of sea fans have so far been discovered from the Indian seas. Initially ten species were reported from the Andaman and Nicobar, but subsequent years saw 51 species reported 44 of which were new (Yogesh Kumar et al., 2014 ).
The earliest reports of jellyfish in the Indian Ocean were from Lakshadweep, Maldives, Sri Lanka and Okhamandal Coast of Kathiawar. India accounts for 34 species of the world total of 200 species. (Chakrapany, 1984).
Lesser-known marine animals: India accounts for only 2,587 species, that is 4.37 per cent of the world’s lesser known marine animals. India has 12 species of comb jellies (Ctenophotes), and 61 species of the world’s sea anemones. It also accounts for 46 (free-living) flatworm species and 883 species of polychaetous annelids., 38 species of oligochaete worms, (Salinas, 2007), and 32 species of arrow worms or glass worms, 10 water bears or tardigrades (ZSI, 1998). However, there is little research done on these animals, and hence their numbers and distribution patterns remain uncertain.