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Giant Clams


What are Giant Clams?


Giant Clams, as popularly known, are among the most specialized bivalves. These are the largest living bivalves, growing to over a metre in length. They have a narrow range of distribution and occur exclusively within tropical reefs under the Indo-Pacific faunal region. Giant Clams appeared during Eocene period and are a fairly young family as compared to other molluscan groups. There are ten living species under the family Tridacnidae within two genera, namely Tridacna and Hippopus.

Giant Clams grow in clear, highly saline and shallow waters, are slow growing and long lived bivalves. They can be seen mostly up to the depth of 20 m. A few individuals can be seen up to the depth of 40 m. They are filter feeders and feed on zooplankton and phytoplankton. The most striking part of the Giant Clam is its bright and fluorescent mantle. The mantle colour varies from fluorescent blue to brown, pale yellow to green. Microscopic algae - zooxanthellae stay in symbiosis (endosymbiosis) within the mantle tissues of Giant Clams. Giant Clams are fixed on coral substrate except for juveniles of 10-20 mm size which move to some distance in search of good substrate to anchor. They are known to live for over 100 years. The largest known clam is that of T. gigas and measures 137 cm in shell length.

Giant Clams occasionally develop pearls. The largest known natural pearl ‘Pearl of Allah’ was recovered from T. gigas. The pearl weighed fourteen pounds and was nine and half inch long and five and half inches wide.


Green Mantle of Giant Clam


Brown mantle of Giant Clam


Blue mantle of Giant Clam

Status of Giant Clam


Of the ten species of Giant Clams found worldwide, nine are included in the IUCN Invertebrate Red Data Book.However, in1996, T. crocea was removed from the Red List of IUCN.

Giant Clams have also been listed in CITES Appendix II since 1985. Species listed in Appendix II are deemed not currently threatened with extinction but are at risk of becoming so unless trade is regulated.

Three species of Giant Clams T. maxima, T. squamosa and Hippopus hippopus are also included in the Schedule I of Indian Wildlife (Protection) Act 1972, 2001 Amendment.


Distribution of Giant Clams in India


Small Giant Clam T. maxima (Roding, 1798): It is the commonest of the four species of Giant Clams occurring in India. It is found in Lakshadweep, Andaman and Nicobar Islands.

Fluated Giant Clam T. squamosa (Lamarck, 1819): Co-occurs with T. maxima but in smaller numbers. It is found in Lakshadweep, Andaman and Nicobar Islands.

Crocas Giant Clam T. crocea (Lamarck, 1819): Distributed only in the Andaman and Nicobar Islands. It is the smallest giant clam.

Hippopus hippopus (Linne, 1758): Known only from the Nicobar Islands. No information on its status or population is available.

There is a possibility of having Tridacna gigas from Andaman and Nicobar. But due to lack of survey, its presence is not certain.



Coral Reefs, El Niño and Coral Bleaching


Coral Reefs, El Niño and Coral Bleaching


Among the most neglected habitats, coral reefs probably top the list. Coral reefs are Earth's largest biological structures and have taken thousands of years to form. Home to several thousand species, it is a mosaic of colours, species and abundance. Reefs world over provide livelihood to millions in the form of fisheries, recreation, and other life support systems. They also act as natural barriers to protect coasts and people from erosion and natural calamities like cyclone, storm, tsunami etc. They are world’s largest and most effective carbon sinks. Formed by tiny, microscopic individuals called polyps (relatives of sea anemones and jellyfishes), one coral colony is the work of several hundred years.

Reefs are misused and abused world over. Blasting of coral reefs for cement industry in the past has destroyed reefs of Gulf of Kutch permanently. Reef poisoning for ornamental fishes has devastated reefs of Indonesia, Philippines and the Caribbean. Bottom trawling has flattened reef beds world over. Number of other stresses like pollution, nitrification and acidification of oceans, eutrophication, increased anthropogenic stresses, solid waste, oil spills, cyclones, storms continue to take toll of this critical habitat. Coral bleaching due to El Niño is another critical yet little studied issue. It is a natural phenomenon happening world over. But the damaged caused by it is unprecedented. In 1998, El Nino created havoc globally with over 50-80 percent reefs destroyed in a short span of a month.

What is Coral bleaching

Coral bleaching occurs when the symbiotic relationship between algae (zooxanthellae) and their host corals breaks down under certain environmental stresses. As a result, the hosts expel their zooxanthellae. Lacking the symbiotic algae, the corals expose their white underlying calcium carbonate coral skeleton and the affected coral colony becomes stark white or pale in color. This is known as "coral bleaching." Coral bleaching can be triggered and sustained under various environmental stresses. In recent years, anomalously warm water temperatures have been observed as one of the major causes of mass coral bleaching worldwide. Ambient water temperatures as little as 1 to 2 °C above coral's tolerance level, indicated by summer monthly mean temperatures, can cause coral bleaching (Berkelmans and Willis, 1999; Reaser et al., 2000). Reefs that are partially - to totally-bleached for long periods often die. Severe bleaching events have dramatic long-term ecological and social impacts, including loss of reef-building corals, changes in benthic habitat and, in some cases, changes in fish populations. Even under favorable conditions, it can take decades for severely bleached reefs to recover.


El Niño Southern Oscillation

El Niño-Southern Oscillation, or ENSO, is a climate pattern that occurs across the tropical Pacific Ocean on average every five years and is therefore, widely and significantly, known as "quasi-periodic". ENSO is composed of an oceanic component, called El Niño (or La Niña, depending on its phase), which is characterized by warming or cooling of surface waters in the tropical eastern Pacific Ocean, and an atmospheric component, the Southern Oscillation, which is characterized by changes in surface pressure in the tropical western Pacific. The two components are coupled: when the warm oceanic phase (known as El Niño) is in effect, surface pressures in the western Pacific are high, and when the cold phase is in effect (La Niña), surface pressures in the western Pacific are low.

1998 El Nino and subsequent coral bleaching affected Lakshadweep islands with over 80% coral mortality. Lakshadweep reefs have shown recovery (30-80 %) from this episode in last 12 years. Unfortunately before the full recovery of these reefs could happen, it was again hit by 2010 El Niño causing severe bleaching.

El Niño of 2010

Reefs in many regions are affected the year 2010 due to El Nino Southern Oscillation. India, Maldives, Oman, Muscat, American Samoa, Virgin Islands, Palau are all affected due to 2010 bleaching event. In India reefs at Lakshadweep, Andaman and Nicobar, Gulf of Kutch, Gulf of Mannar and Konkan coast of Maharashtra are badly affected this year. Reefs like Kavaratti and Agatti in Lakshadweep appear like a mosaic of white structures, indicating total bleaching. Besides corals, most of the zooxanthallae associated species like sea anemones, giant clams, are completely bleached. Tough we could not enumerate the damage at this point of time, impacts of the same will be known soon.

Monitoring coral bleaching

Coral Bleaching Virtual Stations web page is designed by NOAA Coral Reef Watch to provide near-real-time information on thermal stress that induces coral bleaching for 24 selected reef sites around the globe. The information is extracted from near-real-time satellite remotely sensed global sea surface temperature (SST) measurements and derived indices of coral bleaching related thermal stress from 50 km water pixels surrounding or close to these reef sites.

The five status levels of thermal stress are defined as No Stress, Bleaching Watch, Bleaching Warning, Bleaching Alert Level 1, and Bleaching Alert Level 2. These levels are defined in terms of the Hot Spot and Degree Heating Weeks (DHW) values at the selected reef sites. When thermal stress is present at a reef site, i.e., current SST exceeds the maximum monthly mean SST, a Bleaching Watch is issued through Coral Reef Watch's Satellite Bleaching Alert (SBA) System.

The time of peak bleaching season varies among ocean basins and hemispheres. Generally, the peak season for both the northern Atlantic Ocean and the northern Pacific Ocean is July-September and for both the southern Atlantic Ocean and southern Pacific Ocean is January-March. The peak season for the northern Indian Ocean is April-June and for the southern Indian Ocean is January-April. 

Bleaching in Lakshadweep

May-June 2010 proved devastating year for corals reefs in Lakshadweep which witnessed massive bleaching. Acropora showed over 90 bleaching followed by Porites (60%) and other coral species (50%). Besides corals BNHS monitored giant clams in Agatti and Kavaratti islands. Both Tridacna maxima and Tridacna squamosa showed 99% and 98% bleaching respectively. Though bleaching not necessarily results in mortality, the entire ecosystem remains under stress thus affecting new recruitment. Documenting mortality and revival rates are vital in understanding impacts of coral bleaching. This is the first occasion where giant clam bleaching has been systematically documented. Besides corals and giant clams Sea anemones such as Beaded Anemone Heteractis aurora and Bubble Anemone Entacmaea quadricolor also shown extensive bleaching. Some of the images below can provide the insight to the extent of the damage.



Tridacna maxima: Natural colours                              Tridacna maxima: Bleached



Magnificent Anemone Heteractis magnifica: Natural and Bleached


Bleaching and mortality in Gulf of Mannar

The coral reef areas in the Gulf of Mannar (GoM) are predominantly shallow, between 0.5 - 4.5 meter depth, and high sea surface temperatures (SST), around 29ºC, prevail throughout the year. During summer (April - June), the temperature varied between 31.00C and 33.50C. Coral bleaching, as a result of elevated SSTs, has been observed during summer every year in GoM since 2005. Average percentage of bleached corals during 2005, 2006, 2007, 2008 and 2009 was 14.6, 15.6, 12.9, 10.5 and 8.93 respectively. Generally in GoM in the last 5 years, the water temperature started to increase in March and once it reached 31°C in mid April, coral bleaching occurred, however depends on rainfall and winds, recovery usually recorded during June-July and completed in 1-4 months.

In 2010, the rapid survey by SDMRI Reef Research Team in Gulf of Mannar observed that the elevated temperature (32.2 to 33.2ºC) prolonged for 4 months (April to July), resulting mortality to over 9.5% of live corals. Mortality was high in adult branching coral colonies of Acropora formosa, A. nobilis, A. cytherea, Montipora foliosa and M. divaricata. The dead corals are mostly covered by turf algae. However, there was very minimal impact on massive corals. Detailed information on the present coral status, mortality, species most affected, size class, depths variations, recovery, recruit and associated resources has to be collected.



Porites lutea: Natural                                                Porites lutea: Bleached






On several occasions, you may not see the animal, but just a track and/or sign left by the animal. With some practice you can identify an animal from its track and/or sign. This in itself is a fascinating and learning experience. Most of us are familiar with crawl marks of sea turtles. With practice you will be able to distinguish between tracks of a green, hawksbill, and olive ridley turtle very easily. However, tracks and signs of invertebrates are not easy to identify. Nonetheless, with little persistence, you will soon learn to read the language of animals. Some tracks and signs are illustrated here.

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Octopuses are the considered as the smartest invertebrates due to their well developed nervous system.

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