Ghostly clams

Have you seen a bleached giant clam?

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Bleaching in cultured juvenile Tridacna maxima at the St John’s Island Marine Laboratory.

In the (not so many) years of my research, I have not personally encountered bleaching giant clams, despite the severity of the major global events in the last two decades. This includes experiences locally (Singapore) and overseas. Unfortunately, we had that opportunity to observe first-hand clam bleaching in our aquarium tanks. 😦

My colleagues had first observed slight paling in the mid-May but was not yet alarmed. It was in early June, when more clams started to bleach out and ‘turn white’. The severity of clam bleaching is as follows (starting with worst performer): Tridacna maxima > Tridacna gigas > Tridacna squamosa. Note that all species are cultured species, which means that they are not naturally wild giant clams. We also observed mortality in the most severely bleached giant clams. 😦 Fortunately, the intertidal team had yet to report a bleaching clam! Phew!

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Bleaching in giant clam results the loss of symbiont, zooxanthellae. This individual here showed partial bleaching.

Global bleaching events (1998, 2010, 2014-2016)

The global coral bleaching phenomenon has been well-documented – the first event in 1998 and the second event in 2010. The current bleaching event that began since October 2015 has not only been the longest event recorded, but has caused some of the worse (and possibly) devastating impacts on global coral reefs.

One such example is the Great Barrier Reef, where almost 93% of the world’s largest reef has been affected, and almost a quarter of the reef on 2,300km stretch is now dead. Reefs in the Caribbean, Indian Ocean, South China Sea, and Pacific are also suffering in this bleaching event.

In Singapore, sea surface temperature (SST) became persistently high throughout April and May. The SST reached as high as 32°C! By early June, we began to observe bleaching in hard corals, and within a span of one week, it was catastrophic for us. On the reefs, hard coral bleaching incidence was almost 40% and other zooxanthellate cnidarians such as sea anemones and zoanthids showed complete bleaching. For more information on bleaching patterns, you may visit Bleach Watch Singapore.

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SST profile of Singapore during April 2016. Taken from Bleach Watch Singapore.

Recap: What is coral bleaching?

Based on the definition from NOAA – It is a phenomenon when corals (both hard and soft) experience stress caused either by increased temperature, increased light, or increased nutrients, expelling their symbiotic micro algae living in their tissues, causing them to turn completely white. These corals are NOT dead, but may become even more stress over time due to their weakened state.

Similar to zooxanthellate corals, giant clams are susceptible to heat stress too. They would lose their brown colour, which is the zooxanthellae cells, and turn pale to white. Giant clams may also bleached under other stressful scenarios such as stress of transportation and shading (Norton et al. 1995).

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How to identify a bleached giant clam?

It’s quite simple (and similar to zooxanthellate corals). The brown colour you see in giant clams are mostly derived from the symbiont micro algae, and during a heat stress event, the clam loses the brown colour and ‘turn white’. Yes, they look like ‘ghost’ underwater! What surprises me is that their natural coloured pigments appeared to have ‘disappeared’, but not entirely as you can see the slight luminescent blue in the following picture. The discussion of clam colours is deserving of another post in the near future.

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100% bleached clam, whose natural blue-colour pigments have been masked.

Are there any reports of giant clam bleaching?

Yes, there have been a handful of reports of giant clam bleaching but not plentiful, despite the severity of global coral bleaching event. Here’s quick summary of locations with reports of giant clam bleaching:

1998 – Bolinao Ocean Nurseries, Philippines: Seawater temperatures reached 34.9°C and 34.1°C in the months of June and July 1998 respectively. Mortalities were observed for all species cultured in the nurseries, with worst performer being Tridacna gigas (Gomez & Mingoa-Licuanan, 1998).

1997-1998 – Great Barrier Reef (GBR), Australia: Seawater temperatures increased from 22°C to 32°C within several weeks, accompanied by heavy rainfall that lowered the salinity to 18 (Buck et al., 2002). Extensive bleaching of cultured clams and wild stocks in the central region of GBR (Grice, 1999). More than 8,000 out of the 9,000 Tridacna gigas individuals were bleached.

1998 – Takapoto atoll, French Polynesia: Seawater temperatures rose above 30°C for over five months in 1997/8 (December 1997 through April 1998), with a peak at 31.8°C in mid-March. Extensive bleaching of Tridacna maxima in Takapoto atoll, with some partial recovery in May 1998. Abundance of giant clams decreased by >80% since 1993 (Addessi, 2001).

2009-2010 – Mannai Island, Thailand: Seawater temperatures reached a peak of ~32.5°C in May 2010, and prolonged temperatures of >30°C over February to September 2010 (Junchompoo et al. 2013). Extensive bleaching of wild clams (Tridacna crocea and Tridacna squamosa) was observed, with almost 60% mortality after the event.

2010 – Phi Phi Islands, Thailand: Seawater temperatures rose above 31°C in May 2010. Bleaching of giant clams was observed, and surveys indicate that Tridacna maxima were affected with various degrees of bleaching severity. The most bleached clams showed mortality within a few weeks (Sangmanee et al. 2010).

Are bleaching events doomsday for giant clams?

I don’t know yet. There is very little information on giant clam bleaching, and the mechanisms of expulsion of symbiont cells are not certain. Under heat stress scenarios, is it the clam that kicks the symbionts out, or vice versa? One thing I do know is that unlike corals where zooxanthellae cells reside near the surface of their body tissues, the giant clams possess a complex tubular network to enclose the zooxanthellae (Norton et al. 1992). Studies have shown that the amoebocytes (a type of defense cell) could ingest zooxanthellae cells not found within the tubular network  (Fankboner 1971; Fankboner & Reid 1981), hence suggesting that the network is quite critical in protecting zooxanthellae from being eaten by the host clam!

Another study by Leggat et al. (2003) noted that bleaching could reduce the fitness of zooxanthellate marine invertebrates, and result in poor growth and survival. Experiments conducted showed that giant clams cannot digest (or more accurately, assimilate) ammonium. Leggat et al. (2003) also showed the bleached clams provided with organic and inorganic nutrients enhanced recovery rates! Clearly, the physiological mechanisms of giant clams before, during and after a bleaching event are very different.

There is hope – if we can define this pathway of building up resilience. For now, we hope our remaining giant clams push through this tough period…

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Reference List:

Fankboner (1971) Intracellular digestion of symbiotic zooxanthellae by host amoebocytes in giant clams (Bivalvia: Tridacnidae), with a note on the nutritional role of the hypertrophied siphonal epidermis. Biological Bulletin 141: 222-234.

Fankboner & Reid (1981) Mass expulsion of zooxanthellae by heat-stressed reef corals: a source of food for giant clams? Experientia 37: 251-252.

Norton et al. (1992) The zooxanthellae tubular system in the giant clam. Biological Bulletin 183: 503-506.

Norton et al. (1995) Atrophy of the zooxanthellal tubular system in bleached giant clams Tridacna gigas. Journal of Invertebrate Pathology 66: 307-310.

Gomez & Mingoa-Licuanan (1998) Mortalities of giant clams associated with unusually high temperatures and coral bleaching. Reef Encounter 24: 23.

Grice (1999) Studies on the giant clam – zooxanthellae symbiosis. Thesis. James Cook University of North Queensland, Townsville, Australia.

Addessi (2001) Giant clam bleaching in the lagoon of Takapoto atoll (French Polynesia). Coral Reefs 19: 220.

Buck et al. (2002) Effect of increased irradiance and thermal stress on the symbiosis of Symbiodinium microadriaticum and Tridacna gigas. Aquatic Living Resources 15: 107-117.

Leggat et al. (2003) The impact of bleaching on the metabolic contribution of dinoflagellate symbionts to their giant clam host. Plant, Cell and Environment 26: 1951-1961.

Sangmanee et al. (2010) MS8-37 Giant clam bleaching on coral reefs of Phi Phi Islands during the 2010 coral bleaching event. In Mini-symposium 8-Biology and ecology of coral reef organisms. The 2nd Asia Pacific Coral Reef Symposium, June 20-24 2010, Phuket, Thailand.

Junchompoo et al. (2013) Changing seawater temperature effects on giant clams bleaching, Mannai Island, Rayong Province, Thailand. KURENAI 2013-03: 71-76.

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