Beautiful iridescent colours of a clam mantle. Photo © Marcelokato

Clams and their beautiful coloured mantles are just one of the many amazing marine animals that attract millions of visitors each year to the Great Barrier Reef. I studied clam populations on reefs around Heron Island as an undergraduate Marine Studies student at the University of Queensland. It was a great opportunity to observe the variation in mantle colour across reef locations.

Clams belong to Class Bivalvia, Family Tridacnae, and range in size from 15 cm long in Tridacna crocea to more than one metre long in Tridacna gigas. Similar to corals, zooxanthellae (Symbiodinium microadriactum) live inside a clam’s tissues. These symbionts can also be found in gorgonians, sea anenomes and jellyfish. The activity of zooxanthellae are the reason why clam behaviour is often compared to plants.

Similar to coral species, clams need sunlight for symbiotic algae to photosynthesise and produce food for the clam. Photo © Csharker

Just like plants, clams need sunlight. In the mantle, clams possess what are known as, eyespots or iridophores. These pigment cells can sense light conditions and trigger a response in the clam. For instance, if light is poor, the clam improves conditions for the zooxanthellae to photosynthesise by extending its mantle out or using its shell to reflect sunlight back onto the mantle, by holding its valves wide apart. If sunlight is too intense, the iridophores also protect the clam’s zooxanthellae from harmful ultraviolet radiation by reflecting light away.

Mantle colour of clam species is a poorly researched topic. The notion that mantle pigmentation in clams differs across coral reef communities has not been researched widely. Interestingly, studies have shown clams are sensitive to the blue (450nm), blue-green (490nm) and ultraviolet (360nm) colours in the spectrum. Light quality on the reef may affect mantle colour, with clams expressing different pigments, or perhaps a clam’s genetic adaptation to a particular reef habitat influences the type of zooxanthellae inhabiting its mantle tissues. I noted a trend in colour for different reef zones during my fieldwork and related this data to previous reports, which have suggested clams might share symbionts with other invertebrate species in a particular zone.

Clam species growing in a tropical coral reef habitat. Photo © Kris Cashman

Further investigation into the mantle colour of clams by applying molecular biology might reveal why particular colour patterns occur. Research could also concentrate on how clams can adapt to changing light or temperature conditions by genetic variability.

Clams are quite a stable organism and can adapt to changes in their habitat over time. However, fishing pressure has dramatically reduced populations of clams in the pacific region, leading to the extinction of several species. Conservation methods that use aquaculture assist the remediation of wild clam populations on the reef and supply aquariums with different species.

Clams are interesting to observe in their natural environment. During my research, I noted variations in colour mantle between clam populations across the reef zones and between the two sites on Heron Island. While your exploring the reef, it is definitely a feature worth looking out for.

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Written by Gabrielle Ahern

Salty Wave Blue – Into all things ecology.

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