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. These zooxanthellae are the reason why clam behaviour
is often compared to plants.
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 in clams across a coral reef 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.
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.
For
more information, check out the world-wide-web.
Written by Gabrielle Ahern
Salty
Wave Blue – Into all things ecology.
Follow
@SaltyWaveBlue on @Instagram and @Twitter
If
you would like to see beautiful images of clams in their coral reef habitat,
please take a look at my Pinterest site: https://www.pinterest.com/saltywave
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