Today’s post is by Mareike Sudek, our ALMP Chief Science Officer. Mareike led the recent expedition to Aitutaki and has also recently surveyed other lagoons and reefs of the Cook Islands:
Imagine diving down into the clear blue waters of a tropical coral reef and suddenly being smiled at by an attractively coloured set of soft lips. These lips belong to the giant clam (genus Tridacna). Giant clams are probably one of my most favourite animals in the ocean. They have this beautiful iridescent coloration in various shades of blues, greens and browns as a result of pigment cells within their tissue.
Before coming to the Cook Islands, I had never seen a giant clam. I had admired their beauty on photographs, and so I was very excited that I would finally be able to see one in real life. This probably sounds really nerdy, but the first time I spotted one, I almost squealed into my snorkel.
When people hear the word giant clam, they usually think about the true giant of the clams (Tridacna gigas) which can be more than 1 m in size and weigh over 200 kg! I am sure everyone has heard the scary myth that these clams can snap shut and trap a diver underwater. However, this is really just a myth! Giant clams are neither aggressive nor particularly dangerous, and the closing action of the shell is too slow to really pose a threat. While free diving in Aitutaki’s vast lagoon, I had the privilege to encounter one of these friendly giants. Its enormous size was truly impressive, and this clam probably started its life around the time my great-grandmother was born.
However, this species of giant clam is not native to the Cook Islands and is not commonly encountered, although some have been brought in as tourist attractions. The clam you want to look for in the Cooks is the maxima clam or small giant clam (Tridacna maxima). This species is much smaller than the true giant clam, with adults reaching not more than 20 cm in size (still big for a clam!). In Cook Island Maori, giant clams are called paua.
All giant clams from the genus Tridacna differ from most other clams in that they have tiny single-celled algae living inside their tissue (the fleshy, prominent mantle) that provide a large portion of their nutrition. These tiny algae cells, also referred to as zooxanthellae, use photosynthesis to produce sugars and amino acids which are released to the clam. So even though the clams filter out particles from the water (like all other clams do), they don’t depend only on this food source. This is also the reason they are so colourful! The colour of the clams is both an indication of the algae living inside and likely also protection against the harsh tropical sun, kind of like a human tan.
Sadly though, giant clam populations have declined world-wide, mostly due to over-harvesting and pollution. Giant clams have been traditionally used as a food source, and more recently, the beautifully coloured species have been sought after by the aquarium trade. Older local Cook Islanders tell stories about the huge abundance of paua (giant clams) that used to be found in the lagoon when they were children. Nowadays, you have to swim around and really look to spot a giant clam. I have only lived in the Cooks for a few months now, but even I directly experienced the huge decline that paua populations have suffered. During a research cruise to the southern Cook Islands, I had the opportunity to snorkel in Manuae’s lagoon (an uninhabited island). When I first jumped into the water, I almost didn’t trust my eyes. The floor of the lagoon was covered in paua (the small giant clam), and there were even reef structures (bommies) solely built by these clams! Seeing this made me realize how extensive the population decline was in Rarotonga and Aitutaki (the two most populated and visited islands).
Today, giant clam hatcheries exist in most Pacific nations with the intent to grow clams for reseeding depleted reefs, as a food source relieving pressure on wild populations, and for sale as aquarium species. In the Cook Islands, a clam hatchery and nursery can be found on Aitutaki. Clams are hatched and raised in large saltwater basins, and once they reach a certain size (after a couple years), they are transplanted into the nursery inside the lagoon. We saw tiny ones, smaller than a fingernail – but they were almost two years old! It’s great that these clams can be grown in captivity. However, most of these raised clams are sold into the aquarium trade.
But as happens in most aquaculture facilities, some individuals escape and so help to reseed the diminished population inside the lagoon. Near the clam nursery, high numbers of giant clams can be found on the reef, but it is still a long way until the stocks inside the entire lagoon will increase again, particularly because these clams are tasty food for people. Restocking wild populations of giant clams will take time, some sacrifice, and a lot of work, but it is surely worth the effort!