Giant clams are large and beautiful reef animals, the largest bivalve molluscs in the world, commonly reaching more than a metre in length.
Like reef-building corals, they have symbiotic algae in their tissues, and under extreme heat stress can bleach like corals do.
This results in the symbiotic algae being ejected from their tissues and they turn white.
Currently, a giant clam bleaching event is unfolding at Magnetic Island, 14km off the coast of Townsville, North Queensland.
Inshore waters of Cleveland Bay (the stretch of water between Townsville and Magnetic Island) reached 32 degrees in February 2020, resulting in bleached clams and corals.
Scientists from Magnetic Island and Queensland Museum are now closely monitoring fourteen 34-year-old giant clams in Geoffrey Bay and Nelly Bay which feature on the Tourism Magnetic Island snorkel trails (six at Geoffrey Bay and eight at Nelly Bay).
Two of these animals are severely bleached white and in danger of dying whilst the other twelve are pale – a clear sign of heat stress.
These clams have survived several coral bleaching events over their 34 years, including in 1998, 2002 and 2016/17, but have only shown signs of bleaching once before — in 1998 and were able to make a full recovery.
The clams were bred in 1986 at Seafarm, Innisfail, and raised at the giant clam farm at Orpheus Island (Palm Islands) by Dr Rick Braley, marine scientist and Magnetic Island resident.
When the clam farm closed in 1990 some clams were re-homed to Magnetic Island, and in 2013 fourteen of them were placed on the snorkel trails for all to enjoy.
Many locals and visitors have viewed these iconic animals whilst snorkeling Magnetic Island over the past seven years and their welfare is a concern for scientists, residents and tourists alike.
We will monitor the fourteen giant clams on the snorkel trails and we will continue to post photos with updates on their condition in this blog series.
Written and Compiled by Dr Robyn Cumming, Collection Manager (marine) and Bryozoan taxonomist, Biodiversity & Geosciences Program, Queensland Museum and Dr Rick Braley, Aquasearch, Magnetic Island.