A step in the right direction for Magnetic Island’s giant clams

This is the third installment of a blog monitoring a bleaching event currently occurring in reefs off Magnetic Island, 14kms from the coast of Townsville in North Queensland.

Since February 2020, a team of local marine biologists have been monitoring 14 giant clams along the snorkel trails of Geoffrey Bay which were showing signs of severe bleaching.

The team returned again in March and observed that the level of bleaching and number of clams affected had sadly increased.

However, good news came this month when the team returned and reviewed the clams on 4 and 20 April.

All of the clams are regaining some colouration, the result of zooxanthellae algae returning to the mantle tissue. This is remarkable considering that many of the nearby reef-building corals are still stark white.

The seawater in Cleveland Bay has cooled to 27–28o C (recorded by the Australian Institute of Marine Science), down from 32o C in February 2020.

Some clams are recovering much faster than others. The most impressive recovery is by G1, which was 90% white on 15 March, 30% white by 4 April and only <5% white by 20 April.

N6 has also shown good recovery from 60% white on 20 Feb down to <5% by 20 April.

Others, like N4, have shown slower recovery, with approximately half the mantle still white.

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Partially (N5) and fully (N4) bleached clams at Nelly Bay on 7 March (top), 4 April (middle) and 20 April (bottom). N4 has gained significant colouration around the edges of its mantle, and N5 (top left) has returned to normal colouration.

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Partially bleached clams N6 (lower) & N7 (upper) on 7 March 2020 (top), 4 April (middle) and 20 April (bottom). Both have gained some colouration.

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Bleached (G1) and unbleached (G2) clams at Geoffrey Bay on 7 March 2020 (top), 4 April (middle) and 20 April (bottom). G1 has made a rapid recovery.

Table 1: Level of severe bleaching over time for each giant clam at Nelly Bay. Numbers in the table are % of the mantle tissue that is white.

DATE N1 N2 N3 N4 N5 N6 N7 N8
Feb 20 0 0 0 >90 0 60 0 0
March 7 >90 70 70 >90 0 50 0 0
March 15 >90 50 35 90 10 50 30 30
April 4 45 15 10 85 <5 20 20 20
April 20 45 15 0 50 0 <5 0 <5

Table 2: Level of severe bleaching over time for each giant clam at Geoffrey Bay. Numbers in the table are % of the mantle tissue that is white.

DATE G1 G2 G3 G4 G5 G6
Feb 20 70 0 50 25 50 0
March 7 80 0 70 25 70 0
March 15 90 0 90 25 50 0
April 4 30 0 70 10 25 0
April 20 <5 0 50 0 <5 0

More updates to follow as the team continue to monitor the clams’ recovery process.

DSCF8176 - April 6 2020 - Beat Lehmann
Site of the Geoffrey Bay snorkel trail with Townsville and Castle Hill to the South. Two of four yellow buoys marking the trail are in the foreground.  Image: Beat Lehmann

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.

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