Environmental stresses, including warmer and more
acidic seawater, may be affecting the development of the ear bones in
young reef fish, causing the fish to get lost at sea during a crucial
stage of their development.
Research by fish ecologists Dr Monica Gagliano (AIMS
and James Cook University) and Dr Martial Depczynski (AIMS, Perth),
with Dr Stephen Simpson from the University of Edinburgh and James
Moore from JCU in Townsville, has found that fish with asymmetrical
ear bones struggle to return to the reef.
The implications could be profound for the survival of
reef ecosystems, which depend upon a rich biodiversity for effective
function and health.
The research has been published today (Friday 7 March
2008) in the prestigious UK scientific journal Proceedings of the
Royal Society*.
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The stresses causing ear bone asymmetry may be
closely linked to a combination of rising sea surface
temperature and acidity, both caused by high atmospheric carbon
dioxide levels, along with a range of more localised stresses.
Abnormalities in fish hearing structures may be
interfering with a vital part of the animals’ life cycle. Most
reef fish spend some time in the open ocean after hatching,
before finding a place on the reef to settle and breed.
Researchers have only recently established how
important sound is in guiding young fish to their homes. It is
now known that fish at the end of their ocean stage "home in" on
reef-associated sounds, such as the gurgling of fish and the
snapping of crustaceans.
A sophisticated hearing system that enables fine
distinction between frequencies is needed by young fish to
determine where to go. Fish are not the helpless victims of
currents and tides – they actively navigate.
The project examined damselfish, which are
abundant on many reefs, including Queensland’s Great Barrier
Reef and Western Australia’s Ningaloo Reef. As a group,
damselfish are well-understood and provide a good model for
other kinds of reef fish.
The scientists collected representative samples
of hatchlings at their reef of origin. They later traced fish
from the same cohort arriving on the reefs after the ocean
phase, attracting them to traps broadcasting various sound
frequencies, from high to low.
At hatching, 41 per cent had symmetrical ear
bones (otoliths) and 59 per cent asymmetrical. When the team
examined the otoliths at the settlement stage a few weeks later,
far fewer asymmetrical individuals were found to have made their
way back to a reef. The scientists also found that those with
asymmetrical ear bones that did make it to the reef took longer
to do so than their symmetrical counterparts.
"In our opinion, ear bone asymmetry in the early life stages of
reef fish interferes with their capacity to find and settle on
coral reefs," Dr Gagliano said. |
Newly hatched damselfish larva. The
otoliths are visible in the head region, between the eye and
yolk sac (the two dark areas).
Adult
damselfish on the reef.
Microscopic image of damselfish eggs just before hatching.
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Vertebrate animals make sense of sounds by comparing
differences in the acoustic signal between their two ears. To do this
well, their ear structures must be relatively symmetrical.
Asymmetrical ear bones do not appear to make the fish deaf, but they
may interfere with the ability of the fish to hear effectively.
Dr Gagliano said that fish otoliths were a sensitive
tool for studying the effects of environmental stress in fish.
"Asymmetry has been used as a stress indicator for a long time,
although in some contexts it remains controversial," Dr Gagliano said.
"In our case, it looks like it is very reliable.
Preliminary data indicate that if we increase the stress, the
asymmetry of the otoliths will increase," she said.
"There is a degree of asymmetry that is acceptable in
the population – some is natural," co-author Dr Depczynski said. "Not
all the babies are created equal and not all of them are going to make
it, even in pristine environments."
The problem now is that an already high mortality rate
among reef fish hatchlings is likely to rise even higher if young fish
can’t navigate by sound.
At least part of the problem is likely to be linked to
ocean acidification, although much more research needs to be done to
examine the link. Fish ear bones, like their skeletons and many other
kinds of structures such as reef-building corals, are made from
calcium carbonate. When seawater becomes more acidic, there is less
calcium carbonate available for building any calcium-based structure,
including ear bones.
Acidity appears to be having a two-fold effect,
creating a hostile marine environment and also robbing the environment
of the building blocks of calcium-based structures. This has a direct
effect on fish development and on their food sources, as many
creatures the fish eat are also dependent on calcium.
While stress is part of life for reef fishes, new
stresses are now being piled on top of existing ones and fish are
showing the effects, according to Drs Gagliano and Depczynski.
*The Proceedings of the Royal Society paper is titled
"Dispersal without errors: symmetrical ears tune into the right
frequency for survival".
Link
here.
Note to media: AIMS will hold a media
briefing on ocean acidification research plans at its headquarters
near Townsville on Tuesday 11 March at 10am and a public forum at the
Museum of Tropical Queensland in Townsville at 5.30pm on Friday 14
March. Please contact Wendy Ellery, below, for further details.
