| Swarms
of Worms
By Kate
Osborne
AIMS marine scientists on a
recent monitoring trip to the Great Barrier Reef observed one
of nature’s spectacles - thousands of polychaete worms
swarming at Rebe reef.
During a trip to survey sites
in the Whitsunday sector, the AIMS Research Vessel Cape
Ferguson was anchored at night at Rebe reef when thousands
of polychaete worms began swarming at the surface, attracted
by the bright deck lights (Image 1).
Polychaete worms belong to the phylum
Annelida and have an anterior head and long, cylindrical
segmented body, with few other features visible to the naked
eye.
There are many species living
on coral reefs, where they have a variety of roles. They
provide a food source for other animals, erode recently dead
coral and recycle mucus (1).
| Coral reef polychaetes
spend most of their life cycle as bottom dwellers,
living in dead coral rubble or sand. Their presence at the
ocean surface is an example of a dramatic shift in behaviour
that occurs during the reproductive phase of their lifecycle.
Some species also exhibit profound changes in physical
characteristics during this time such as the development of
swimming appendages and greatly enlarged eyes (1).
|
Image
1: Polychaete worms
swarming at the surface
|
This process is known as epitoky and may
involve the transformation of the entire individual or the
separation of the rear portion of the animal (2).
In this photo (Image
2) taken from the back deck of the RV Cape Ferguson
we can identify
the head, trunk and the enlarged portions at the rear of the
body that contain the reproductive material.
Mass spawning events are common in coral reef animals that
have a planktonic life history phase and has the advantage of
greatly increasing the chances of fertilisation of the
reproductive material. The resulting planktonic larvae live in
the water column for a period of time and thus get dispersed
around the reef before settling back to the reef surface and
metamorphosing into adults.
| How the adult polychaetes obtain the cue to swim to the
surface is not known for certain. Within the individual
hormone changes regulate reproductive development. However it
is likely that the hormone changes are themselves regulated by
changes in the external environment.
Data collected from the
northern Great Barrier Reef in 1986 shows peaks of worms
captured in a light trap just after the full moon throughout
the summer months.
The number of worms caught in November 1986
was an order of magnitude higher than other months (1).
|
Image
2: Polychaete worms
|
This suggests that both lunar rhythms and
water temperature are important cues.
An experimental study on
a temperate worm species exposed worms to artificial moonlight
over an extended period and found that swarming behaviour was
induced in the waning phase of the moon (3). The
polychaetes we observed were seen on the 6th
November, 6 days after the full moon.
The great number of animals was an impressive sight and a
reminder of how much of the biodiversity on coral reefs is not
visible on a day to day basis, yet makes a large contribution
to the functioning of the ecosystem as a whole.
References
(1) Hutchings, P. and L.Howitt
(1988) Mass spawning of Polychaetes on the G.B.R in Proc.
Sixth Int. Coral Reef Symposium, Townsville, Australia.
pp739-744
(2) Barnes, R.D (1987)
Invertebrate Zoology, CBS College Publishing
(3) Fong, P. (1993) Lunar
control of epitokal swarming in the polychaete Platynereis
bicanaliculata (Baird) from central California. Bull. Marine
Science, 52(3):911-924
For
more information contact:
Kate Osborne
Tel: +61 7 47534354
Email: k.osborne@aims.gov.au
web@aims.gov.au
 AIMS home
Last updated - December 18, 2008
Copyright
©1996-2002 Australian Institute of
Marine Science
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