Since the discovery of massed coral spawning twenty
five years ago, the event has been filmed to death in every coral
reef film ever broadcast. The sight is indeed spectacular, but
we have seen it so many times and I have it on good authority,
we will be seeing it once again in an upcoming series of programmes.
What is seldom shown in any detail is the very complex nature
of the packaging that accompanies each release and the subsequent
orgy of sexual activity and predation that follows hot on the
heels of the main event.
Early research showed that, in species that liberate
their gametes (not all do), the eggs are released by the coral
polyps at the appropriate combination of water temperature and
lunar phase (see above). In time it was realised that eggs were
in fact clusters of many eggs - maybe ten or so squeezed , in
most species, into one spheroidal mass. Now it is realised that
the hermaphrodite species of coral glue the eggs together with
a matrix of viscous, but as yet still inactive and unprimed, sperm.
Coral egg cluster
When the "eggs" are released in many species of
branching and meandering coral, the whole mass floats to the surface
of the sea and there absorbs water. The individual eggs soften,
swell and round off. The glutinous sperm thins as water dilutes
the mass and steadily billions of live, wriggling spermatozoa
swim free and cause the glistening sheen sometimes noticed when
calm conditions accompany the event. Amongst this soup of sex
cells, fertilization is expected to take place, but sophisticated
chemical systems seem to prevent self-fertilization. Sperm from
another package must ideally find their way to an unfertilised
egg, if the union is to be successful. In Bermuda we have seen
pilchard spawning coincide with coral spawning, so producing a
scum that extended the length of the fringing reef and smelt strongly
of cod liver oil.
Coral and fish egg aggregation
On the Australian Great Barrier Reef it has been
recently recognised that vast vertical migrations of deep sea
Myctophid fish feast upon the spawn and trigger a food web concentration,
now referred to as the "aggregation", that Japanese fisherman
began to exploit nearly ten years ago. The western Australian
reefs, in contrast, have for many years been seen as the focus
of a similar, but different, event, the climax of which is the
arrival of some hundreds of plankton-feeding whale sharks.
This article was written
by Peter Parks
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