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International Journal of Marine Science 2015, Vol.5, No.3, 1-7
http://ijms.biopublisher.ca
5
may act directly or, in the case of photoperiod, set
circannual clock mechanisms. However, swarming is
controlled by daytime, tide, weather conditions and
sex pheromones
. Among marine
invertebrate species, reproduction requires the
synchronous spawning of neighboring individuals in a
population. The fact that, in some species, entire
populations can spawn abruptly, sometimes within a
few days, suggests that spawning is coordinated by
external environmental factors. Among the
environmental variables, temperature and day length
may interfere with the reproductive cycle without
even triggering the spawning period
.
Field studies have clearly shown that, in this case,
swarming and spawning are often correlated with
particular phases of the lunar cycle. However,
temperature may interfere with lunar timing, so
swarming may occur at different times of the lunar
cycle in order to enhance breeding success at higher
water temperatures. The spawning of polychaete
worms generally appeared at midnight as recorded by
earlier reports
. They appeared in small
numbers usually an hour or two before midnight and
were mingled with spawning swarms. It may be said,
in general, that more intense swarming occurs usually
between the full moon and last quarter
.
In the present observation, more intense swarming
occurred in the first quarter.
Swarming of marine worms has been associated with
the lunar phase
. Conversely, according to
swarming was more correlated
with temperature than lunar phase. As mentioned
above, if, in some cases, environmental factors
provide the information necessary for the observed
synchronization, these factors alone do not seem
sufficient to synchronize the reproduction of marine
polychaetes. This is particularly the case in epitokous
Nereididae, whose reproduction requires exact spatial
and temporal synchronization, leading to the
formation of swarms in which partners recognize each
other by means of chemical signals. Many of the
Nereididae species undergo drastic behavioural,
morphological and physiological changes during their
sexual maturation (epitoky)
.
This metamorphosis prepares benthic worms for a
brief pelagic existence devoted to mating and, after
spawning, individuals of both the sexes die.
In the
Nereis (Neanthes) virens
of family Nereididae,
the gravid males leave their burrows and swarm
before spawning and releasing gametes in the sea
while the females apparently spawn in their burrows,
minimizing predation and avoiding adverse
environmental conditions that males encounter.
However,
D. aestuarina
worms first come to the
nuptial party where they are scattered as individuals.
The isolated small heteronereis females with ripen
eggs suddenly appear from below and rise to the
immediate surface, swimming a few moments there in
spirals and loops. Males continue unabated in
numbers and vigor as the females appear a little later
to join the dance, a dance which seemingly is a climax
that marks the end of their lives. This swimming in
increasingly small circles while emitting genital
products called nuptial dance (Zeeck et al. 1996 ).
This swimming behavior limits the dispersion of
gametes and ensures their release in close proximity to
one another and, consequently, a good fertilization
rate. In mature worms, the behavioral sequence
leading to and including the nuptial dance suggests
that different chemical signals are implicated; these
are, successively, a mate-recognition pheromone
(MRP), an egg-release pheromone (ERP), and a
sperm-release pheromone (SRP). The Nereid coelomic
fluid contains sex pheromones that not only induce
prolonged electrical activity in the central nervous
system of epitokous individuals of the opposite sex,
but also elicit the nuptial dance (Zeeck et al. 1996).
In the present study, swarming occurred on the
concrete steps submerged in the riverbank, where
hundreds of people come to perform the last rites for
their ancestors. The gathering of a large human
population has brought huge amounts of organic
matter that might have favoured polychaete swarming.
D. aestuarina
has been recorded in high abundance
from polluted parts of the estuaries around
metropolitan areas
. The present study
area was a purely freshwater environment with
moderately low dissolved oxygen level (2.9 ml L
-1
)
and slightly acidic (pH 6.5). Water temperature was
27.6°C with low turbidity (5.7 NTU). The observation
that epitokous Nereididae species show significantly