International Journal of Marine Science 2016, Vol.6, No.30, 1-8
5
physical, chemical and biological factors (Swar and Fernando 1980). Hutchinson (1967) cited numerous studies
which indicated that temperature regulated the birth rate, longevity and other population characteristics of
zooplankton. In a previous study, in the same estuary, availability of nutrients such as nitrate and silicate and
dissolved oxygen were reported to be higher but salinity and turbidity were lower in surface water in post
monsoon season (Divakaran
et al.,
1982). The effect of salinity in the distribution of plankton has also been
discussed by Madhupratap (1978) and Pillai
et al.,
(1973) in the Cochin estuary.
Zooplankton is subjected to wide range of seasonal fluctuations with major peak during monsoon and minor peak
in post monsoon period. Previous studies from the same study area, revealed that copepods emerged as the major
group of zooplankton population (Nair and Abdul Azis 1987). Imbalance in zooplankton population arises from
the fluctuations in the environmental conditions resulting in poor upwelling, rise in sea surface temperature, under
water disturbances, altered monsoon and water currents, which are the main natural causes whereas, one of the
major man made causes for the imbalance is pollution especially due to oil spills (Sharma and Wilma 2007).
Furthermore, copepods can reproduce rapidly and any population reduction can soon be restored (Nair 2001).
Among the functions pointed out by previous authors, reduction of dispersal by currents seems to be important for
the swarms; permitting these copepods, especially adult females producing offsprings, to stay in their preferred
habitats by maintaining their position against the currents.
Swarming as a reproductive strategy has several advantages. Swarm formation seems to be adaptive to enhance
mating success, because encounter rates are accelerated in dense aggregations (Buskey 1995, Ambler 2002,
Jayachandran
et al.,
2015). It also has a greater mating success because potential mates are abundant in swarms
(Ambler
et al.,
1991). In the present study, swarms of copepods were typically composed of adult males and
females, of which
Acartia
species especially
Acartia southwelli
were observed with spermatophore suggesting
that it might be a mating aggregation. Here, the individuals that made up the entire swarm contain most of adult
Acartia
species with a restricted size range (1.14±0.07 mm). In several studies for some invertebrate taxa, the
persistent zooplankton aggregations were directly observed in the field at small spatial scales. Even though the
reasons for swarming remain unknown; the proposed advantages of swarming could include a less predation,
persistence in favorable environment, and proximity to mates. In addition to this, some zooplankton in fact may
respond directly to water temperature and salinity (Buskey
et al.,
1995, Wishner
et al.,
1988). Temperature plays a
single most important physical parameter structuring an ecosystem like rise in temperature influencing water
column stability, nutrient enrichment and primary production that in turn can affect the abundance, size
composition, diversity, and trophic efficiency of zooplankton. Temperature and salinity have often been
considered as the main cause of temporal succession as well as the spatial segregation of the
Acartia
species
(Galleger
et al.,
1996, Conover 1956, Jeffries 1962, Tranter and Abraham 1971). Temperature, salinity, and food
supply are some of the important factors that influence the aggregation of the local population of Acartiidae in
estuarine environments (Greenwood 1981, Uye
et al.,
2000, Islam
et al.,
2006, Hubareva
et al.,
2008).
Aggregation or swarm usually refers to zooplankton densities of the order of 100-1000 animals.m
-3
(Milione
and Zeng 2008); commonly occurring during daylight hours on a diel cycle and contain mostly of adults. There
are several reports that are available on swarming from oceanic realm where, Emery (1968) observed distinct
swarms of copepods
Acartia spinata
,
A. tonsa
,
Oithona oculata
, and
O. nana
in coral reef environments using
SCUBA diving. Marshall and Orr (1952) reviewed the early literature on observations of
Calanus
species
swarming at the surface in summer months. Hamner and Carleton (1979) made further observations on copepod
swarms on and around coral reefs and recorded aggregations of
A. australis
,
A, bispinosa
, and
Centropages
orsinii
. Direct underwater observations on copepod swarms have been rarely reported from other than coral reef
environments by Bainbridge
(1952), whereas several direct observations on copepod swarms were made by
Hamner and Carleton (1979), Russell (1928), Wada (1953), Kitou (1956) and Kawamura (1974) from ships
during cruises or from the sea shores.
