1 Introduction

Swarming of zooplankton at various temporal and spatial scales has been observed generally; aggregative behavior has been well studied in larger aquatic organisms (fish, krill) and terrestrial animals (birds, insects), but little is known for copepod aggregation. Even though swarming behavior of copepods in temperate region is available, estuarine region in the tropics has remained almost unknown for many reasons. Furthermore, there is little quantitative knowledge available on the reasons for aggregations in zooplankton and the purpose of these aggregations. Several reports are available on the swarming of zooplankton like amphipods along the west coast of India, (Nair 1972) and ostracods, salps, medusa and pelagic amphipods from off Kutchh region, Gujarat (Paulinose and Aravindakshan 1977). 

In aquatic zooplankton community, copepoda is the most dominant and second largest crustacean taxa, representing 70% of the oceans biomass; consumed by a wide range of predators forming the important phytoplankton and micro zooplankton grazers, which is a major trophic link to many predatory invertebrates and fishes (Atkinson 1996). Among the calanoids, members of the genus Acartia are the major constituents of the holoplankton communities in coastal zones, estuaries and other semi-enclosed marine areas (Conover 1956, Abraham 1969, Alcaraz 1983, Lakkis 1994). Family Acartiidae is distributed worldwide and are mostly common and abundant in estuarine and coastal waters. The most commonly occurring calanoid copepods, especially A. southwelli and A. centrura, are the inhabitants of estuaries that have the capacity to grow fast and breed continuously with high reproductive capacity. Acartia southwelli, the only representative of the subgenus Euacartia that was the major component of the swarm reported; originally described incompletely by Sewell from Kilakarai (9°25' N, 78°50' E), Tamil Nadu, South east coast of India (Sewell 1914).

2 Materials and Methods

The mass swarm of calanoid copepods was observed in Ashramam (8°54'13''N - 76°34'45''E) in Ashtamudi estuary during the late monsoon season (September) of 2015. Ashtamudi estuary, the second largest aquatic system in Kerala, is located in the Kollam District, with a water spread area of about 32 km² (8°53′-9°2′N and 76°31′–76°41′E) (Figure 1) . It is a palm shaped extensive water body with eight prominent arms, adjoining the Kollam town. The major river discharging is the Kallada River, originating from the Western Ghats; formed by the convergence of three rivers, the Kulathupuzha, the Chendurni and the Kalthuruthy. The estuary opens to the Arabian Sea at Neendakara, southwest coast of India. Several major and minor drainage channels loaded with waste products from municipal and industrial (mainly fish processing units) sources join the estuary at the southern end. Coconut husk retting for the production of coir fiber is predominant at several locations in the eastern arms of the estuary.

The mesozooplankton samples were collected using bongo net (mesh size 200 μm) with a mouth area of 0.28 m². A calibrated flow meter (General Oceanics model number-2030 R, 2012) was attached to the net and was towed horizontally just below the surface at a fixed speed of approximately 1 knot for 10-15 minutes. The samples were fixed in 4 % buffered formalin and 95% ethyl alcohol. Zooplankton biomass was estimated by displacement volume method and expressed as ml.m^-3 (Harris et al., 2000, Johnson and Allen 2005). Dissolved oxygen concentration was estimated by modified Winkler’s method and Mohr-Knudson method for measuring salinity (Strickland and Parsons 1972, Grasshoff et al., 1999). Free carbon dioxide was estimated by APHA (2005). The water samples for estimation of the inorganic nutrients, nitrate-nitrogen, nitrite-nitrogen, ammonia-nitrogen, phosphate-phosphorus and silicate-silicon were acidified with conc.HNO₃ and analyzed based on standard methods (Grasshoff et al., 1999). Samples were sorted at group level for major zooplankton taxa (Omori and Ikeda 1984, Tait 1981, Todd and Laverack 1991) enumerated and density was expressed in ind.m^-3. Each copepod was identified to species level using standard keys (Sewell and Seymour 1947, Kasturirangan 1963, Wellershaus 1974). For determination of sex ratio, 150 individuals were removed from each other and sexed. The sex ratio [number of males (M) to number of females (F)] was tested by the Chi- square analysis (X²) using the software, Statistical Package for the Social Sciences (SPSS) version 20.

3 Results

The average mesozooplankton density recorded in Ashtamudi estuary was 150533 ind.m^-3. Highest density of mesozooplankton was recorded in Ashramam (251733 ind.m^-3), followed by Ashtamudi (115933 ind.m^-3) and lowest was in Kavanadu (83933 ind.m^-3). In Ashramam, swarm was composed of very small numbers of organisms other than the members of swarming species of calanoid copepods (97%). Fish eggs contributed 2.4% of the total planktonic population while other organisms such as cyclopoid copepods (0.2%), crustacean nauplii  (0.3%), molluscan larvae (0.1%) and cumaceans were scarce in the sample. In Kavanadu, calanoid copepods contributed 85.3% followed by crustacean nauplii (9.6%) and cyclopoid copepods (2.6%), whereas in Ashtamudi, calanoid copepods contributed 69.3% of the total zooplankton population followed by crustacean nauplii (9.4%), cladocera (6.6%), cyclopoids (3.6%), copepodids (3.6%) and fish eggs (2.2%).

The calanoid copepod density was 244200 ind.m^-3 in Ashramam; represented by Family Acartiidae (98%) and Paracalanidae (2%) (Figure 2). Swarming of Acartia southwelli (34%) was observed along with Acartia plumosa (16%), Acartia centrura (16%), Acartia tropica (16%), Acartia bilobata (9%) and by Bestiolina similis (9%) in Ashramam (Figure 3). Adult copepods generally constituted the major part of the swarm; especially, A. southwelli, A. plumosa, and A. centrura that consisted almost entirely of adults. Adults constituted more than 85% of the copepods in the swarms, reported in Ashramam. Among adults, females generally outnumbered males in the swarms and Acartia species, representing mostly adults showed extreme values of adult sex ratios.  Sex ratio of the swarm was 1:3 in the Ashtamudi estuary during the study period. Chi square test (X²) indicates that the sex ratio significantly differed from the expected sex-ratio of 1:1. Therefore, the overall sex –ratio of calanoid copepod was significantly in favor of females (X²=9.8; p<0.01). The sex ratio in adult calanoid copepod populations was typically biased with the dominance of females.

The physico chemical characteristics of Ashtamudi estuary is presented in Table 1. Temperature in the water column ranged from 28°C to 30°C; salinity from 20 ppt to 26 ppt and the pH from 5.1 to 6.8, that was slightly acidic.

4 Discussion

Aggregation of zooplankton mainly appears to be influenced by various factors. They can be passively concentrated or actively motivated. When considering the nutrient profile, usually monsoon showers bring nutrients from allochthonous sources into the systems enriching the phosphate, nitrate and silicate concentrations in water (Bijoy Nandan 2008). Apart from nutrients, the present study revealed that salinity was the most fluctuating factor recording maximum at the swarming area, Ashramam. Although most zooplankton species survive under a wide range of environmental conditions, their growth and density depend on a number of 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.

Studies on swarms of Acartia species have been reported from tropical to temperate latitudes; A. spinata and A. tonsa from turtle grass beds, coral rubble and mangrove prop roots in Florida and Belize (Bainbridge 1952); A. australis from the lagoon in Davies Reef (Great Barrier Reef), A. bisponsa in Palau lakes (Emery 1968) and A. hamate in fringing coral reefs in Okinawa and A. plumosa, A. steueri, A. japonica and A. omorii in temperate bays of Japan (Hamner and Carleton 1979). Aggregation of Family Acartiidae is likely linked to the fact that some Acartia species have the ability to disperse by producing resting eggs, leading to their sudden appearance when favorable conditions are encountered (Udea 1983, Uye and Fleminger 1976). They are likely adapted to high concentrations of food in coastal waters where the copepods are a major food of fishes (Uye et al., 1979). Swarming behavior can also be affected by the interactions of swarming animals with light cues, water current and turbulence, behavior of predators and prey. Photo taxis maintains swarms of cyclopoid copepod Dioithona oculata in shafts of light between mangrove roots (Yoo et al., 1991) and similar responses to light gradients are known in several other species (Ambler 1991, Hamner and Carleton 1979, Hebert 1988). Attraction to food odours and increased turn in food patches which aids foraging and increased forager density have also been reported in a number of species (Udea et al., 1983, Williamson 1981, Poulet and Ouellet1982, Tiselius and Jonson 1990). Swarming also occurs in turbid water when turbidity increases planktonic omnivorous copepods that can encounter more prey. Dissolved oxygen could also potentially affect the spatio temporal variation of copepods in an estuary. During the study, as a result of swarming comparatively low dissolved oxygen and higher carbon dioxide concentrations were recorded. Copepods are unresponsive to oxygen-saturated water; they usually avoid hypoxic water, when dissolved oxygen (DO) is naturally depleted. In most cases, hypoxic conditions are observed in eutrophic or polluted areas and in waters dominated by high stratification and residence time in summer (Soetaert and Rijswijk 1993, Park and Marshall 2000). However, hypoxic conditions may cause habitat shrinkage, which was not seen in the Acartia populations during this study. The other factors influencing the distribution of copepods might be determined by preferences to environmental factors or tolerance to other species (Uye et al., 2000).

Thus, the overall localized habitat shifts in zooplankton population, arising from marked changes in environmental variables, such as temperature, salinity, nutrients, carbon dioxide and other climatic and anthropogenic factors could result in swarming conditions occurring in such coastal water bodies.

Acknowledgements

The authors gratefully acknowledge the financial support provided by University Grants Commission to conduct this study and also to the Head, Department of Marine Biology, Microbiology and Biochemistry, School of Marine Sciences, Cochin University of Science and Technology for the facilities.

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