Occurrence of Mass Swarming of Family Acartiidae (Calanoid Copepods) (Zooplankton) in Ashtamudi Estuary, Kerala

Aggregation and swarming of different species of zooplankton has been reported worldwide from estuaries and coastal systems. But such swarming behaviors are seldom reported from tropical water bodies. Mass swarming and aggregation of Acartia species (91%) in mesozooplankton were observed in Ashtamudi estuary on the west coast during late monsoon season (2015). The mesozooplankton density recorded was 106,578 ind.m -3 ; composed of calanoid copepods dominated by females (97%), fish eggs (2.4%) and cyclopoids (0.2%). Even though swarming of copepods appears to be a common phenomenon in temperate shallow waters, reports from tropical estuaries are less. Here, females were outnumbering males, with sex ratio of 1:3 (male to female) and observed with spermatophore in the collections suggesting aggregation of copepods that might be for mating purpose. Since the males are usually short lived, swarming will facilitate high rate of fertilization and it helps the copepods in existence and propagation. They are also able to disperse easily by producing resting eggs leading to their sudden appearance in water column when temperature rises and salinity increases. Presence of such swarms indicates habitat shifts due to climatic change, water pollution and eutrophication issues in the coastal environment.