4.1. CAB diversity between the copepod genera
Calanus spp. are filter feeders and mostly herbivores, but they do feed on ciliates and other heterotrophic protists during reproduction and energy shortfall [39, 40]. This might be a reason behind their high H index. Most of the gene sequences used for this meta-analysis were from Calanus finamarchicus . However,Centropages sp. feeds on different sources from microalgae to fish larvae [41]. Acartia spp. are primarily omnivorous (with a high degree of carnivore behaviour), feeding on phytoplankton, occasionally ciliates, and rotifers[42], whereas Temora spp. frequently switches its feeding behaviour, i.e. an omnivore or herbivore based on the season and food availability [43]. The bacterial alpha diversity analysis in the Temora sp. revealed a significantly lower Shannon diversity. However, in an early study, no difference was reported in the alpha diversity between the Temoraspp. and Acartia spp. [38]. This can be explained based on the source of copepods involved for the study, Wega et al. [38], which was based only on a single source, i.e. the central Baltic sea; however, in our case the CAB sequences for Acartia spp. were from the central Baltic sea[38] as well as the Gulf of Maine[10]. The occurrence of high Faith’s_PD inPleuromamma spp. maybe due to their range distribution in the water column and few species within Pleuromamma spp. are known to migrate vertically [11, 44] or might also be due to their food uptake, which includes phytoplankton, microzooplankton(ciliates and flagellates) and detritus [11, 45].
The consensus phylogram revealed that, at genera level, Calanusspp. was phylogenetically closer to Pleuromamma spp. and form two distinct clusters in the PCoA plot. Further, the difference in dissimilarity percentage of CAB between Pleuromamma spp. andCalanus sp. may be attributed to the difference in vertical migration, life stages and the feeding behaviour between the two copepod genera. Pleuromamma spp., an omnivorous feeder[11, 45]. can migrate vertically up to 1000m[11, 44] whereas Calanus sp., mostly herbivores, but occasional omnivores [37, 38], can migrate up to 600m [46, 47]. Also, it may be due to the difference in the life stage of Calanus (the microbial communities varied between the diapausing and active feeding)[2].