The diversity of freshwater calanoid copepods from different habitats in Vietnam is investigated based on our findings from a field expedition in 2012–2013 and literature reviews. We collected 160 samples from 87 sites, including lakes, ponds, roadside canals, rivers, and rice fields in eight provinces of southern Vietnam. A total of 13 species belonging to eight genera and three families were recorded. Among these, four were recorded for the first time in Vietnam (Mongolodiaptomus malaindosinensis, Mongolodiaptomus mekongensis, Vietodiaptomus blachei, and Pseudodiaptomus siamensis). One unidentified taxon (Tropodiaptomus sp.) probably belongs to an undescribed species. Both Eodiaptomusdraconisignivomi and M. malaindosinensis were the most frequently encountered species (28.74% of the sampled sites), followed by Mongolodiaptomus botulifer (24.14%), while Neodiaptomus yangtsekiangensis, Tropodiaptomusoryzanus, and Tropodiaptomus sp. are rare species found in a single locality. To date, 40 calanoid species (33 in the family Diaptomidae) have been recorded from Vietnam, and an updated list is presented. Seven species are potentially endemic to Vietnam. At the same sampling dates, the species richness of the calanoids was a range of 1–5 species per locality. The results of the Canonical Correspondence Analysis showed that pH and conductivity tended to be positively related to the calanoid distribution.

The freshwater calanoid copepod Mongolodiaptomus malaindosinensis is an endemic species of the lower Mekong basin, including Malaysia. The relationships between environmental factors and life history traits have been reported for many copepods, while they have never been assessed in this tropical species. Post-embryonic development time (post-EDT), body size and survival rate of M. malaindosinensis were determined under different algal concentrations of Chlamydomonas reinhardtii and Cryptomonas tetrapyrenoidosa at 25°C, to clarify the effects of algal species and carbon supply on their somatic growth and survival. Post-EDT until adult in M. malaindosinensis decreased with increasing food carbon supply regardless of the algal species tested. Although naupliar developments were the shortest at 10⁴ cells mL⁻¹ of C. reinhardtii, food shortage delayed the development time and also induced high mortality during the copepodid stages. The highest survival rate of 93% was obtained in the copepods fed on C. tetrapyrenoidosa at 5×10⁴ cells mL⁻¹, whereas the survival rates in the remaining lower carbon supply were <23%. The most sensitive stages were the 1st copepodid stage, as indicated by high stage-specific mortalities, because of large morphological and physiological changes in this transitional stage. Prosome length of adult M. malaindosinensis positively correlated with food carbon supplies, though females were larger than males. Somatic growth rate exponentially increased with increasing food carbon supplies with a threshold at around 4.0 mgC L⁻¹. These results suggest that tropical copepods may be adapted to higher food quantities due to their high metabolic rates under high temperature conditions.