Respiration rate and swimming activity of larvae of two sub-tidal nassariid gastropods under reduced oxygen levels: Implications for their distributions in Hong Kong waters

The effects of hypoxia on the larvae of two sub-tidal nassariid gastropods, Nassarius siquijorensis and N. conoidalis were compared so as to understand how the species-specific tolerance to hypoxia might have resulted in changes in the abundance and distribution of these two species in the hypoxic Tolo Harbour, Hong Kong, since the 1980s. Respiration rates of N. siquijorensis and N. conoidalis larvae were reduced at 4.5mg O₂ l^-1, or below, as compared with the normoxic control. Significant reduction in swimming velocity was also observed for 10-day old larvae which were exposed to <2.0mg O₂ l^-1 for N. siquijorensis and <1.0mg O₂ l^-1 for N. conoidalis. The 48h LC₅₀ values of N. siquijorensis and N. conoidalis larvae were 0.7 and 1.7mg O₂ l^-1, respectively. The results suggested that N. siquijorensis are more tolerant to hypoxia than N. conoidalis.