An approach for assessing the suitability of Ceriodaphnia rigaudii as an indigenous tropical toxicity test species

Although the selection criteria for toxicity test species are well-documented, they do not provide a clear approach for establishing the suitability of a species for regulatory testing. This study looks at the approach used to determine if Ceriodaphnia rigaudii, an indigenous tropical cladoceran species, may be designated as a suitable freshwater test species for regulatory testing in Trinidad and Tobago. The approach involved species identification, life cycle characterization, salinity tolerance, toxicological responses to standard chemicals, and comparative sensitivity evaluation with an established test species. The results showed that C. rigaudii (0.45 mm) was significantly smaller in size, had a shorter life cycle and matured faster than temperate cladoceran species such as Ceriodaphnia dubia and Daphnia magna (3–6 mm). Ceriodaphnia rigaudii has a life span of about 10–15 days and reached sexual maturity within 2 days, whereas D. magna had a life span of about 40–50 days and attained sexual maturity within 6–10 days. The LC₅₀ values for C. rigaudii were significantly less than D. magna for the six compounds tested. Interspecies correlation also showed a low positive correlation, suggesting that the sensitivities of both species were not similar for the compounds tested. The sensitivity factors of C. rigaudii and D. magna for the toxicants ranged 0.01–12.3, suggesting that C. rigaudii was more sensitive than D. magna. This study was useful in defining a stepwise approach to help establish C. rigaudii as an indigenous tropical toxicity test species in Trinidad.