The sea cucumber Holothuria (Halodeima) grisea Selenka, 1867 is a tropical, epibenthic, non-estuarine echinoderm inhabiting intertidal and subtidal rocky shores from the Caribbean region up to southern Brazil (Santa Catarina State). The commercial interest, the recent threats of unregulated capture, and the potential use of sea cucumber in integrated multitrophic aquaculture, higlighted this species as a candidate for aquaculture in Brazil. The knowledge of the limits of tolerance to low salinity is one of the important factors for adequate aquaculture site selection. However, little information is available about the effects of low salinities exposure on the mortality and physiological stress of H. grisea.
Adults were collected at Porto Belo (SC) (Lat. 27o 09’ 18”; Long. 48o 33’ 48” W) in April/ May 2022 (License Brasil/MMA/SISBIO no. 68215). Two laboratory assays were carried out to determine mortality and ammonia excretion at reduced salinities. The salinities tested in the first assay were 34, 30, 25, 20 and 15 PSU. A second assay tested the salinities 20, 15, 10, 5 and 0 PSU. The ambient temperature was controlled in both assays (20,5oC ± 1,5), pH and ammonia were monitored daily. Experiments were carried out in 10-L aquaria with four replicates per treatment. Observations were made at 6, 12 and 24 h of exposure, and subsequently every 24 hour up to 144 hours. Individuals were considered dead, when podia were not attached to the container walls and no contractile responses of the tentacles were displayed after touching with a glass rod. Death was confirmed after returning them to 34 PSU and no activity was recovered.
In the first assay, no mortality was recorded after 144 hours of exposure at the tested salinities, however at 20 and 15 PSU individuals were weakened at the end of the experiment. Whereas in the second assay, 100% mortality was attained after 6h at 0 PSU; 24 h at 5 PSU and 48 h at 10 PSU. Ammonia excretion (figure 1) results from protein metabolism, and its increase can be considered an indicator of stress resulting from amino acid catabolism for intracellular osmotic regulation. H. grisea can tolerate exposure to salinity as low as 15 PSU for up to 6 days, which indicates that this species is relatively resistant to low salinities, however, the high rate of ammonia excretion below 25 PSU indicates physiological stress. This information will be valuable for future development of sea cucumber aquaculture in Brazil.