Coping with salinity change: How does the cyclopoid copepod Apocyclops royi (Lindberg 1940) do it?

Abstract

The cyclopoid copepod species Apocyclops royi has attracted significant attention due to its importance in marine food webs and its role as a vital food source for many marine organisms, particularly marine fish larvae. This study aims to understand the activity patterns, osmoregulation mechanisms, and physiological adaptations of A. royi in response to acute decreasing salinities. In total three experiments were conducted. The first two experiments both investigated behavioural change and survival as a function of acute decreasing salinities in the range from 32 to 0, with steps of salinity reductions of five. The third experiment investigated the correlation between internal and external osmolality in A. royi, by using a novel method developed for the experiment. The first experiment indicated that A. royi behaviour and survival were not affected at salinities from 20 and higher. Surprisingly, some copepods were able to survive an acute decrease in salinity from 32 to 0. The second experiment utilized, for the first time for this copepod species, an in situ Multispecies Freshwater Biomonitoring system, to further observe A. royi's behaviour. The results showed that the system was able to monitor A. royi activity level. The system both documented that A. royi exhibit a statistically significant increase in activity levels in response to light. Furthermore, it provided knowledge about the temporal activity level of A. royi as a function of acute decreases in salinities, providing insights into that A. royi has an ∼3 h acclimatization time to an acute decrease from 32 to 0 salinity. In the third experiment, the osmolality of the copepods' body fluids with relation to external osmolality was examined using a vapor pressure osmometer. In this context a new method to extract body fluids from A. royi was developed. The body fluid osmolality of copepods exposed to three different salinities 10, 20 and 32 was examined. The results showed that A. royi is an osmoconformer at a higher salinity 32 but initiates hyperregulation at a lower salinity 10. Furthermore, it was observed that when copepods were exposed to a salinity of 10, 1000 individuals (stage: C5 or adults) were needed to obtain one sample of body fluid (10 μL) whereas when exposed to a salinity of 32, 3000 individuals were required to extract the same amount of body fluid. Overall, the findings demonstrated that A. royi has a high tolerance for acute decreases in salinity, showcasing behavioural adaptations and osmoregulatory capabilities, at extreme salinities. These results contribute to our understanding of copepod physiology and their ability to thrive in various habitats. Further research is needed to fully comprehend the physiological mechanisms underlying A. royi's adaptation abilities to acute decreases in salinity.