Impact of temperature and salinity on the morphological variability of lab-grown Cyprideis torosa (Jones, 1850) (Ostracoda)

Keywords: Culture experiment, Size, Proxy development, Phenotypic adaptation

The carapace of ostracods (small crustaceans) protects their soft body from harsh conditions. Its size can vary distinctly within one species but major causes for this variability remain uncertain. Cyprideis torosa is a widespread brackish water ostracod-species with high morphological variability, making it a well-suited study object relating environmental conditions quantitatively to its morphology. Since this species lives in habitats with great spatio-temporal variability, lab-cultures are highly valuable for studying its phenotypic adaptation to different conditions.

In June 2024 location water and sediment samples were collected from the Marano Lagoon (North Italy). The samples provided three levels of salinity (PSU 7.7, 16.1 and 29.6) which were used to start experimental cultures (sediment from PSU 7.7 served as origin of all ostracod specimens). All three levels of salinity were incubated at four different constant temperature conditions ranging from 15 to 35°C. One set of cultures was placed outside the building, being exposed to fluctuating temperatures. Size and other morphological features were analysed to identify specific environmental influences on its morphological characteristics.

Our study reveals that temperature and salinity play an important role on size variability and variance of the carapace. Individuals living in higher salinities and cooler temperatures grow bigger. The opposite is true for extreme conditions (high temperatures) or low salinities. While high temperatures cause significantly smaller carapaces at high salinities, they lead only to higher variances in lower salinities without affecting the average size. The average size of one outside culture (PSU 7.7) reveals that size may be unaffected by diurnal changes. The results were compared to C. torosa valves collected from ostracods grown in the lagoon (PSU 22.7). The length of the individuals of this sample correlates best with rather extreme conditions in our cultures. The length/height ratio of left valves of natural environments (such as lagoon sample, permanent culture and naturally grown ostracod culture starters) are similarly low to each other, corresponding only to the experimental grown ostracods in high mesohaline salinity at 30°C, while other experimental grown individuals show a higher ratio in average.

Our results indicate that salinity as well as temperature influence the size of C. torosa simultaneously and requires further morphological analysis to separate these factors.