Introduction
Recently, mass mortalities of Manila clam, Ruditapes philippinarum, were verified in the lagoons of the North Adriatic Sea. In summer, they could be linked to thermal stress from heatwaves (Bertolini & Pastres, 2021). High mortalities in autumn may be due to the low salinity levels, caused by extreme precipitation events and related high freshwater discharges. This study aimed at elucidating the sublethal effects of temperature and salinity on Manila clam behaviour using the valvometer "SmartShell", a system developed by the Istituto Zooprofilattico Sperimentale dell’Abruzzo e del Molise “G. Caporale” (Di Giacinto et al, 2021).
Methodology
Fifty (50) R. philippinarum specimens were sampled in Venice Lagoon, Chioggia (VE). The first acclimatization period lasted 1 month in automated system (400 Liter aquaria) at 20 °C with artificial seawater (Instant Ocean®). Microalgae suspension of a mixture of Tetraselmis spp. and Dunaliella spp. were used for animal feeding. Experimental setting up was settled in four experimental groups composed by 2 individuals inserted in the SmartShell sensors in closed aquaria at different temperature and salinity conditions:1) 20°C ± 2 °C at 22 psu; 2) 20°C ± 2 °C at 36 psu; 3) 28°C ± 2 °C at 33 psu; 4) 20°C ± 2 °C at 33 psu (control group). The test lasted 14 days, during the first 7 days the behaviour of all molluscs was pre-evaluated at control conditions, then they were put under different salinity and temperature conditions. Four replicates were carried out.
Data were grouped in five classes of valve gapes (VG), namely: VG ≤ 20%, 21–40%, 41–60%, 61–80% and ≥ 81% (Redmond et al., 2017). Both in control and exposed mollusc groups, the average amount of time percentage spent in each VG class has been evaluated. Preliminary analysis showed some effects on periodicity, therefore Fourier analysis was used to analyse differences in periods and amplitudes of oscillations. Fourier analysis was done for each individual for both the pre-exposure and exposure periods separately.
Results and discussion
All molluscs had clearly distinguishable periods of activity and inactivity, repeated within the 24 hours. This was confirmed with the periodograms showing the highest peak around the 24h. In the low salinity group this value was slightly lower indicating a slightly higher frequency of oscillations. In the high temperature treatment, the molluscs spent more time opened than the control group, with the amplitude of oscillations significantly dampened.
In the control group, molluscs showed a flapping behaviour, spending more time at VG category of 61–80%. At lower salinity, the animals spent more time at VG of 61–80%. The most represented VG categories at higher salinity and temperature were 61–80% and 41 – 60%, respectively.
These results, first for clams, confirmed that high temperature influenced the behavioural patterns of molluscs, consistently with results for other species (e.g. mussels, Bertolini et al. 2022).
References
Bertolini, C., & Pastres, R. (2021). Tolerance landscapes can be used to predict species-specific responses to climate change beyond the marine heatwave concept: Using tolerance landscape models for an ecologically meaningful classification of extreme climate events. Estuarine, Coastal and Shelf Science, 252, 107284. https://doi.org/10.1016/j.ecss.2021.107284
Bertolini, C., Capelle, J., Royer E., Milan M., Witbaard R., Bouma T.J., Pastres R. (2022) Using a clustering algorithm to identify patterns of valve-gaping behaviour in mussels reared under different environmental conditions, Ecological Informatics, https://doi.org/10.1016/j.ecoinf.2022.101659
Di Giacinto, F., Berti, M., Carbone, L., Caprioli, R., Colaiuda, V., Lombardi, A., Tomassetti, B., Tuccella, P., De Iuliis, G., Pietroleonardo, A., Latini, M., Mascilongo, G., Di Renzo, L., D’Alterio, N., Ferri, N. (2021) Biological Early Warning Systems: The Experience in the Gran Sasso-Sirente Aquifer. Water, 13, 1529. https://doi.org/10.3390/w13111529
Redmond Kirsten J., Berry Mark, Pampanin M. Daniela, Andersenb Odd Ketil (2017) Valve gape behavior of mussel (Mytilus edulis) exposed to dispersed crude oil as an environmental monitoring endpoint. Marine pollution bulletin 117:330-339.