The timing of marine heatwaves during the moulting cycle affects performance of decapod larvae

Abstract

Marine heatwaves (MHW) pose an increasing threat and have a critical impact on meroplanktonic organisms, because their larvae are highly sensitive to environmental stress and key for species' dispersion and population connectivity. This study assesses the effects of MHW on two key moulting cycle periods within first zoea of the valuable crab, Metacarcinus edwardsii. First, the changes in swimming behaviour during zoea I were recorded and associated to moult cycle substages. Then, larvae were exposed during the zoea I to (1) control temperature of 12 °C, (2) Early MHW, occurring in intermoult, (3) Late MHW, occurring in premoult and (4) 14 °C, representing MHW during whole development. Additionally, optimum temperature was estimated from thermal performance curves through swimming behaviour of one-day zoea I. The timing of the MHW within the moulting cycle significantly affects larval fitness. Early MHW led to improved survival rates (72%) and reduced developmental times (9.8 days) compared to those exposed to Later MHW (63% and 10.3 days, respectively). As optimum temperature was higher than 12 °C, MHW events maybe favouring larval performance. These results highlight the importance of interaction between the moult cycle and environmental variables as a factor of sublethal effects on population dynamics.

Fig. 1

Time series of daily sea surface temperature (SST, black line) from 2015 to 2020 along the coast of Valdivia (40°S, 73.75°W). The blue line represents the 35 year-climatology (1982–2016), while the green and red lines indicate the Marine heatwaves (MHWs) thresholds, corresponding to the 99th (P99) and 90th (P90) percentiles, respectively. The green and pink shaded areas indicate MHW events exceeding the P99 and P90 percentile thresholds, respectively. The grey bars highlight the spring seasons. SST data are based on ERA5 reanalysis project. MHWs were detected following the methodology described by Hobday et al..

Fig. 2

The average distance moved per individual in an hour tracked throughout the days of the first zoea at 12 °C. The black brackets represent the standard error between the 4 female’s larvae. Bars showing the same letter denote no statistical differences among days after hatching.

Fig. 3

Thermal performance curves fitted with the Flinn model, based on (A) the distance moved and (B) the percent of time in movement by zoea freshly released in a 5-minute record. Blue and red dashed lines represent the lower and upper thermal optimal limits, respectively. The green dashed line denotes the optimal temperature. Top-right table show the values corresponding to the three optimum parameters highlight by the dashlines. Black dots and brackets represent the mean and standard error between 48 larvae performance per temperature.

Fig. 4

Shared-cox model curves of survival of Metacarcinus edwardsii. Coloured lines represent the estimated survival probability for each treatment of thermal regime through the days of zoea I development. The shaded colours show confidence interval (level 0.95).