Come rain or come shine: environmental effects on the infective stages of Sparicotyle chrysophrii, a key pathogen in Mediterranean aquaculture.

Background: Evidence concerning the environmental influence on monogenean transmission and infection processes is widely accepted, although only the effects of a limited number of abiotic factors on particular monogenean species have been explored. The current context of climate change calls for further research both on this subject, and also that concerning monogenean hosts, especially in aquaculture.

Methods: In this study, four experiments were used to assess the response of the infective stages of Sparicotyle chrysophrii, a pathogenic monogenean from gilthead sea bream (Sparus aurata) cultures in the Mediterranean, to variations of temperature (from 10 °C to 30 °C), pH (7.0 and 7.9), photoperiod (LD 12:12, LD 0:24 and LD 24:0) and salinity (from 27 ppt to 47 ppt).

Results: Thermal variations cause the strongest responses among the infective stages of S. chrysophrii, which reduced development and survival times as temperature increased. The optimal thermal range for maximum hatching success was found between 14 and 22 °C, whereas temperatures of 10 and 30 °C probably represent biological thermal limits. Reductions of development time and hatching rates were recorded at the lowest pH level, but hatching success remained above 50%, suggesting a certain degree of tolerance to slight pH variations. Photoperiod acts as an environmental cue synchronising the circadian hatching rhythm of S. chrysophrii with the first four hours of darkness. Response to a wide range of salinities was negligible, suggesting a high tolerance to variations of this abiotic factor.

Conclusions: Larval development and hatching of S. chrysophrii are modulated according to environmental factors, mainly temperature, thus parasite-host coordination and successful infections are enhanced. Therefore, abiotic factors should be broadly considered to design treatments against this monogenean. The high tolerance to the predicted environmental variations over the next century reported for gilthead sea bream and herein exposed for S. chrysophrii suggests that neither will be notably affected by climate change in the western Mediterranean region.