New data on the seasonality of Flabellina affinis (Gmelin, 1791) and Cratena peregrina (Gmelin, 1791) (Gastropoda Nudibranchia) in the Ionian Sea, Central Mediterranean

Flabellina affinis (Gmelin, 1791) and Cratena peregrina (Gmelin, 1791) are two common nudibranchs in the Mediterranean Sea. However, there are only a few studies on their seasonality which reported these species principally in summer and in well-lit shallow areas. Instead, through the present study carried out throughout three years (from 2017 to 2019) in three areas sited along the Ionian coast of Sicily (Italy), it has been observed that: 1) both species may be present in any season of the year with a high number of specimens; 2) F. affinis in the study areas is more competitive than C. peregrina; 3) both species showed a less photophilous lifestyle than that usually reported in literature, since in this study both species were found in a deeper bathymetric range; 4) F. affinis and C. peregrina could be considered warm-water species and their strong presence in cold seasons might be used as an indicator of the increase in the seawater temperature of the Mediterranean Sea.