Application of menthol anesthetics for body size measurements of the sea angel Clione elegantissima (Mollusca: Gymnosomata)

Understanding body size variation has been a central goal in ecology for almost half a century. However, it is difficult to measure the size of soft-bodied invertebrates accurately, such as members of the genus Clione, which expand and contract their bodies. To allow for accurate body length measurements, anesthetic menthol was used to temporarily relax the bodies of Clione elegantissima collected from the southern Okhotsk Sea. A standard solution was prepared using 1.4 g l-menthol dissolved in 250 mL ethanol. Three test solutions were then prepared using 0.1, 0.25, or 0.5 mL of the standard solution and dissolved in 20 mL of filtered seawater. Using a menthol-based anesthetic, the animals extended their body parts (including foot lobes, wings, and head tentacles), remained motionless, and showed no reaction to pricking with forceps. All tested animals returned to their actual sizes within 1–15 min after placing them back in filtered seawater, indicating that the menthol-based solutions were effective and harmless. The best concentration for measuring the body size C. elegantissima was 0.1 mL of the standard solution. It allowed not only bodies but head tentacles to stretch, providing to the full-length of Clione samples for body size measurements. We suggest that this anesthetic can be used as a new standard for body measurements and morphological observations in Clione species.