The Heliconoides Modified Dissolution Index-HMDI: Do methane seepage environments affect the preservation state of Heliconoides inflatus?

•Pteropods are holoplanktonic molluscs presently endangered because their fragile aragonitic shell is very sensitive to ocean acidification.•We have revealed the nature and origin of the yellowish pteropod shells present in sediments from the Eastern Mediterranean.•We categorize the preservation of Pteropods and established a modified Indices: Heliconoides Modified Dissolution Index.•We have shown the applicability of the HMDI to normal marine but also to methane seeps environments.