160-9 Body Size and Extinction Threat in Modern Mollusks with Comparisons to Cenozoic Patterns

Abstract:

Mollusks are are on of the most species-rich and functionally diverse groups of animals in the modern ocean. However, marine mollusks are severely underrepresented on the The IUCN Red List of Threatened Species. The World Register of Marine Species lists more than 61,000 marine mollusk species. However, only 2,098 species of marine mollusks have been added to the Red List, which constitute just over 3% of know species. Coverage of marine mollusks is in stark contrast to other groups such as marine tetrapods and terrestrial/freshwater mollusks, with approximately 100% and 26% of know species listed on the Red List, respectively. Despite the low coverage of marine mollusks on the Red List, the situation has been improved, with 804 species added since 2016 decade and 210 species were added in 2026.

This study takes advantage of a database of extant and extinct marine mollusk body sizes to evaluate potential body size biases in the species that have been added to the Red List and to compare extinction threat in the modern to the Cenozoic extinction selectivity. The data show that the large-bodied bivalves and gastropods are overrepresented on the Red List. Red List Cephalopods appear to be a random sample with respect to body size—primarily because 759 of the 869 know species are on the Red List. Body size selectivity patterns are mixed among classes. Threatened bivalves species do not show body size selectivity while larger cephalopods are more threatened and smaller gastropod species are more threatened. Gastropod selectivity is the strongest among the three classes and persists even though there is a bias in the Red List towards larger species.

The Red List species list was "degraded" to a genus-level dataset to compare with extinction selectivity across the Cenozoic. For Red List species with more than one species per genus, the most threatened species was used for the whole genus. Over the Cenozoic, smaller genes of bivalves and gastropods are more likely to go extinct than larger genera. For both classes, the modern extinction selectivity is outside the range of Cenozoic background, suggesting the ecological impacts of future extinctions could fundamentally reorganize marine ecosystems.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.