LAUSR

Abstract This study compares species composition, abundance, biomass, and several other factors by which sedentary megafaunal and macrofaunal assemblages at depths between 14 m and 55 m on the Hueneme Shelf in southern California, USA, can be characterized. It details fundamental differences between these assemblages in this study area and demonstrates how little we know about a diverse range of attributes that are important in describing and evaluating infaunal assemblages. It provides strong support for a recent claim that: “The soft-sediment seafloor of the open continental shelf is among the least-known biomes on Earth, despite its high diversity and importance to fisheries and biogeochemical cycling” (Tomasových and Kidwell, 2017). Basic and applied research after the 1950s transformed infaunal ecology from a discipline focused primarily on long-lived sedentary megafauna (analogous to ‘trees’ on land) into a field concentrating on opportunistic or ephemeral macrofauna (analogous to ‘weeds’ on land). Based on findings of this study, macrofaunal assemblages play a subordinate role in ecosystem dynamics. Most research in recent decades has overlooked contributions by sedentary megafauna. This focus leads to a poor understanding of the dynamics of these widespread ecosystems, severely undervalues their ecological importance and production, underestimates impacts of anthropogenic activities on unconsolidated sediments as well as the duration of recovery trajectories following disturbances. Sedentary megafaunal and macrofaunal assemblages were quite different in virtually all aspects assessed during this analysis. Although macrofauna in this study had far greater species richness and were far more abundant than sedentary megafauna, conservative estimates for megafaunal biomass and secondary production indicate these organisms contribute substantially more energy to higher trophic levels than macrofauna. In addition, the megafaunal assemblages are characterized by many influential ‘ecosystem engineers’. Consequently, the differences in numbers of species and individuals are irrelevant with regard to determining value to the ecosystem. Megafauna in this area also exhibited more sensitivity in environmental factors. They live substantially longer than macrofauna, likely leading to greater stability in megafaunal assemblages. These differences indicate that current approaches focusing on macrofauna severely undervalue infaunal resources and contributions to higher trophic levels. They imply that studies of megafaunal assemblages should provide considerably greater insight into ecosystem dynamics and secondary production than macrofaunal assemblages and would have greater power to predict or assess environmental degradation or change, and document or predict recovery trajectories.