Holocene records of an irregular echinoid unveil 19th century population decline in the northern Adriatic Sea

The consequences of historical and long-term human impacts on non-commercial marine benthic species are poorly known due to temporally limited monitoring data. Skeletal remains preserved in sediment cores provide a historical archive for reconstructing ecological baselines prior to anthropogenic impacts and can document population fluctuations not captured by recent ecological surveys. Here, we traced a unique Holocene record of the echinoid Echinocyamus pusillus from sediment cores and grab samples in the northern Adriatic Sea, a region affected by human disturbances over millennia. The exceptional preservation of the small luminacean archives long-term persistence of this species over the past ~11000 yr, providing a baseline for understanding long-term dynamics of echinoid populations. The radiocarbon age-dating of multiple specimens from the surface sediment layer revealed high abundance of E. pusillus tests from the 18 th and early 19 th century but an absence of individuals that died within the last ~100 yr. This pattern suggests a dramatic decline in test production and thus population densities since the late 19 th century, likely linked to intensified siltation, trawling-induced sediment disturbance, eutrophication, and hypoxia in the northern Adriatic Sea. Our results highlight the vulnerability of non-commercial or poorly studied benthic species to human-driven habitat degradation, which is often difficult to assess without palaeoecological data due to the historical legacy of many human impacts.