Dermal denticle assemblages in coral reef sediments correlate with conventional shark surveys

Abstract

It is challenging to assess long‐term trends in mobile, long‐lived and relatively rare species such as sharks. Despite ongoing declines in many coastal shark populations, conventional surveys might be too fleeting and too recent to describe population trends over decades to millennia. Placing recent shark declines into historical context should improve management efforts as well as our understanding of past ecosystem dynamics. A new palaeoecological approach for surveying shark abundance on coral reefs is to quantify dermal denticle assemblages preserved in sediments. This approach assumes that denticle accumulation rates correlate with shark abundances. Here, we test this assumption by comparing the denticle record in surface sediments to three conventional shark survey methods at Palmyra Atoll, Line Islands, central Pacific Ocean, where shark density is high and spatially heterogeneous. We generally found a significant positive correlation between denticle accumulation rates and shark abundances derived from underwater visual census, baited remote underwater video and hook and line surveys. Denticle accumulation rates reflected shark abundances, suggesting that denticle assemblages can preserve a signal of time‐averaged shark abundance in low‐energy coral reef environments. We offer suggestions for applying this tool to measure shark abundance over long time‐scales in other contexts.

Response to Comment on “An early Miocene extinction in pelagic sharks”

“…However, we note and agree with Naylor et al that there are distinct taxonomic groupings of denticle morphotype diversity, which allows them to be used as broad-scale indicators of elasmobranch evolution (3,4,6). Indeed, taxonomically distinct denticles in reef sediments correlate with the relative abundance and presence of those shark species in the local area (7) and have been used to reconstruct historical shark communities on coral reefs (8).…”

Morphological characterization of dermal denticles of the Broadnose Sevengill Shark Notorynchus cepedianus (Elasmobranchii: Hexanchidae)

“…In particular, it has been studied as shark species identification tool (Tanaka et al, 2002;Valenzuela et al, 2008;Marshall, 2011), community's shark reconstruction, especially based on fossil records (Kriwet and Benton, 2004;Kriwet et al, 2008;Dillon et al, 2017;Ferrón et al, 2019;Rangel et al, 2019) and recognition of morphological patterns associated with their ecology (Reif and Dinkelacker, 1982;Reif, 1982;Muñoz-Chápuli, 1985;Reif, 1985a;Ferrón et al, 2014;Ferón and Botella, 2017;Ferrón et al, 2018;Dillon et al, 2020).…”

Fossil dermal denticles reveal the preexploitation baseline of a Caribbean coral reef shark community

“…Denticles are shed naturally and accumulate in marine sediments, where they preserve as fossils (16). Denticle accumulations reflect shark abundances in low-energy reef habitats (17), and denticle morphology varies across sharks with different ecological modes, as it is coupled to denticle function (18-21) (Fig. 1).…”

“…1). Because sharks have several orders of magnitude more denticles than teeth, denticles are far more abundant in reef sediments, facilitating statistical analyses (17,20,21). As such, denticle assemblages can yield rigorous ecological information about past shark communities.…”