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

Abstract

Preexploitation shark baselines and the history of human impact on coral reef–associated shark communities in the Caribbean are tpoorly understood. We recovered shark dermal denticles from mid-Holocene (∼7 ky ago) and modern reef sediments in Bocas del Toro, Caribbean Panama, to reconstruct an empirical shark baseline before major human impact and to quantify how much the modern shark community in the region had shifted from this historical reference point. We found that denticle accumulation rates, a proxy for shark abundance, declined by 71% since the mid-Holocene. All denticle morphotypes, which reflect shark community composition, experienced significant losses, but those morphotypes found on fast-swimming, pelagic sharks (e.g., families Carcharhinidae and Sphyrnidae) declined the most. An analysis of historical records suggested that the steepest decline in shark abundance occurred in the late 20th century, coinciding with the advent of a targeted shark fishery in Panama. Although the disproportionate loss of denticles characterizing pelagic sharks was consistent with overfishing, the large reduction in denticles characterizing demersal species with low commercial value (i.e., the nurse shark Ginglymostoma cirratum) indicated that other stressors could have exacerbated these declines. We demonstrate that the denticle record can reveal changes in shark communities over long ecological timescales, helping to contextualize contemporary abundances and inform shark management and ecology.

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 study of the oral denticles of the porbeagle shark Lamna nasus

“…Morphological studies classified the dermal denticles into five functional morphotypes: drag reduction, abrasion strength, ridged abrasion strength, generalized functions and defence (Dillon et al , 2017, 2021; Ferrón & Botella, 2017; Raschi & Tabit, 1992; Reif, 1978, 1985). The results showed that L. nasus oral denticles can be classified as abrasion strength as its denticle is diamond‐shaped, lacks lateral cusps and has less pronounced ridges, which are restricted to the anterior portion of the denticle.…”

“…According to Dillon et al . (2021), drag reduction and abrasion strength morphotypes are mostly found in pelagic and active sharks such as Carcharhinidae and Sphyrnidae, whereas ridged abrasion strength is observed in demersal sharks such as Ginglymostomatidae. In addition, Reif (1978) classified most Carcharhiniformes as bearing the drag reduction morphotype, e.g., Negaprion , Galeocerdo , Triaenodon , Galeorhinus , Prionace , Sphyrnidae, Carcharhinus spp., and also some Lamniformes such as Alopias vulpinus and members of the family Lamnidae, that is, a morphotype related to active and pelagic life habits.…”

“…Regarding the analysis of dermal denticles of elasmobranchs, recent studies have been employing micro CT scan to describe their morphological aspects (e.g., Tomita et al, 2020) or to investigate their biomimetic properties (Ankhelyi et al, 2018;Domel et al, 2018;Wen et al, 2015). Although histology and SEM are the most usual tools for this kind of work (Compagno, 1988;Dillon et al, 2017Dillon et al, , 2021Popp et al, 2020;Poscai et al, 2017Poscai et al, , 2021Rangel et al, 2016Rangel et al, , 2017Rangel et al, , 2018Reif, 1974Reif, , 1985, CT scan method works as a complement to elucidate the morphology of dermal denticles, which is not possible with the use of traditional techniques.…”

Emergent research and priorities for shark and ray conservation

“…Research on pre-scientific accounts and museum collections has helped identify the presence of 2 sawfish species in the Mediterranean Sea, a region previously deemed to have unsuitable environmental temperatures for these species (Ferretti et al 2016). Dermal denticles from sediment cores allowed reconstruction of pre-human elasmobranch community baselines in coral reef ecosystems (Dillon et al 2017(Dillon et al , 2021.…”