An environmental DNA tool for monitoring the status of the Critically Endangered Smalltooth Sawfish, Pristis pectinata, in the Western Atlantic

Abstract

The Critically Endangered Smalltooth Sawfish, Pristis pectinata, was once widespread in the tropical and subtropical waters of the Atlantic Ocean, but following substantial declines over the past century, the core population is currently confined to southwest Florida in the U.S. and the Bahamas. Recent research and verified public encounter reports suggests that this core population may be stabilizing and, potentially, expanding into formerly occupied areas of their historic range in the Western Atlantic; however, the status of this species in non-core waters is not well understood. Environmental DNA (eDNA) methods provide a relatively cost effective and rapid assessment tool for monitoring species occurrence in aquatic habitats. Here, we have developed an eDNA tool: a species-specific Droplet Digital PCR (ddPCR) assay targeting a 100-base pair portion of the mitochondrial NADH dehydrogenase subunit 2 gene in P. pectinata, with the ability to reliably detect as little as 0.25 pg of target DNA. The assay was validated by collecting and analyzing a water sample from known P. pectinata nursery habitat in Florida, which was found to contain an average of 11.54 copies of target DNA/{micro}L (SE = 0.72) in the reaction. The assay was then further tested by placing a juvenile sawfish in an ex situ tank and analyzing water samples collected at time intervals. The implementation of this eDNA tool into field surveys will provide additional, reliable data to assess species recovery and aid in prioritizing localities beyond the core range in which to focus research and education initiatives.