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Illuminating the dark diversity via eDNA

Updated: Aug 22, 2022

Locating wedgefishes and giant guitarfishes in Fiji’s coastal waters

Bycatch of unidentified guitarfish from Fiji

Since December 2021, Swiss marine biologist and elasmobranch researcher, Kerstin Glaus, has been conducting a two-year eDNA-project to investigate the occurrence and species composition of wedgefishes and giant guitarfishes in Fiji’s coastal waters.

Based on anecdotal evidence and reported catches, guitarfishes and wedgefishes occur in the waters around Fiji. However, except for Rhychnobatus australia, additional species of guitarfishes and wedgefishes are yet to be identified. Experts even assume the presence of endemics, being species that exist only in Fiji and nowhere else in the world.

  • Wedgefishes and giant guitarfishes have overtaken sawfishes as the most imperiled marine cartilaginous fish families globally.

  • All but one species of sawfish, wedgefishes and giant guitarfishes are considered as critically endangered on the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species assessment.

  • Their demonstrated vulnerability to exploitation requires the development of management strategies, which in turn depend on accurate population assessments.

To date, the occurrence of 75 species of elasmobranchs is documented in Fijian waters, of which about two-third have an elevated extinction risk. Therefore, Fiji is of particular interest for shark and rays biodiversity surveys, as additional at-risk species can be identified which further corroborate the need for conservation management measures.

Unidentified guitarfish from Fiji

Dark diversity

In biodiversity research, we speak of dark diversity when species that should be present within a certain region cannot be found there. Have they actually disappeared?

  • Or are they so elusive that they cannot be found when using standard methods?

  • To date, it is unclear whether this dark diversity of wedgefishes and giant guitarfishes results from local species extirpations or from failure to detect remaining individuals.

A substantial portion of megafauna dark diversity is composed of mobile, rare, elusive, and threatened species that are difficult to detect. Yet the development of management strategies for elasmobranchs depends on accurate species and population assessments. Survey methods, such as fishing by long-lining or gill-netting, acoustic monitoring, baited remote underwater video, underwater visual census and fisheries-dependent population surveys, are often resource intensive, selective, dependent on taxonomic expertise, and are sometimes invasive.

Bycatch of a Whitespotted Guitarfish from Fiji

Also, like most terrestrial vertebrates, some elasmobranchs exhibit learning abilities linked to avoidance behavior, and repeated exposure to negative anthropogenic interactions may increase their elusiveness. This suggests that their prevalence in marine habitats, even close to humans, may be greater than assumed, with individuals being less detectable, therefore overestimating the apparent level of dark diversity. Accordingly, an ongoing concern is the chance of non-detection of remaining individuals by traditional sampling methods.

The answer to this question has significant implications in terms of management and conservation.

Illuminating dark diversity: Environmental DNA (eDNA)

Environmental DNA (eDNA) analysis is based on the retrieval of genetic material naturally released by organisms into their environments. It is an emerging method to detect and identify even rare and elusive species.

This project represents the first targeted attempt to apply eDNA metabarcoding to specifically infer the presence and diversity of wedgefishes and giant guitarfishes in Fiji. Evidence-based and effective shark and ray conservation regulations have yet to be adopted in the country. The results of this project have crucial implications in terms of management and conservation. The presence of previously undetected individuals may require immediate action to prevent extirpation of remnant individuals, whereas the confirmed absence of species requires different management considerations.

Preparation of a SPYGEN filter for sampling Kerstin Glaus after a successful sampling.

Each individual sample contains DNA filtered from 40 litres of seawater.

Specific aims of the project

By 2023, the presence and diversity of wedgefishes and giant guitarfishes in Fiji’s coastal waters will be identified at the species level. In addition, Kerstin Glaus will test the potential of this methodology for multi-species detection.

Species inventories and assessment of geographical distributions based on eDNA metabarcoding could potentially represent an important tool for rapid environmental monitoring. These developments will drastically enhance our knowledge on elusive elasmobranch’s communities and lead to improved conservation strategies.

June 2022


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