Abstract
The freshwater mussel fauna of Borneo is highly endemic, with at least 11 species being unique to that island. Most of these species have not been recorded for at least 50 years owing to a lack of sampling effort and large-scale habitat destruction and degradation. Surveys conducted in 2016 across much of Malaysian Borneo failed to locate four out of five native species historically recorded in the study area. The present study aimed to determine the diversity and distribution of freshwater mussels of Brunei and adjacent Limbang Division, Malaysia. In 2018, we conducted interviews with locals, recorded environmental data and surveyed mussels at 43 sites, and conducted interviews at a further 38 sites. Only one population of native mussels, i.e. Ctenodesma borneensis, was found in a small tributary of the Limbang River situated in a patch of intact rainforest, representing the first record of this Bornean endemic genus since 1962. In addition, Sinanodonta lauta was found in a pond in Lawas district, representing the first record of this species outside its native East Asian distribution. Our data suggest that C. borneensis can sustain populations in relatively undisturbed habitats and is likely to have suffered population losses across northern Borneo. The first molecular phylogenetic analysis (COI + 28S) including an endemic Bornean freshwater mussel genus revealed that Ctenodesma is phylogenetically divergent from all other previously sampled lineages, rendering it a particularly valuable conservation target.
Highlights
Freshwater mussels (Unionida) are one of the most threatened animal groups, with 45% of assessed species being listed as near-threatened, threatened or extinct in the IUCN Red List (IUCN 2019)
This severe lack of knowledge is a result of the lack of targeted surveys, it may reflect that Bornean freshwater mussels have become more rare and, in some cases, potentially extinct in the course of intense deforestation and habitat destruction since the 1960s (Brook et al 2003; Iwata et al 2003; Gaveau et al 2014)
The species was identified as Ctenodesma borneensis by morphological characters
Summary
Freshwater mussels (Unionida) are one of the most threatened animal groups, with 45% of assessed species being listed as near-threatened, threatened or extinct in the IUCN Red List (IUCN 2019). The conservation status of 53% of the 135 freshwater mussel species present in Southeast Asia (Zieritz et al 2018b) has either not been assessed to date or is currently classified as ‘‘data deficient’’ by the IUCN (IUCN 2019). The freshwater mussel fauna of Borneo is diverse and highly endemic, with 3–4 of its 10 genera and 11–13 of its 17 species being unique to the island (Table 1; Zieritz et al 2018b; Graf and Cummings 2019). Our understanding of the Bornean freshwater mussel assemblage is very limited and most species have not been recorded for several decades (Zieritz and Lopes-Lima 2018). The non-native invasive species Sinanodonta cf. woodiana showed positive associations with human influence and is the most widespread and common freshwater mussel in the study area
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