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DNA barcoding sheds light on cryptic silverfish species

3 min read
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The application of DNA barcoding by Australian Museum (AM) researchers has been used to unravel the species complexHeterolepisma sclerophyllum, in addition to investigating silverfish phylogenies in the remote islands off Eastern Australia.

Silverfish are a fascinating group of insects. Most infamous for invading our homes, they belong to the ancient order Zygentoma. Possessing primitive characteristics, they are believed to have evolved over 400 million years ago. Small and wingless, their scaly bodies taper to a point ending in three distinct appendages.

Silverfish are found all over the globe, and call a broad range of environments home. There are species living in the driest of deserts, where they absorb moisture from the air through their anus in order to survive the conditions. Some are close-knit neighbours of termites and ants, found living amongst them in their mound-like homes. Several species are found in the remote and semi-tropical islands off the east coast of Australia. And lastly, there are numerous that are blind, inhabiting the dark depths of caves and seemingly inaccessible rock cracks.

The identification of silverfish is notoriously difficult as they continuously moult, even following sexual maturity. This perpetual moulting can result in substantial morphological differences amongst individuals of the same species, making taxonomic evaluations tricky.

Prior to two 2019 studies by AM Researchers Dr Graeme Smith and Dr Andrew Mitchell, 24 species of Heterolepisma were known from around the world. Based on morphological characteristics Heterolepisma sclerophyllum was described as a single species in 2014, with a range from the tip of Queensland to the southern reaches of New South Wales.

Since its discovery, numerous Heterolepisma sclerophyllum specimens with similar characteristics have been collected, spanning the Australian east coast. Graeme and Andrew analysed 68 of these specimens for DNA barcodes, also comparing the sequences 16S and 28S (these nuclear and mitochondrial rDNA sequences are commonly used in phylogenetic studies). The data showed considerable differences between QLD and NSW populations, as well as within state populations. A detailed morphological examination was also undertaken. Following these analyses, two new species were described; one from southern Queensland (Heterolepisma cooloola) and one from Glen Davis NSW (Heterolepisma coorongooba).

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The results of the genetic analysis also aided in the determination of which morphological characteristics are most useful in differentiating species within Heterolepisma. Scale shape, the absence of large bristles from the forehead and the number of pairs of abdominal styli were found to be the most important traits.

Without DNA barcoding, disentangling this species complex using morphological characteristics alone would have been all-but-impossible. This is due to numerous species sharing similar traits, as well as considerable variability between individuals of the same species, due to continuous moulting.

Not satisfied with discovering two novel species, Graeme and Andrew took their silverfish fervour to several islands off Eastern Australia. A lack of wings and love of the desert has not prevented these primitive insects from colonising remote islands. Lord Howe Island, Norfolk Island, Balls Pyramid and the tropical Herald Cays are home to a diverse fauna of silverfish. The pair examined 14 Heterolepisma specimens from these islands, with DNA and molecular analyses supporting two new species. Using morphological criteria, with the aid of molecular data, a new genus was also described! The genus, Maritisma, was discovered on the Herald Cays coral atoll. Unfortunately, Maritisma, along with a new species also described from the low-lying Herald Cays (H. heraldense) are at risk from becoming endangered due to rising sea levels.

Although they are not the most popular of insects, silverfish have lasted the test of time. They have endured in the toughest of habitats, survived two mass extinctions and are an incredibly diverse group. Increased use of DNA barcoding in taxonomic studies is bound to further reveal cryptic species, as well as increase our understanding of the silverfish phylogenetic tree.

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