Saturday 4 June 2022

Ameronothrus retweet: A second species of Japanese Marine-associated Oribatid Mite discovered on Twitter.

Most Arachnids are associated with terrestrial or freshwater ecosystems (Sea Spiders, Pycnogonida, and Horseshoe Crabs, Xiphosura, are generally viewed as Chelicerate Arthropods closely related to, but outside the Arachnida), although some species have colonised inter-tidal zones, notably the Marine Spider, Desis marina, of New Zealand, and a variety of Oribatid Mites from the Superfamily Ameronothriodae. 

Four separate families of Ameronothriod Mites are considered to have colonised the inter-tidal zones, the Fortuyniidae and Selenoribatidae, which are found in tropical and subtropical zones, the Podacaridae, found in the southern temperate and polar zones, and the Ameronothridae, found in northern temperate and polar zones, although some experts consider the Ameronothridae and Podacaridae should be treated as a single family.

The Ameronothridae as it is currently defined comprises a single genus, Ameronothrus, which currently contains 14 valid species, the most recently identified of which Ameronothrus twitter, was described in 2021 after pictures of an unknown Mite from the inter-tidal zone of the Chiba Peninsula on Honshu Island, Japan, were posted on the social media platform Twitter.

In a paper published in the International Journal of Aracology on 19 May 2022, Tobias Pfingstl of the Department for Biodiversity and Evolution at the University of GrazShimpei Hiruta of the Center for Molecular Biodiversity Research at the National Museum of Nature and ScienceIris Bardel-Kahr, also of the Department for Biodiversity and Evolution at the University of Graz, Yuito Obae of the Graduate School of Sustainability Science at Tottori Universityand Satoshi Shimano of the Science Research Center at Hosei University, describe a new species of Ameronothrus discovered as a result of the discovery of Ameronothrus twitter being discussed on the social media platform after which it was named.

The new species is named Ameronothrus retweet, due to the way in which it was discovered, on the basis of photographs posted in response to a tweet (Twitter post) announcing the discovery of Ameronothrus twitter. The photographs were taken by Yuito Obae of of some mites he discovered on Iwado Rock Beach on the Japan Sea (north) coast of Honshu, which he thought might be another population of Ameronothrus twitter, but were subsequently recognised as another new species.

Specimens of Ameronothrus retweet range from 641 to 859 µm in length, and are dark brown or black in colour, with a densely granulated cuticle, the granules being larger on the lateral sides of the body. As with other Ameronothriod Mites, Ameronothrus retweet appears to feed on Algae, Fungi, or Lichen found in the inter-tidal zone.

Photographs of male (upper row) and female (lower row) Ameronothrus retweet specimens in dorsal (left side) and ventral view (right side). Pfingstl et al. (2022).

Unusually for an Oribatid Mite, Ameronothrus retweet shows sexual dimorphism, with the females showing strongly folded gastronotic integument and considerably shorter epimeral, genital, and aggenital setae than seen in the males. While sexual dimorphism has not previously been recorded in an Ameronothriod Mite, it has been recorded in a number of other Oribatid Mite species associated with aquatic or intermittently wet environments, so the presence of such a trait in a species of Ameronothrus should not be seen as a complete surprise. The purpose of sexual dimorphism in Oribatid Mites found in aquatic environments remains unknown, although increasing the number of species in which this is known may help to solve this mystery.

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