Scientists Find a Spider Wearing a “Pearl Necklace” – and It’s Made of Living Parasites

A chance observation of an unusual spider led scientists to identify a new parasitic mite species in Brazil.

While sorting through spiders and scorpions in the Zoological Collections Laboratory at the Butantan Institute in São Paulo, Brazil, researchers noticed something unusual. A spider only a few millimeters long appeared to be wearing a strand that looked like a pearl necklace, so they asked a colleague who specializes in mites to take a look.

Ricardo Bassini-Silva, a researcher and curator of the Acarological Collection in the same laboratory, quickly recognized the “necklace” as mite larvae. Until then, Brazil had only a single reported case of mites that parasitize spiders, and that earlier record involved a different mite family.

Using a combination of microscopy methods, including scanning and light techniques, the team studied the animal’s morphology and confirmed a major find. Their analysis led to the description of Brazil’s second known spider-parasitic mite species and the first in its family to be documented in the country.

Backed by FAPESP, the work was published in the International Journal of Acarology.

The research brings together two FAPESP-supported projects. One is led by Bassini-Silva, and the other is coordinated by Fernando de Castro Jacinavicius, a co-author of the paper and a professor at the Institute of Biology at the University of São Paulo (IB-USP).

Expanding the Known Range of a Genus

The mite, named Araneothrombium brasiliensis, belongs to a genus first described in Costa Rica in 2017. Finding it in Brazil points to the possibility that related species occur across other neotropical countries as well.

Each larva measures about 500 micrometers, or half a millimeter. The spiders they were attached to are only a few millimeters long. For now, the species is known only from its larval stage; the larvae were found parasitizing juvenile spiders from three different families. All of the specimens were engorged, meaning they had fed so much that their bodies became greatly enlarged.

“For this group of mites, it isn’t uncommon to know many parasitic species only through their larvae, since in adulthood they become free-living predators, living in the soil and feeding on small insects and even other mites, which makes them very difficult to find,” says Bassini-Silva.

The spiders parasitized by the new species of mites were collected in Pinheiral, a municipality in the state of Rio de Janeiro. The environment was close to caves and grottos, similar to that of the first Brazilian species of spider-parasitic mite, Charletonia rocciai.

In a paper published in 2022, the researchers redescribed the species first described in 1979. They added new morphological characteristics, biological data, locations of occurrence, and hosts, including spiders.

Pearl necklace

Spider mites feed on lymph, a fluid that circulates through the bodies of certain arthropods. They suck the fluid through the pedicel, which is the region between the cephalothorax (where the spider’s eyes and mouth are located) and the abdomen.

“This is the spider’s most vulnerable region since other parts have a lot of chitin, which forms an exoskeleton difficult for the mites‘ fangs to penetrate,” the researcher explains.

Their presence on juvenile spiders may indicate opportunistic behavior, as young individuals are more vulnerable to parasites and predators. Additionally, the species could potentially parasitize other arthropods, such as insects. This is the case with Charletonia rocciai, which parasitizes at least two orders of insects.

“With more than 3,000 species of spiders alone, Brazil has immense potential for discovering new parasitic mites,” says Bassini-Silva.

The study also highlights the importance of zoological collections in studying biodiversity. The spiders had been stored for years, and no one had noticed the mites until now.

Through partnerships with researchers and environmental consulting companies that work in the field, Bassini-Silva hopes to soon receive more mites associated with other animals and describe new species.

Reference: “First species description of Araneothrombium Mąkol, Felska and Król, 2017 (Trombidiformes: Microtrombidiidae) parasitizing spiders in Brazil” by Ricardo Bassini-Silva, Cal Welbourn, Ronald Ochoa, Antonio Domingos Brescovit and Fernando de Castro Jacinavicius, 7 October 2025, International Journal of Acarology.
DOI: 10.1080/01647954.2025.2566344

Funding: São Paulo Research Foundation

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