Under the sand dunes of Californian coasta tiny architect hidden for millennia has just been discovered. The scientists ofUniversity of Californialed by Emma Jochim and Jason Bond, have discovered that what for over a century was considered a unique species of trapdoor spider – Aptostichus simus – actually hid a second species: Aptostichus ramirezae sp. Nov. The first, Aptostichus simus it’s a trapdoor spider (Euctenizidae family) which lives exclusively in the coastal dunes between Baja California and Monterey Bay. These environments, threatened by erosion, urbanization and rising seas, are the habitat of a unique but fragile fauna. The trapdoor spider builds tunnels lined with silk and closed by a camouflage “lid” of sand. Although the first genetic analyzes date back to the 1990s, only thanks to latest generation genomic techniques the authors of the study published on Ecology and Evolution they were able to clarify the identity of the spider found and separate the two species, which are almost identical to the eye indistinguishable.
Indistinguishable to the naked eye, but genetically different
When the researchers compared the DNA of spiders collected along the California coast, they realized that they did not all belong to the same population. Genetic analyzes (over 4 million genetic sequences per individual) have in fact revealed three very distinct groups — one in the north, one in the central area and one further south — separated by profound differencesso much so that they no longer share genes with each other. In numerical terms, the global genetic distance (denoted G’ST) is found 0.68, a very high value which confirms an independent evolution; among the southern and central groups it even reaches 0.77.
To check if there were still exchanges of genetic materialthe authors used a test called ABBA-BABAa sort of “mixture detector”: the result showed that no more recent intersections occur among populations.
From an external point of view, however, the differences are almost imperceptible. Yet, measuring the males’ paws and palps with millimetric precision, 9 out of 17 characters they are different; in females, 8 out of 15 change significantly. In practice, genetics tells a story of ancient separation, while appearance continues to deceive the human eye.
The new species: Aptostichus ramirezae
The northern and central populations, genetically distinct from the southern group, were recognized as a new species: Aptostichus ramirezaededicated to the biologist Martina G. Ramireza pioneer in the study of trapdoor spiders.
In the males of the new species, the reproductive organ called embolus — a small structure at the end of the palp that serves to transfer sperm — has an unmistakable shape: a serration which has four to ten serrations, like a tiny irregular blade. Even the front legs and the tibial palpa fundamental segment for mating, are slightly shorter (approx 1.76mm long and 1.05mm wide) compared to those of Aptostichus simus.
The femaleshowever, are distinguished by a microscopic characteristic: they possess over 300 small cusps on the endite, a part of the mouth that they use to handle food. Their forelimbs are also a little shorter than their “sister” species.
This spider lives exclusively along a limited stretch of the Californian coastbetween Moss Landing State Beach and El Segundo Beach, and on two offshore islands, Santa Cruz and Santa Rosa. A restricted range which makes it even more precious and vulnerable.
The habitat of these spiders is rapidly declining
There has been a genetic separation, but despite this, the two species remain almost identical in appearance: a perfect example of cryptic speciationwhere evolution occurs without major visible changes.
California’s increasingly eroded and urbanized coastal dunes represent a rapidly declining habitat. Some populations, such as those on Broad Beach (Malibu), have already disappeared. At the current rate of sea rise – 2.13 mm/year over the last century, expected up to 6.2 mm/year by 2030 – researchers fear that entire genetic lines may become extinct before being studied.
