The secret of the snail that is not a snail
1 November 2018
Photo: UHH/CeNak, Wiggering
The snail shell-like appearance of its quiver clearly distinguishes the fly from its relatives.
Small, gray, inconspicuous - but a shining example of the miraculous workings of evolution. Our treasure of the month November is the young of the caddisfly Helicopsyche helicifex. Some specimens of this species are in the malacological collection of LIB and have been used for research.
The snail shell-like appearance of its quiver distinguishes Helicopsyche helicifex from its relatives otherwise equipped with elongated quivers and abdomens and secures it a decisive evolutionary advantage. This form of adaptation to other organisms, which is presumably present here, is known in technical language as mimicry and in this case has been described in detail for the first time by researchers at CeNak.
Evolutionary history of the species illuminated comprehensively for the first time
That the shell shape of the caddisfly larvae of the helicopsychids may be a form of mimicry has never been discussed in detail before. Specimens of this caddisfly family have often been incorrectly described by researchers as new species of snails. Benedikt Wiggering and Matthias Glaubrecht have now for the first time discussed in detail the possible evolutionary history of these small adaptive artists. The evolutionary-biological scenario they developed can now be tested in detail.
Most caddisfly larvae build their caddis as a cylindrical tube around their soft body. But not Helicopsyche helicifex, which lives in Portugal. Although it too uses materials from its environment to build its quiver, it is coiled like a snail shell. This makes it look confusingly similar to some water snails, for example species of the feather gill snails (Valvata). These tend to be less popular with birds, which are also predators of caddisfly larvae, and are only on the menu in lean times.
Good camouflage, stronger reproduction
For the caddisfly larva, this enclosure offers double protection. On the one hand, it is well camouflaged by the materials of its quiver and hardly stands out from its surroundings. Second, by adapting to the appearance of water snails, it is less likely to be preyed upon by predators. This type of mimicry is more accurately termed Batessian mimicry and describes the imitation of a defensible or inedible organism by one that is edible and non-venomous.
Such adaptation occurs through natural selection and over a long period of time. In the case of Helicopsyche helicifex, Benedikt Wiggering and Matthias Glaubrecht, both researchers at the Centrum für Naturkunde, explain for the first time the emergence of the shell through a two-stage evolution: "Through mutation, a sinuous shell may have prevailed in the ancestors of the Helicopsyche living today, as it offers higher stability in fast-flowing waters and when buried in the aquatic substrate." After the first sinuous caddis thus emerged, Benedikt Wiggering theorizes, such individuals that resembled snails were less likely to be preyed upon by enemies. "This probably allowed them to reproduce more. As a result, this appearance prevailed in the descendants."