Midges Divide Time and Space

Lunar rhythms combined with the tides can help separate midge strains and set the stage for new species to evolve

To the point

• Two strains of the same species: Clunio marinus strains reproduce at full moon or new moon—yet inhabit the same coastline.
• Timing translates into egg-laying location: The time of reproduction determines where the larvae live in the intertidal zone.
• Temporal niche meets spatial niche: The combination of lunar timing and habitat separation enables coexistence.
• Scientific evidence: Genotyping and mathematical modelling provide the first explanation of the long-term coexistence of Clunio strains.

How can different ecologically specialised populations of the same species—so-called ecotypes—coexist within the same habitat and even evolve into distinct species over time? This fundamental question of ecology and evolutionary biology has now received new insight from a research team led by Tobias Kaiser of the Max Planck Institute for Evolutionary Biology in Plön and Hanna Kokko of the University of Mainz. Their work focuses on the marine midge Clunio marinus, and they have identified a causal link between the timing of adult emergence and the spatial distribution of larvae—an interaction that enables the coexistence of different ecotypes.

Clunio marinus inhabits the intertidal zone along the European Atlantic coast. Its larvae live deep in this zone to remain submerged as much as possible. However, adult midges depend on the habitat becoming dry so they can lay their eggs—something that reliably occurs during spring tides, at full and new moons. The development and maturation of Clunio are therefore regulated by a lunar clock (a so-called "circalunar clock"). This clock ensures that adult midges only emerge during spring tide low tides – they mate and oviposit immediately and die only few hours later in the incoming tide. A few years ago it was shown that two strains of Clunio coexist in Roscoff, Brittany (France): one reproduces only at full moon, the other only at new moon.

Coexistence of full moon and new moon strains

In an initial study [1], Runa Ekrem, Lotte de Vries, Tobias Kaiser and Hanna Kokko used mathematical modelling to show that the coexistence of full moon and new moon strains is not to be taken for granted. Although the adult midges reproduce at different times, their larvae inhabit the same space and therefore compete—as a result, over time, one strain would be expected to outcompete the other.

Coexistence was only possible in the model if larvae were assumed to switch from one (food) resource to another at a very specific point in their development. However, such a shift in diet has not been observed in real Clunio larvae, leaving the strains’ coexistence unexplained.

Stable coexistence of the two strains

A second study [2] by Runa Ekrem, Alexander Jacobsen, Hanna Kokko and Tobias Kaiser solved the puzzle. The researchers discovered that the new moon strain does not reproduce at the optimal time just after the new moon, but instead a few days earlier. At that time, the low tide is not as extreme. This led to the hypothesis that the new moon strain must lay its eggs higher up in the tidal zone than the full moon strain. If true, this would mean that their larvae likely occupy different spatial zones.

By collecting larvae in the field and genetically assigning them to the full moon or new moon strain, the team was able to confirm this spatial separation. In short: the “temporal niche” of the adult midges, shaped by their timing of emergence and its interaction with the tides, results in a “spatial niche” for their larvae. Once this spatial element was incorporated into the mathematical model, the stable coexistence of the two strains became possible.

Ongoing research is now exploring the genetic mechanisms that support this coexistence, as well as other ecological factors—arising from the spatial separation of larvae—that may drive further evolutionary adaptation and ultimately support the formation of new species.